Introduction

This document will describe the basic operation and application of the Leica Total Station surveying equipment as well as discuss the process of downloading the data and producing detailed topography maps using the Liscad software. It is meant to explain the operation based upon the Active Tectonics members’ experiences in using the equipment, and should supplement the reading of the manual and your intuition. This document contains the basic set-up and operating instructions for our electronic surveying equipment. It also contains discussions of how the instrument works, and suggestions for methods and strategies of use.

Equipment

Description

Most of this equipment is stored in the locked cabinet, or alongside it, in the Active Tectonics lab, room 475 of the Physical Sciences H-wing. An equipment list and check out sheet are posted on the door of the cabinet.


Table 1. Location of the Total Staion and its components.
Picture of all items and associated checklist.

Item

Description/Location

Leica (Wild) TCM-1100 self-contained Motorized Coaxial Electronic Total Station

This is the core of the surveying equipment, controlling the EDM (Electromagnetic Distance Measurement device), manipulating and storing the data, and measuring horizontal and vertical angles. It has an optical plumb for centering over a station and two levels. The EDM is implemented around the theodolite's telescope so both sighting optics are coaxial. The EDM measures the slope distance to the reflector, and communicates the data to the theodolite. We have two TCM-1100 that are kept in the cabinet in H475.

Reflector

Corner-cube type. Two of these, along with a reflector cap, are kept alongside the cabinet in H475.

Plumbing pole/reflector staff

The reflector attaches to the top of this and it telescopes in height from 1.30 m to 2.15 m. It is light and a useful alpine staff, although Roland Bürgmann stabbed himself before on the pointed bottom of a similar plumb pole, so be careful. Two of these are kept alongside the cabinet in H475.

Tripod for Theodolite

Two of these are kept alongside the cabinet in H475.

Small Internal Batteries

We have two small batteries. They are inserted into a compartment on the side of the Total Station. Each of these when fully charged within a few days of the survey effort should be good for at least 300 to 800 shots, depending on use of motors. Each small battery weighs ? kg (? lb). These are kept in the locked cabinet or in its compartment in the Total Station.

Gel-cell Batteries and cords

The gel-cell batteries can support the Total Station for 7x the time of the internal battery. They weigh about 8 pounds (3.5 kg). They can be left on the smart charger indefinitely. We have two of these, along with padded cases. We also have two cords that plug into the side of the Total Station where a gutted battery takes place of the real one. These are also found in the locked cabinet in H475. This battery system owes its origination to Lee Amoroso, our battery specialist.

Battery Charger

Two that plug into an electrical outlet; two that run off the car battery

Cables (see checklist)

Rechargeable battery (gel-cell) to Total Station (2); outlet to Total Station(2); adapter cable for inverter(2).

Portable Power Inverter

We have two for the batteries.

International Adapter Plugs

important if traveling overseas (2 sets).

Strip Surge Suppressor

We have two.

Verbatim SRAM Card 2MB(I think it stands for Static Read-Only Memory, but check)

The SRAM card acts as a memory module for coordinates and attributes recorded with the Total Station. SRAM cards can hold variable amounts of data (e.g., 2 MB SRAM cartridges, 8 MB cartridges, etc.) and allow for the Total Station operator to collect a large amount of data, given that s/he has enough SRAM cards to store the project's work. The SRAM card interfaces with a PC via the PCMCIA port; therefore, transfer of data is quick and easy. A 2MB SRAM card is capable of storing approximately 8000 blocks of data.

2-Way Radios

We have 4 Motorola Radius SP50 and 4 BELL Phones (#9914-1) FRS UHF 2-Way Radios


Leica (Wild) Surveying Equipment

The Leica (Wild) equipment is well known as some of the best surveying equipment available. It is often referred to as "the Cadillac" of surveying equipment. "All surveyors either own a Leica (Wild) or wish that they did" (Hanz Haselbach, Jr., personal communication, 1991). Leica (Wild) makes top of the line equipment that is used on all expeditions and has better mechanics, optics, factory support, ease of repair, and ability to maintain resale value than other manufacturers. Furthermore, the company supports older equipment by providing a modular set-up, allowing selective upgrades:

We bought our system from:

Surveyor’s Service Company
4317 N. 16th Street
Phoenix, AZ 85016
(800) 938-0608
(602) 274-2052

They should be consulted for upgrades, repairs or with questions.

Preparation

In order to properly prepare for a survey effort, checking the equipment in the lab/office before leaving and proper reconnaissance before set up are important. On the next page is a list of equipment and the checks that should be made before leaving the office.

Planning and Reconnaissance

Before leaving the office, look at the base map or airphoto of the area and define the mapping project(s). Consider access. If you need permission to get on the property, you should get that well in advance. The equipment is heavy and bulky. Two people can carry it with difficulty for 1 or 2 kilometers, but it is a demoralizing experience. Therefore, try to plan appropriate vehicle access, but don't drive unnecessarily through the landscape destroying the beauty and making a bad name for geologists! You must have a field assistant. Operating this equipment by yourself is pretty silly, although it can be done. You will end up walking at least two times the total slope length of the shots you make. The distance will add up quickly! Your helper need not be a geologist, but should be sturdy, careful, and interested.

Once in the field, walk over the area with the field assistant. Look for suitable instrument stations. Much time and precision may be lost with multiple set ups. Therefore, plan the minimum number of instrument stations. A high position somewhere near the survey area should suffice. Remember that the instrument has an effective working range of at least 1 km, so if you have walkie-talkies or body signals and can communicate (see communication techniques) over long ranges, much of the survey may be completed from a single well chosen site. Consider gullies and ridges, and other obstacles that may obscure the survey. Remember that the reflector plumb staff telescopes to 2.15 m, so that may help with visibility. Be sure to communicate to the instrument operator changes in reflector height (click here to see how to enter into Total Station) and record them on the Survey Record Form and in the Thoedolite! While reconnoitering the area, consider the problem being addressed by the mapping project, and choose an appropriate and optimal strategy: What is the optimum number and distribution of points necessary for this project? What is the desired precision? Will I ever want to reoccupy this network? Be sure that the field assistant understands the strategy too, so that you do not have to have major discussions about strategy once you have started working, although the plan should be flexible if an obvious modification becomes apparent.

 

Table 2. Equip and location.

Y

Item

Check/comments

 

Essential Total Station Equipment: Total Station and parts photo checklist

 

Theodolite

Check smoothness of horizontal and vertical spindles. Turn on and off to check electronics.

 

Tripod

Set up and make sure that it opens smoothly. There should be no play between the various components. If necessary, moderately tighten the three Allen screws under the tripod plate. When the tripod is lifted by its head, the legs should just remain spread out. Adjust the hinge screws if necessary. The yellow plastic head cover for the tripod contains an Allen key for the tripod screws.

 

Verbatim SRAM card

This is the memory. It should be stored in the side of the Theodolite in its compartment. Check with previous users of the instrument to determine if they have backed up any data they might need in the module so you can clear it if necessary before starting your survey. Note: It must be inscerted correctly: Look at the front with the right-side-up and that side goes on the bottom of the case. So when the case is shut the front is facing the outside of the Total Station and it is right-side-up.

 

Reflector

Ensure it is clean and stored in a clean place.

