Active Tectonics, Quantitative Structural Geology and Geomorphology

Scan and Rectify an Air Photo in Imagine 8.3

1. Scan air photo.

Available scanners for geology are located in H461 (Mac), F551 (Mac), and F675 (PC). The scanner in H461 is best for resolution, availability, and free disk space. For Mac, launch DeskScan II. Photoshop also has a scan plug-in but it is usually missing or inoperable. Place air photo in scanner, aligning it as square as possible. Click Preview button. Adjust contrast and brightness accordingly or click on auto contrast button. Note contrast and brightness values. Adjust the marquee box around the image, making sure to include the fiducial marks. Click on Final button, give the image a name (use a consistent naming convention if you have several photos), and save as tiff (*.tif). For subsequent scans, try to make the contrast and brightness similar so your final mosaicked image will not have distinct borders.

2. Reassemble air photo (optional).

If your air photo is wider than the scanner bed (8.5 in. usually), first scan one side, then the other, keeping the image aligned as best as possible. In this case, the left and right sides should have identical brightness and contrast. In Photoshop (or Canvas), open up the left and right images. On one image, enlarge the canvas size by several inches in width for the missing swath. Copy the missing swath from the other image, making sure to have an inch of overlap. Paste swath into widened image, making sure there is enough room so the swath won't be cropped off. The swath will automatically become a new layer. Go to Window->Show Layers and toggle transparency to 50% for the top layer. Zoom in to one of the overlap boundaries. Align the boundary with the underlying layer by shifting it with a click hold. Toggle transparency back and forth and make sure all pixels line up by scrolling along the boundary. If not, you will need to rescan both sides of the photo and try again. Increase top layer transparency to 100%. Flatten the two layers with Layer->Flatten Image. Crop any unnecessary extra area making sure to retain the fiducial marks.

3. Save image and ftp to Alai.

Save as *.tif. Transfer file from Mac desktop (or MacData) to Alai using Fetch. Start Fetch. Host is, enter your user ID and password on Alai and click OK. Click on binary. Click on Put File. Select the correct filename. Enter the filename you wish it to be saved as on Alai (make sure this is in *.tif format). (It is generally best to keep the same filenames to avoid future confusion.) Change format to Raw Data. Click OK and watch the little dog run until the file is transferred. If you have several files in a folder you ca save tim by ftping a folder of files. Make sure the folder maintains Unix naming convention of no spaces, punctuation, or long names.

4. Import into Imagine.

Log in to Alai. Launch Imagine (Applications->ERDAS Imagine). Click on the Import icon. Change Type menu to TIFF. Change Media menu to File. For input file, click on the folder button and go to the Alai directory where your topo image is located. (For successive files located in the same location, select the Recent button.) Imagine will automatically create an output name as *.img. Select the location you want the *.img file to be saved. Click OK. When finished processing, click OK, then close import window.

5. Ground control point selection.

Open a second viewer by clicking on the Viewer Icon. To view both side by side, go to Session->Tile Viewers. In Viewer #1: Go to File->Open->Raster Layer and open the unrectified image (air photo). In Viewer #2: Go to File->Open->Raster Layer and open the georeferenced image (previously georeferenced topo map). In Viewer #1, start the Geometric Correction Tool by going to Raster->Geometric Correction. Select the polynomial option and click OK. When the polynomial