 

Reflector plumb staff

Make sure it telescopes smoothly.

 

Batteries

Ensure that at least one of the large ones has been charged. To fully charge, it may take 8 to 12 hours. Also charge the on board battery on the Total Station.

 

Computer

Optional. Make sure that you have all of the cables and parts (mouse, keyboard, etc.). Software necessary for Liscad plots include Excel, Canvas, Liscad, and the Liscad dongle. Bring some floppy disks to back up data in a few places.

 

Brunton Compass

Used to approximately level the instrument, and to determine reference azimuth.

 

Backpack, Briefcase

These should be loaded up with the gear.

 

Altimeter

Measures barometric pressure for scale correction

 

Thermometer

Measure temperature for scale correction.

 

Extra survey Equipment:

 

Flags

Do you have enough for your plan?

 

Flagging tape

Could be useful.

 

Survey Record Form

Are the information columns adequate?

 

Clipboard

 
 

Pencils, ruler, protractor

 
 

Liscad Dongle

Necessary to run the Liscad plotting program.

 

Printer and paper

Good for mapping and plotting results

 

Cables

Printer, laptop

 

Binoculars

Helps to spot instrument or rod person.

 

Walkie-Talkies

Make sure batteries are fully charged and if necessary have extras.

 

Calculator

Useful for checking measurements and calculations.

 

Metric grid paper

Useful for checking plots and measurements, and for mapping.

 

Rebar, steel rods, or stakes

For benchmarks. You may also want cement.

 

Sledge hammer

Pounding benchmarks. You can use a rock hammer, too.

 

Appropriate base map or topographic map/airphoto

Use these to locate benchmarks and reference locations for the survey.

Set-up

Choose an adequate instrument station as described above. In detail, ensure that you can safely operate the instrument without knocking it over. It is necessary to have the center of the instrument, which is the point of intersection of the transverse axis and the vertical axis of the instrument, directly over a given point on the ground (the Instrument Station). Also, the circle and the transverse axis must be horizontal (Moffitt, 1987). Remove the yellow plastic cap from the tripod, and leave the instrument in the case until the tripod is nearly level (see below). The tripod legs are spread (not too vertical) and their points are placed so that the tripod is approximately horizontal and the telescope is at a convenient height for sighting (this is important: you don't want to get a sore back from it being too low, nor do you want to get sore calves and fall over from it being too high). The instrument should be within 20 cm of the desired point, but no extra care is taken to set it up closely at once. If the location of the instrument station is not predetermined, set up first, and then use the optical plummet to locate the benchmark: this will save time, because it takes extra care to accurately set up over a benchmark. When setting up on rough ground, two legs of the tripod should be set at about the same elevation and the top of the tripod is made level by changing the length of the third. Consider the common sighting direction and try to have one leg pointing that way so that there will be a gap between two legs behind, and the instrument operator will not have to straddle a tripod leg for the entire session. If the instrument is more than a few inches from the given point, the tripod is shifted bodily without changing the inclination of the legs and it is set near as possible to the point. Check the level by placing a Brunton compass on top of the tripod and adjusting gently until the Brunton bubble level indicates that the top of the tripod is level. Tread tripod shoes firmly into the ground, making sure to press along the leg and not verically down (Anderson and Mikhail, 1989). If shoes do not penetrate to an equal depth, re-level by extending or retracting the tripod legs. Before attaching the instrument, check that the clamps of the telescopic legs are tight.

Once the tripod is nearly level and nearly above the desired point, remove the Theodolite gently from its case and carry it with one hand on the upper handle, and one hand below, to the top of the tripod, and immediately screw the tripod fixing screw firmly into the bottom of the theodolite. The levels on the instrument are very sensitive, and therefore, the instrument should be completely set up in order to ensure that it will be properly leveled. This will also give the tripod more time to thermally equilibrate with the atmosphere and incident sunlight, since differential thermal expansion of the tripod legs will effect the levels.

Now you are ready for fine leveling of the instrument. Use the three black footscrews on the tribrach (the device on the bottom of the Theodolite to which the tripod fixing screw is attached) to center the circular bubble. This is a coarse level. Turn the outer ring on the eyepiece of the optical plummet until the crosshairs are in focus. Turn the inner ring of the optical plummet until the ground point is in focus. Slacken tripod fixing screw and move instrument over tripod plate until cross hairs coincide with ground mark. Only translate the instrument over the tripod plate, do not rotate it. Finally retighten fixing screw. Now do the fine leveling.


Fine leveling instructions

1) Set plate level (the one in the tube--this is the fine level) parallel to two footscrews. Center plate level bubble by adjusting these two screws in equal and opposite directions.

2) Turn Theodolite through 90°. Center bubble with third footscrew.

3) Turn Theodolite through 180° (in the same direction as above). Note position of bubble. Turn the third footscrew (same one used in step 2) to bring the bubble to a point halfway between the position noted and the centered position. Use the adjusting pin (in silver handle in the Theodolite case) to turn the adjustment screw until bubble is centered (see page 44 of the Theodolite manual for a picture).

4) Repeat until bubble remains centered within a single division for any position.

Note that the instrument will drift out of level as the day progresses. Repeat the above steps as needed and enter change by pressing the level button (see Operation). If the optical plummet is not above the ground point desired, slacken tripod fixing screw and move instrument over tripod plate until cross hairs coincide with ground mark. Only translate the instrument over the tripod plate, do not rotate. Retighten fixing screw. Now press the ON button and the startup menu will come on. Press the white LEVEL (figure 1) key along the bottom of the key panel followed by continue. When the levels are set press the continue button.

Now that the instrument is leveled up, set up and test the electronics. First, insert the Verbatim SRAM card into the slot in the Theodolite. Ensure that the arrow on the back of the card faces down when you insert it. See below for a suggested initial key sequence for the Theodolite and see the TPS-System 1000 Short Instructions System manual in the Theodolite case for a list of commands for the Theodolite. The keypad on the Theodolite allows for maximum utility with the minimum number of keys. Therefore, most of the keys have several functions, depending upon the class of operation. One way to remember the meaning is by noting the background color of the keys.

Table 3. Classes of keypad operation for Theodolite (TCM-1100) (Short Instruction Book) See Figure 2.

Class of operation

Key color

Example

Fixed Keys

White

Functions which are always available. Returns to last diologue. ON OFF.

Function Keys

Orange

These keys correspond to the bottom line on the display. Pressing SHIFT followed by an F key displays other options.

Navigation Keys

Green

Vertical scrolling, position the cursor to edit, insert or delete numerical data and characters, and position colums.

Numeric Keys

Yellow

1...9 Numerical input. ". " and +/- set decimal place and sign. The Enter key to complete the data input of confirm a selection out of a data list. Use the CE key to delete the last entered digit or character.


Figure 1. Keypad.

The ENTER button is important in that it executes the preceding commands or answers in the affirmative, while the CE button often clears the last command. Before anything, start with a fresh copy of the Survey Record Form, and fill out the important details at the top. Use this form to take notes while surveying. Remember to record all observations and considerations, since the next time you look at this data, it might 6 months from now in the Dungeon! Record the observations so that someone else can understand. After all, someone else might use the data.

Table 4. Suggested command sequence for Theodolite start up

 

#

Key sequence

Description

1

 

Switches Theodolite on. Screen briefly displays software version.

 


2

 

After the bubble is level press the Continue button. You can check and adjust level all day.




3
   

 

Select the setup file by pressing F5 (Setup) and using the arrows to select file. Press F6 to get the list of files. Push enter to select (button with curved stem). It is also possible to set user templates based on many different users or survey types with different needs. We will go through the template setup later in the sequence. Press Continue to go to the SETUP/ STATION DATA menu.

4

x 4

 

Enter the station number (is it your first one of the network?, or is it a new station setup within a network?). Use the down button and enter instrument height. You measure this using the reflector staff and holding it parallel with the station and up to the small hole (1 mm diameter) under the Leica name on the right side of the Total Station. Arrow down and enter station easting (we use 1000), station northing (we use 1000), and station elevation (we use 100). Record this information on your Survey Record Form. Make sure you are in meters, and if not, we will go through the sequence below.

5

   

Press F4 to enter in the reference azimuth. Point the instrument at the backsight or reference azimuth and input the value. For example, 335.5 will set the reference azimuth and horizontal circle to 335.5°. All subsequent measurements will be made relative to that direction. Record Hz0 on the Survey Record Form. Press continue followed by F3, record.




6
       

 

Now you are back at the main menu. Press F4, Data, to see what is in file. Scroll down with arrows to File, selct F6 to list files or F5 (search) to examine individual files. By using the F3 and F4 keys, you can scroll through the file history. Press Continue when finished.

 

 

7

     

You should be back at the main menu. Press F3 (CONF) to system configuration. Press enter for user configuration. Press F4 (SET) if you need to change the language or units. Press F6 to list the selections and enter to select it. We use ENGLISH, Metre, 3 Dec., 360 °, 3 Dec., °C, mbar, Easting/Northing, Clockwise (+), and V-drive left. Press CONT when completed, and CONT to get back to main menu.


8
  (  or   

 

We are going to set the user template. The first step is to set the recording mask. From the main menu press F3 (CONF). Press Continue. From here you can change the recording mask sequence(F2) or the display mask (F3). Both are set in the following sequence: Press F6 (LIST) to list the possibilites followed by the enter key to select and move to the next choice. When complete press CONT and set the other mask if desired.

9

  

Now we are just about ready to shoot points. From Main Menu press F6 (MEAS). Enter the reflector height by pressing F4 (TARGET) and arrow down to Refl.Height and enter in the reflector height in meters. Any time the reflector height changes it must be changed in the Total Station and on the survey record form. Record height on the Survey Record Form. Press CONT to save this change and you will return to MEASURE MODE. For operation refer to chart E.

Operation

Now that the instrument has been set up physically and electronically, you are ready for operations.

Reflector

Check to make sure that the reflector is properly mounted on the plumb pole by squeezing the small button on the lower back portion of the reflector as you slip it down over the top of the plumb pole. When you point the telescope at the reflector, the cross hairs must intersect on the yellow reflector plate as shown in Figure 1. This offset from the actual reflector is the same as the offset between the center of the telescope and the EDM.

We also have a Triple Tilt Prism Assembly that is used for longer distances .

Focusing and sighting

Focus the reticle cross hairs by pointing the telescope at some other uniformly light surface (the sky), and rotate the inner black portion of the eyepiece until the cross hairs are sharp and black.

Point telescope toward reflector by means of optical sight. Use of this sight will speed up acquisition of the reflector significantly. With both eyes open, have the image of the white cross hair in the optical sight in one eye, and the reflector in the other. When the two are superimposed, you are approximately on target. This saves a significant amount of time searching for the reflector with the actual telescope.

The horizontal and vertical rotations of the Theodolite are controlled in two ways. First you can rotate the Theodolite horizontally by hand by placing two hands on either side of it and rotating it. To rotate vertically, rotate the EDM unit up or down by hand by nudging in up or down. Manual rotation works best and is less demanding on the battery if there is a distance change from point to point of several meters. Otherwise fine-tuning is done by turning the horizontal and vertical knobs on the sides of the unit.

The instrument updates angle measurements within 0.3 seconds, so the angles can be quickly displayed. In order to see the horizontal and vertical angles of the current pointing, push F2 (DIST). Before the station is moved press F3 (REC) to record the point.

Measurement and recording

With the instrument pointed at the reflector, ensure that the reflector operator is holding the reflector still and level (by using the bubble level on the plumb pole), and then push F1 (ALL). The EDM will measure the slope distance in about 3 seconds, and it will display on the LCD of the EDM if you push (DIST). Up to this point, the data has not been recorded. To record the entire block of measured data, push (REC). If the format used is not the standard recording format (and it is not if you followed the suggestion in step 8 above), the query "OK?" appears on the display before recording the first data block. Press OK to confirm and record the first data block. This will happen each time the instrument is turned on. As you can see, with many shots, you would end up pushing often. To make things go quicker, measurements that you don't want to check can be speeded up by pushing . The instrument will measure the distance and automatically record the block of measured data and increment the current point number by 1. However, it will not display the distance or other calculated values like Easting or Northing in the LCD. To see these to check the shot, you would have to press (DATA) (SEARCH) and then to scroll through the points.

As you are surveying it may be advantageous to switch operators occasionally. Another important hint is to have simple hand signals to communicate the status of the measurement to the reflector person or the type of topography to the instrument person without having to shout or use the two-way radios for simple communications (See Communication techniques). I like to have the instrument person hold his or her arm straight up when starting to focus and measure to indicate to the reflector person that he or she should hold the reflector level. As soon as the distance measurement appears in the LCD, the instrument person's arm is held sideways to indicate that the shot is complete and the reflector person should move to the next target point. With this scheme, approximately 100 points per hour can be acquired. Our record for one day is 707 shots with three people rotating through the two positions. If the target points are further apart or in rough terrain, you may have two reflector people, and alternate shots between them. This will speed up operations, but it requires more coordination and concentration, especially on the part of the instrument person.

Communication Techniques

Your surveying team may not be blessed with radio communication. However, there are some basic arm signals that we have developed for geomorphic features.

"Shoot topo points."

"I'm ready to shoot point, please level the staff."

"End that series."

"That was the first point for a linear feature (fault)."

"That was the first point for a breakline."

Coding specific points and breaklines

Coding specific points or breaklines adds sort-of a dimentional character to your map. We generally use the Total Station to produce maps or profiles that contain features-such as topographical breaks in slope-that, when accentuated with a polyline, provide a three dimentional view on the map. This section will discuss the coding system and cover the keypad procedure.

There are basically three types of codes: specific survey point codes (i.e., landmarks), polyline codes (i.e., topographic breaks), and physical feature codes (i.e., flotsam, trees). Specific survey points, such as a benchmark or survey mark are denoted by a symbol other than a dot. Polylines connect a given set of points to denote a ridge, stream, scarp, etc., and provide a line of which topolines will "break" along when plotted using the Liscad software. Also, physical features such as vegetation, roads, buildings, and electric can be symbolized on a map.

To assign a point or set of points to a code, first shoot the point as described in Table 4, only make sure to press F2 DIST. Next, you must set that point to a code number.

See this link for coding details: coding.

Data downloading, manipulation, and software

I want to emphasize again the importance of taking good notes while surveying. A few quick annotations may save a major hassle down the road. Use the Survey Record Form to organize the observations, and your organization and manipulation of the data should go smoothly.

Data storage format

The memory card stores two types of information units: measurement blocks and code blocks. With our present software set up, we don't use code blocks. You are welcome to experiment with them. Other more sophisticated software systems use the code blocks as part of the data organization. The measurement blocks contain measured data and a point number. Each block includes a consecutive unique block number which is recorded automatically. If possible, record both the point number and block number on the Survey Record Form in order to avoid ambiguity later.

The memory card data is made up of data blocks, each of which is made up of words with a fixed length of 16 characters per word. Each block may contain up to 8 words. A measurement block has the following data format:

Word 1

Word 2

 

Word n

 

Point number

 

....

 

End character

The first word of a measurement block always contains the point number. The remaining words of the block are determined by the measurement format of the Theodolite. Here is an actual data block:

110001+00000000 21.103+09803900 22.103+08849970 31..00+00258533 51....+0100+000 81..00+00255904 82..00-00036142 83..00+00007194

While it appears quite cryptic, you can see the word identifiers in there at the beginning of each word indicating the data (31 for example is slope length). The first word, 110001+00000000, contains the word identifier, 11, meaning point number (click here for more codes; 21 is the horizontal angle, 22 is the vertical angle (measured from zenith, so that is why it is usually around 90 for the typical nearly horizontal shot0, 31 is the slope length, and 81, 82, and 83 are the easting, northing, and elevation respectively), then it is followed by the block number (0001), and then the point number (00000000). Each of these will be incremented automatically. The running point number may be changed or input as surveying proceeds, but the block numbers are unique. See coding for corresponding features.

Downloading

The data should be downloaded from the SRAM card when the surveying is complete. The SRAM card should then be cleared of data for the next surveying trip. The Macintosh computer systems that we use are not the type supported by the surveying community; therefore, our scheme is original and somewhat circuitous, but effective. In the motel, truck, or office, remove the SRAM card from the Total Station, and insert it in the laptop. Open Microsoft Excel and open file. Excel will ask you to choose from several options: plate 1- select delimited; plate 2- select space and tap delimiters; plate 3- finish. All your data will be in the form indicated below in the raw data block. This form needs to be changed to either be viewed by Liscad or as northing, easting, and elevation in excel.

Note that these days (2004), I typically just parse the raw file in excel and put a column break in on the left side of each plus as well as at the spaces, and then you can delete the columns you don't want and will have millimeters for the unit for the distances and easting, norrthing, and elevation which you can easily correct.

Filtering the data

Obviously the raw data is of little use for analysis.

Raw data block:

110549+00006000 21.323+07291800 22.323+09123700 31..00+00180854 81..10+01172836 82..10+01053110 83..10+00996292 87..10+00001300 410550+00000005 42....+00001101

Block #

Point #

Easting

E

Northing

N

Elevation

H

Horiz. angle

Hz

Vertical angle

V

Slope length

D

549

6000

117.2836

105.3110

99.6292

72.91800

91.23700

180.854

To view as easting, northing, and elevation data in excel, you must first delete rows that are not needed. The word identifiers (the first two numbers at the beginning of the word) indicate the code of the word. We keep the point number, 41 (point number), 42 (code), 81 (easting), 82 (northing), and 83 (elevation). Delete all other columns. Be careful not to delete columns where additional data was recorded causing a shift of words. You also need to add this to the header (first line):

Wild REC-TotalStation

Here is a sample that works. It is important to note that the data should be in this order because the program (liscad) will take the second three words and assume that they are Easting, Northing, and Elevation.

Wild REC-TotalStation
110549+00006000 81..10+01172836 82..10+01053110 83..10+00996292
410550+00000005 42....+00001101
110551+00006001 81..10+01177605 82..10+01046419 83..10+00992448
110552+00006002 81..10+01185919 82..10+01040090 83..10+00986772
110553+00006003 81..10+01193146 82..10+01036721 83..10+00985793
110554+00006004 81..10+01196477 82..10+01035305 83..10+00985896
110555+00006005 81..10+01211466 82..10+01026636 83..10+00979522
110556+00006006 81..10+01215893 82..10+01023119 83..10+00978902
110557+00006007 81..10+01221285 82..10+01019707 83..10+00977450
110558+00006008 81..10+01225470 82..10+01015623 83..10+00976024
110559+00006009 81..10+01240939 82..10+01016348 83..10+00975100
110560+00006010 81..10+01253217 82..10+01013742 83..10+00971985
110561+00006011 81..10+01271393 82..10+01012739 83..10+00969438
110562+00006012 81..10+01286564 82..10+01008193 83..10+00966146
110563+00006013 81..10+01290373 82..10+00997422 83..10+00964371
410564+00000005 42....+00000000
110565+00006014 81..10+01296148 82..10+00970876 83..10+00961116
110566+00006015 81..10+01302319 82..10+00944821 83..10+00958195
110567+00006016 81..10+01278370 82..10+00934876 83..10+00957227
110568+00006017 81..10+01272799 82..10+00954352 83..10+00959228
110569+00006018 81..10+01261752 82..10+00980963 83..10+00962315
110570+00006019 81..10+01265783 82..10+01001572 83..10+00965681
410571+00000005 42....+00002101
110572+00006020 81..10+01265783 82..10+01001563 83..10+00965681
110573+00006021 81..10+01253140 82..10+01001680 83..10+00966532
110574+00006022 81..10+01238534 82..10+00999553 83..10+00966963
| 110575+00006023 81..10+01250007 82..10+00944827 83..10+00957900


Lastly, save excel worksheet with a .ex extension. This file will be the master archive for your data. From it you can manipulate, plot, and contour your data. Print this file out and save the hardcopy somewhere safe.

Care

Our surveying equipment is fairly rugged and designed to last. However, it is also expensive and likes a little attention and Tender Loving Care occasionally. Giving the equipment a good cleaning occasionally and treating it with care will ensure that it will last for a long time. Here are some basic suggestions.

Transport

For transport, use shockproof packaging material for the instruments. We have nice carrying cases for the EDM and Total Station and most of the associated hardware. While these cases add significant weight, they really do protect the instruments, and the one time that you decide to lighten your load and only carry the instruments, will be the one time that they fall down a hill and are severely damaged--a fall which they would have survived had they been in the cases!

Cleaning and drying

Before cleaning, blow dust off lenses and prisms. Handle lenses, eyepieces and prisms with special care. Always use a soft, clean cloth or clean cottonwool. Breathe on glass components, then wipe gently. If necessary, slightly moisten cloth or cottonwool with pure alcohol. Do not use any other liquid. Never touch optical glass with your fingers.

Cables and plugs

Clean periodically. Do not let plugs get dirty. Protect from moisture. Use pure alcohol to rinse dirty cable connectors, then leave to dry thoroughly.

Condensation on prisms

When a prism is cooler than the ambient air it may collect condensation. If this happens, warm the prism for some time by placing it in a warm environment (room, vehicle, or inside clothing). Merely wiping the prism is useless.

Storage

If an instrument has become wet, unpack it on return to base. Carefully clean the instrument, accessories, case and foam inserts. Wipe dry. Repack only after all the equipment is again thoroughly dry.

In the field

The main perils in the field to the instrument are rain, blowing sand, heat, and being knocked over. If it is raining or blowing hard, you will have to postpone the effort. Protect against light mist and moderately blowing sand by placing one of the plastic hoods (stored in the EDM case) over the instrument whenever it will not be used for longer than several minutes. If the temperature is less than -20°C or greater than +50 °C, you should not operate the instrument. Protect against temperature fluctuation with an umbrella. Walk carefully when working around the instrument and when setting up, check to make sure that the area is clear of debris that may be an obstacle.

How it works

Electromagnetic Distance Measurement (EDM)

One of the fundamental measurements in surveying is distance. Obviously with a distance and an angle, one can establish a coordinate system and locate the relative positions of objects or observations. The angle defines the orientation and the distance of the scale. The direct measurement of distance in the field is one of the more troublesome of surveying operations, especially if a high degree of accuracy is desired. Indirect measurements, like the use of a stadia rod, have been developed and used extensively; however, these systems are of rather limited range and accuracy. With the advent of electromagnetic instruments, the direct measurement of distance with high precision is possible (Burnside, 1991).

When a distance is measured using an EDM instrument, some form of electromagnetic wave (in our case, infra-red--IR--radiation) is transmitted from the instrument towards a reflector where part of the transmitted wave is returned to the instrument. Electronic comparison of the transmitted and received signals allows for computation of the distance (Price, 1989). See Figure 3 for a schematic of the method of operation of an EDM.

Figure 3.1 Schematics of EDM system. From Price and Uren, 1989.

Electromagnetic waves

Electromagnetic waves can be represented by a sinusoidal wave motion. The number of times in 1 second that a wave completes a cycle is called the frequency (f), and is measured in

Hz. The length of one cycle is called the wavelength (l), which can be determined as a function of the frequency from

(1)

where v is the speed of propagation of the wave.

The speed of electromagnetic waves in a vacuum is called the speed of light, c, and is taken to be 299,792,458 m s-1.(Price, 1989). The accuracy of an EDM instrument depends ultimately on the accuracy of the estimated velocity of the electromagnetic wave through the atmosphere (Burnside, 1991).

A relationship expressing the instantaneous amplitude of a sinusoidal wave is

(2)

where Amax is the maximum amplitude developed by the source, A0 is the reference amplitude, and f is the phase angle which completes a cycle in 2╣ radians or 360°.

In an EDM system, distance is measured by the difference in phase angle between the transmitted and received versions of a sinusoidal wave. The double path length (2D) between instrument and reflector is the distance covered by the radiation from an EDM measurement. It can be represented in terms of the wavelength of the measuring unit:

2 (3)

The distance from instrument to reflector is D, lm is the wavelength of the measuring unit, n is the integer number of wavelengths traveled by the wave, and Dlm is the fraction of the wavelength traveled by the wave. Therefore, the distance D is made of two separate elements. An EDM instrument using continuous electromagnetic waves can only determine Dlm by phase comparison (Figure 4).

If the phase angle of the transmitted wave measured at the instrument is f1, and the phase angle measured on receipt is f2, then

(4)

The phase angle f2 can apply to any incoming wavelength, so phase comparison will only provide a determination of the fraction of a wavelength traveled by the wave, leaving the total number, n, ambiguous (Price, 1989).



Figure 4. Phase comparison. (a) An EDM is set up at A and a reflector at B for determination of the slope length (D). During measurement, an electromagnetic wave is continuously transmitted from A towards B where it is reflected back to A. (b) The electromagnetic wave path from A to B has been shown, and for clarity, the same sequence is shown in (c), but the return wave has ben opened out. Points A and A' are effectively the same, since the transmitter and receiver would eb side by side in the same unit at A. The lowermost portion also isslustrates the ideal of modulation of the carrier wave by the measuring wave. From Price and Uren, 1989.


For many EDM instruments, an accuracy in measurement between 1 and 10 mm is specified at short ranges, and a phase resolution of 1 in 10,000 is normal. Assuming an accuracy of at least 1 mm, therefore, a measuring wavelength (lm) of 10 m is required. Approximating the speed of propagation, v, by 3 x 108 m s-1, 10 m corresponds to a frequency of 30 MHz. Adequate propagation of an electromagnetic signal of 30 MHz frequency for EDM purposes is not practical, so a higher frequency carrier wave is used and modulated by the measuring wave (Figure 5, (Price, 1989)). In the case of our instrument, the carrier wave is infra-red, with a wavelength of 0.835 Ám, corresponding to a frequency of 3.6 x 105 GHz.

The return signal is usually amplified, and then the phase difference is determined digitally. The signal derived from the modulation triggers off the counting mechanism every time the signal changes from negative to positive. The signal derived from the reflected ray stops the counting mechanism (Burnside, 1991). For our instrument, the integer ambiguity (n) of wavelengths is determined using the coarse measurement frequency of 74,927 Hz, equivalent to 2000 m, and the amplitude modulation of 4,870,255 Hz (30.7692 m) provides the fine measurement (Dlm). In order to achieve the stated accuracy of 5 mm, the phase measurements are accurate to 1 part in 6,154.

Errors in distance measurement

The path of electromagnetic energy is the true distance that is measured by the EDM, and will be determined by the variability in the refractive index through the atmosphere.

(5)

Since the medium is air, the velocity is nearly the same as that of a vacuum, and so the refractive index is nearly one, and for standard conditions may be taken to be 1.000320. The exact value of the refractive index is dependent on the atmospheric conditions of temperature, pressure, water vapor pressure, frequency of the radiated signal, and composition. Therefore, for measurements of the highest accuracy, adequate atmospheric observations must be made. Other potential error sources are from path curvature (similar to the curvature of the earth).(Burnside, 1991).

Scale error

One source of error that is adjusted for in our instrument is a scale correction in units of ppm that adjusts for slight errors in the reference frequency and in the accuracy of the average group refractive index along the line of measurement. The ppm value is set to 0 on the EDM, and adjusted in the theodolite.

Atmospheric correction is one scale error adjustment that takes into account both atmospheric pressure and temperature. It is an absolute correction for the true velocity of propagation, and not a relative scale correction like reduction to sea level (see below). To determine the atmospheric correction to an accuracy of 1 ppm, measure the ambient temperature to an accuracy of 1°C and atmospheric pressure to 3 mb. For most applications, and approximate value for the atmospheric correction (within about 10 ppm) is adequate. This can be obtained by taking the average temperature for the day and the height above mean sea level of the survey site. A temperature change of about 10°C or a change in height above sea level of about 350 m (= 35 mb) varies the scale correction by only 10 ppm. The atmospheric correction is computed in accordance with the following formula:

(6)

where: DD1 = atmospheric correction (ppm), p = atmospheric pressure (mb), and t = ambient temperature (°C). For extreme conditions of 30°C temperature change and 100 mb pressure change, one can expect variations in scale error of 50 ppm in a day. This maximum value is ten times greater than the stated initial accuracy of the instrument, and therefore should be accounted for by adjusting the scale error on the theodolite occasionally during the day's surveying effort (see Figure 6, graph 1 to determine DD1).

The correction in ppm for the reduction to mean sea level is based upon the formula:

(7)

where DD2 = reduction to MSL in ppm, H = height of EDM above MSL, and R = 6378 km (earth radius). This correction is a constant and should be determined at the beginning of the survey effort by consulting Figure 6, graph 2a or 2b.

Corrections in ppm may be made for map projections as well.

Reduction formula

The theodolite computes slope distance according to the following formula:
D = D0x(1+deltaDx10-6) + mm

where D is the corrected slope distance in mm, D0 is the measured (uncorrected) slope distance in mm, (deltaD) = sum of scale corrections (n is the number of scale corrections) in ppm, and mm = prism constant in mm.

The theodolite computes horizontal distance (Dh) and height difference (eh) by accounting for earth curvature and mean refractive index. These corrections are on the order of 10-8 or 0.01 ppm of D0 and therefore not significant compared to the scale correction (~50 to 100 ppm).

Figure 6. Scale correction graphs from Theodolite manual. Use these to determine DD (sum of scale corrections).

Infra-red radiation from a GaAs lasing diode

The source of the IR radiation for our instrument is a Gallium Arsenide (GaAs) lasing diode. This device is made from a small chip of semiconducting material, and is similar in size and appearance to other semiconducting devices (Figure 7). Driven by a forward biased voltage and maintained by different electrostatic potentials in the two halves of the diode, an electron population inversion between the two halves of the diode will provide the energy level transition for stimulated emission of photons by electrons as they fall to the lower energy state (Price, 1989). The energy difference is emitted as radiation (and thus the process and device are called a laser--Light Amplification by the Stimulated Emission of Radiation). The process of stimulated emission enables the laser to emit an intense, monochromatic radiation that travels as a narrow beam for considerable distances before it spreads out (Price, 1989). The intensity of the IR radiation is nearly linearly proportional to the current flow and with virtually an instantaneous response (Burnside, 1991). If an alternating voltage is superimposed upon the normal operating voltage of the GaAs diode, the intensity of its emitted radiation varies in sympathy with the alternating voltage (Figure 8, Price, 1989). This provides a simple and inexpensive means of directly modulating the infra-red beam.

The GaAs diode is widely used in surveying. The small dimensions of the junction in which the radiation is emitted gives rise to poorer collimation of the radiation. Therefore, the GaAs laser emits a beam with a relatively large elliptical spread and the brightness of the GaAs laser is lower than that of other lasers. The spectral width of the radiation emitted is usually 2-3 nm compared with 0.001 nm of the visible light, HeNe gas laser and thus the GaAs laser lacks the monochromacity of other lasers, contributing error to the effective propagation velocity. However, GaAs lasers can be made to operate at orders of magnitude greater efficiency than other lasers, can be made much smaller and more rugged, and are considerably less expensive than other lasers (Price, 1989).

Because of the relatively low power radiated, a beam of sufficient power will not be reflected from an unprepared surface. A special reflector is therefore used in order to ensure a good return signal. A plane mirror can be employed, but it requires accurate setting, so in practice, a corner cube reflector is most often used. This reflector will return a beam along a path parallel to the incident path over a wide range of angles of incidence onto the front surface. A cube of glass is usually used with its edges ground into a corner with accuracies of grinding to within a few degrees of arc (Burnside, 1991). The path length of the signal within the reflector must be corrected for, and that is the reason for the setting of the prism constant within the theodolite. The constant for Wild circular prisms is 0. Note that over short distances, the "cat-eye" type reflector commonly used for bicycles will adequately reflect highly oblique incident signals.

Figure 7. Gallium arsenide diode characteristics. From Burnside, 1991.


Figure 8. Modulation of a Gallium arsenide diode. From Price and Uren, 1989.


Selected Technical Data--Leica (Wild) DI4L

Standard deviation of distance measurement

5 mm + 5 mm/km

Breaks in beam

result not affected

Range with one reflector

1.2 km in strong haze

2.5 km in average atmospheric conditions

(the maximum range I have shot is 1.8 km)

Carrier wavelength

0.835 Ám infra-red

Fine measurement

4,870,255 Hz = 30.7692 m

Coarse measurement

74,927 Hz = 2000 m

Weight--DI4L

Counterweight

Container

Total weight

1.1 kg (2.4 lb)

0.8 kg (2.0 lb)

3.8 kg (8.4 lb)

5.7 kg (12.8 lb)

Beam width at half power

4' (12 cm at 100 m)

Theodolite

The Theodolite is an accurate horizontal and vertical angle measuring device with a telescope and on board electronics for data storage and EDM operation. Fortunately for us, the angles are measured and recorded accurately and electronically, avoiding the need for us to read a vernier and record data manually as is typical on transits and optical theodolites. This electronic theodolite contains circular encoders which sense the rotations of the vertical and horizontal spindles of the telescope, and converts those rotations into horizontal and vertical angles electronically, and displays the values of the angles on a Liquid Crystal Display (LCD) (Moffitt, 1987).

The integrated EDM/Theodolite combination is often called a "Total Station" or "Total Geodetic Station." Output from the horizontal and vertical circular encoders and from the EDM are stored in a data collector. The instrument may convert the data (horizontal and vertical angles and the slope distance) electronically into Easting and Northing coordinates, height difference, and horizontal distance (Moffitt, 1987).

Selected Technical Data--Leica (Wild T-1000)

Standard deviation of angular measurement

3.0' (seconds of a degree) horizontal and vertical

Telescope

Erect image

Magnification

30x

Shortest focusing range

1.7 m

Field at 1000 m

27 m

Displays

2 LCD displays each for 8 digits, sign, decimal point and symbols for user guidance

Keyboard

Weatherproof, 14 multiple function keys, contact pressure 30 g

Automatic power off

About 3 minutes after last keystroke

Angle measurement

Continuous, by absolute encoder

Updates

0.1 to 0.3 seconds

Optical plummet (in tribrach)

Focusing

Magnification

2x

Temperature Range

-20°C to +50°C

Weight--T1000

4.5 kg (9.9 lb)

Container

3.9 kg (8.6 lb)

Total weight

8.4 kg (18.5 lb)

Determination of Easting, Northing, and Elevation

The actual observations made by the Total Station are the horizontal and vertical angles (Hz and V), and the slope length (D)--these are called fundamental measurements. Clearly, from these data, one can determine the relative coordinates of the instrument and reflector (Figure 9). The instrument makes its own reductions based only upon the fundamental measurements. The horizontal angles are made relative to a backsight or reference azimuth or known orientation. This may be determined by a careful compass sighting to a distant, but stable and easily identifiable landmark, or it may be arbitrary. Once the backsight is made, the reference azimuth, (hz0) may be set on the Theodolite. The vertical angle may be called a zenith angle and is measured in a vertical plane down from the upward direction of a vertical or plumb line (Moffitt, 1987).

Instrument Station Reference Location

The point over which the Total Station is set up is called an instrument station. Such a point should be marked as accurately as possibly on some firm object. On many surveys, each station is marked by a wooden stake, spike, or piece of REBAR driven flush with the ground and into the top of which a small (~ mm diameter) dimple is marked (a tack may be put in the wooden stake) (Moffitt, 1987). The Total Station measurements and reductions are made relative to the instrument station (E0, N0, and H0--where E refers to distance East, N distance North, and H elevation, and the subscript 0 indicates a reference value). These values must be predetermined by other means (triangulation, previous survey location, Global Positioning System, etc., or they may be arbitrary). In fact, for most local surveys, with only one survey set up, the instrument station is commonly taken to be (0, 0, 0).

Determinations in the vertical plane

In the lower portion of Figure 9, the geometric reductions in a vertical plane are indicated. From the fundamental measurements and a few more observations, the horizontal distance (HD) and elevation (H) may be determined:

(9)

H* is the elevation difference between instrument and reflector, IH is instrument Height, and PH is the Prism Height.

(10)

Figure 9. Surveying geometry.


Determinations in the horizontal plane

In the upper portion of Figure 9, the map view reductions from the fundamental measurements are shown. Because we are interested in the map relations of the surveyed objects or observations, the distances East (E) and North (N) may be determined in the following way:

(11)

(12)

Foresight and Backsight: the traverse

One other thing that people have trouble with and that I have only recently gotten to the bottom of is the traverse or how you move your station to a new position. The figure below shows how one starts of at the reference position E0, N0, H0, works, and then moves to the new station with a reference position E1, N1, H1. What you have to do is shoot from the first location to the second (foresight) and record the E1, N1, H1, and the bearing Hz0. Then, move to the new location, set up, tell the total station you are at E1, N1, H1, and set the horizontal circle to Hz1 or Hz0+180 degrees (backsight). Then shoot to the reflector set up at the first station and record the position. It should be within a few mm of the original E0, N0, H0.

Precision

The angular accuracy at one standard deviation of the Theodolite is 3", and the linear accuracy at one standard deviation of the EDM is 5 mm ▒ 5 ppm of the slope length. The upper portion of Figure 10 shows a map view of the 1 standard deviation error volume (remember that the horizontal and vertical angles have the same precision). In reality, this volume is really an ellipsoid if we assume that the errors are normally distributed. The significance of this figure and the plot in the lower portion of Figure 10 is that one can anticipate the precision and its changes with slope length, and consider them accordingly. For example, for a 100 m shot, or slope length D = 100 m, the linear error, l, is 5.5 mm, and the angular error, a, is 1.5 mm, and therefore, the error volume is 11 mm along the shot, and 3 mm wide along horizontal and vertical arcs. Note that this is much smaller than reflector placement error.

Planning and executing a surveying/mapping project

The important concern when planning a mapping project is the question that is being asked, or problem being addressed. While mapping and surveying may be intrinsically fascinating and interesting of themselves, unless you want to become a surveyor, they are only methods used to collect data in order to address a geologic problem. Therefore, the technique and methods should be appropriate for the problem. You should collect data at the appropriate scale and precision that will most efficiently shed light upon the problem. Check (Compton, 1985) for an inspiring and essential text to guide you in the field.

Basic detailed mapping procedure

This technique is appropriate for outcrop to kilometer scale mapping (1:10 to 1:1000 scale) for which no adequate base map exists. The basic idea is to shoot in control points with the Total Station, establish a base map, and then use tape and compass and triangulation to interpolate and locate features between the control points.

Figure 10. Surveying precision.

1) Recon the area. Consider the problem, and instrument locations.

2) Flag points. Place flags (numbered) on important features (along large fractures or contacts for example). The density depends upon the scale of the problem and the map. If the flags are too close, you will spend a long time shooting them in and then may be confused while mapping. If they are too far apart, you will spend a long time measuring off distances and bearings between points. I suggest a radial distance between flags of 5 to 10 m.

3) Shoot points with Total Station. Make sure that the point number corresponds either directly or in some noted way with the flag numbers.

4) Produce basemap. This may be done in several ways. It may be done in the field or the office by manually plotting the location and recording the elevation of the point on grid paper. Be sure to use metric grid paper. The other way to produce the basemap is to download the data as described above, and contour and plot the points with their numbers. Print the contour map out to an appropriate size for your mapping. I do this step in Deltagraph Pro by importing a four column (point number, E, N, H) Excel file, and then plot an XYZContour chart (select the data by using the arrow in between the two label words:

c

Label

Label

 

Show symbols, and adjust the size of the chart by selecting Axis Attributes under the Axis choice under the Chart menu. Make sure that the X and Y (E and N) axes are the same scale. Choose an appropriate contour interval. Show the labels of the points (point numbers, for example) by selecting the chart, and then under the Chart menu, select Show Values, and choose a location besides None, and the Text should be Category. This will plot the items in the corresponding cell in the Labels Column adjacent to the symbol.

5) Mapping. The above method should provide a basemap with the control points displayed and labeled. Tape the base map or a portion of it to your map board and overlay a piece of vellum or mylar. Mark the control points and a few more index marks, especially if you will use more than one page for the base map. As you map, observe the locations and orientations of objects and features by noting the bearing and distance (using a Brunton Compass and a tape measure or a well calibrated pace) from one or more control points. You may also shoot a bearing to two or more control points, plot the angle on the map and triangulate your location. Mark your observations carefully with a sharp pencil. As you map, record the topography. If you plotted contour lines, trace and modify them in the field and include the subtleties that you can. In the evenings or whenever you are at a stable point, trace your lines in ink using a very fine pen.

Examples

Below are a few examples of different mapping projects our members of our group have completed recently. These are meant to illustrate the different solutions to different problems that have been achieved.

Topography

My own interest in the geomorphic responses to active faulting provides the following example. During the June 28, 1993 Landers California earthquake, spectacular faulted landforms were produced all along the surface rupture. These landforms provided an important opportunity to document the original shapes and initial modification of faulted landforms. This project posed several interesting survey challenges: 1) producing detailed scarp profiles and longitudinal profiles of gully main and tributary channels, and detailed topographic contour maps of faulted knickpoints; 2) establishing a cheap but hopefully stable control network; and 3) reoccupying the network.

Scarp profiles--coordinate transformation by rotation

These profiles are made perpendicular to the fault scarp and are ideally planar. In the field, we marked the upper and lower ends of the profiles with steel rods, and walk between, shooting points about every 50 cm until we are at the free face of the scarp, where points are shot about every 10 - 20 cm. These data are down loaded as described above, and then the profile is projected to a plane perpendicular to the scarp. The coordinates are transformed by a rotation about the origin so that one axis is the distance along the profile and the other is the deviation from the plane of the profile. The equations for such a coordinate transformation are as follows:

(13)

(14)

The figure above and to the right illustrates this geometry.

 

The following two figures illustrate the initial and rotated coordinate systems for a scarp profile. In the lower plot, the coordinate system has been rotated by -36° (counterclockwise), and the horizontal axis used as the horizontal distance along the profile. The vertical axis indicates that the deviation from a plane is a maximum of 5 m over 30 m.

Longitudinal profiles

The gully longitudinal profiles were determined by observing points along the lowest portion of the active channel in a given gully. The distance along the profile is the sum of the distances between individual points. In other words, the longitudinal profile is not projected to a plane, rather the run, or distance along the channel, is stretched out and plotted as the horizontal axis for the longitudinal profile, while the vertical axis is the corresponding elevation measurements. This plot is important since it indicates the effective slope for the flow, which clearly may not be ideally contained within a single plane.

Contour maps - topographic mapping

Because of the precision and rapidity of data acquisition of our Total Station, as well as the opportunity to quantitatively document the three-dimensional (i.e. volumetric) changes in scarp form, particularly where the gully channels were disrupted by the earthquake surface rupture, we made detailed observations of small portions of the gully channel surfaces. We have observed after one year, and expect significant change within a few years. The data were observed over a relatively small area (125 to 250 m2) at a fairly high density: 1 measurement every 0.5 to 2.5 m2. The lower density shots seemed to optimally address the problem, and we achieved them by setting a goal of 100 shots to complete the map. This seems to be a better psychological tactic than simply observing every apparently important break in slope.

We used the program Deltagraph Pro to contour our data. This program has two advantages: 1) it can contour a set of data with an irregular boundary, enclosing the area of interest with a relatively close fitting polygon, instead of a rectangle, as is common with most contouring programs; and 2) it uses triangle-based terrain modeling (commonly called a Triangular Irregular Network--TIN; Kennie, 1990). This method essentially models the surface as a series of planar, triangular elements, each of which contains three neighboring data points (Figure 11). As you are surveying it is important for the reflector person to attempt to visualize this triangle network in order to assess the appropriateness of a given observation point. The points where contour lines intersect the lines between neighboring points are determined by direct, linear interpolation. The contour lines are then determined by connecting those intersections. Because the contour lines are not smoothed, this method provides a basic contour map that honors each data point directly. However, it does generate a somewhat jagged map which incidentally appears more appropriate for semi-arid landscapes rather than smoother humid-temperate landscapes.

Given a method of plotting the map and taking it in the field, the subtleties of the contours could be adjusted manually. We adjusted our maps slightly in the office, honoring the topography as best as possible, by adding a few more vertices to the contour lines in the drawing program Canvas (Figure 11).

Figure 11. Contour map preparation using Deltagraph Pro and Canvas.

Reoccupation

There are three types of errors: random errors, systematic errors, and blunders (Kennie, 1990). Random errors result from the random normally distributed probability of a

measurement. Systematic errors cannot be detected by redundant measurements because they effect all measurements similarly. However, they can be addressed by making more observations; for example the scale error due to atmospheric changes that is explained above is a type of systematic error that can be minimized by making observations of temperature and pressure and making the appropriate scale adjustment. Blunders are human errors that hopefully are minimized or large enough to be noticed and removed from the data.

Errors within a single survey are rather small. Reoccupation of a survey network in order to repeat measurements may be desirable, but it incorporates greater error. The best way to adjust two sets of data measured of the same network during successive occupations of a survey network is by least squares. In this technique, the squares of the residuals of the observations are minimized:

(15)

where V = li’ - li, in which li is the observation and li’ is the adjusted observation, and W is a weight matrix consisting of the inverse of the covariance matrix of the observations. If the data are not correlated, the matrix will be diagonal, and if the variances of all the data are the same, the W matrix may be dropped from the determination (Kennie, 1990).

Detailed Fracture Mapping

Finally, as an example of the mapping technique I described above, I will briefly discuss the method by which we made a detailed fracture map of a portion of the surface rupture of the Landers earthquake (Antonellini, 1992). See Figures 12 and 13. First, we made a reconnaissance of the area, and noted the important features and discussed the problem and the pertinent observations. Second, we determined the approximate size: ~700 m long by ~250 m wide. We considered the mapping scale by determining the basemap size for various mapping scales: 1:250--the ultimate scale chosen--would result in a basemap approximately 2.8 by 1 m in size. We divided into four mapping groups, so each would have a base map approximately 0.7 by 1 m--probably the maximum comfortable size for a single sheet. Third, we flagged numbered control points along fractures and other obvious and important features. Fourth, the southeastern third of the control points were located using a conventional plane table set up. The northwestern two-thirds of the control points were located using the Total Station in approximately the same time as that for those done with the plane table. We did not have a means of automatically plotting our data in the field, so we did it manually. Here we made things difficult by having grid paper ruled in English units: inches and ten divisions. This made it necessary to convert the measurement read off of the Total Station LCD in metric units to the quirky and irregular English scale. With metric ruled paper, and such a regular mapping scale of 1:250, it would have been much easier to lay off the base map control points. The points were plotted with the point number and elevation adjacent to the symbol, and then four basemaps generated by overlaying sheets of mylar on the control point map, marking the control points (including some shared by the adjacent maps to help with future compilation). With the control points marked in pen on the basemaps, we were ready to do our mapping of the surface fracturing. Each mapping group had a measuring tape and compass, and recorded observations relative to the control points within the mapping area (Figure 12). As mapping proceeded, topography was recorded, and contours generated. The contours generated varied in their precision depending upon the effort of the mappers. The most successful contour generation technique was for one member of the mapping group to put his eye level at the level of a contour and site along a level line determined by the Brunton compass toward the landscape, and the contour continued and mapped in by communication with the mapping partner who interpolated between the control points in a manner similar to that used for mapping other features (Cooke and Christiansen, Figure 12). As mapping progressed, observations and contour lines were inked. The final map was compiled and drafted by Ken Cruikshank (Figure 13). It was presented at the Fall, 1992 AGU meeting (Antonellini, 1992).

Figure 12. Reduced portions of original basemaps from landers earthquake surface rupture mapping (Antonellini and others, 1992). Note the different styles and representations of fractures, topography, landforms, and human impacts.

Figure 13. Final analytical fracture map from Landers study (compiled by Ken Cruikshank)..

Bibliography

Geologic mapping

Compton, R. R., 1985, Geology in the Field: New York, John Wiley and Sons, 398 p.

The best field geology book available. Covers all of the basics from rock identification through basic outcrop procedures to aerial photography. You should own it.

Electromagnetic distance measurement

Burnside, C. D., 1991, Electromagnetic distance measurement: Boston, Blackwell Scientific Publications, 278 p.

Good reference on the subject. A bit arcane in the explanation of the basics, but thorough. Includes a nice description of different equipment (3rd edition, TA601.B87 1991, Engineering Library).

Price, W. F., and Uren, J., 1989, Laser surveying: London, Van Nostrand Reinhold (International), 256 p.

Contains a good description of the basics of lasers, particularly GaAs diodes, and a clear description of the basics of EDMs. Covers other types of laser equipment as well (e.g., timed pulse distance measurement, alignment lasers, and laser Theodolites and levels), and laser safety (TA579.P94 1989, Engineering Library).

Basic Surveying

Moffitt, F. H., and Bouchard, H., 1987, Surveying: New York, Harper and Row, 876 p.

Good reference on the subject. This one has it all (8th edition, TA545.B7 1987, Engineering Library).

Advanced Surveying

Kennie, T. J. M., and Petrie, G., editors, 1990, Engineering surveying technology: New York, John Wiley and Sons (Halsted Press), 485 p.

Thorough overview of latest advances. Contains nice discussions of electronic distance and angle measurement, survey network errors, photogrammetry, and digital terrain modeling

Mapping reference

Antonellini, M., Arrowsmith, R., Aydin, A., Christiansen, P., Cooke, M., Cruikshank, K., Du, Y., and Wu, H., 1992, Complex surface rupture associated with the North Emerson Lake fault zone, caused by the 1992 Landers, CA earthquake: results of detailed mapping: EOS Transactions AGU, v. 73, p. 362.



Comments? e-mail Dr. Ramón Arrowsmith ramon.arrowsmith@asu.edu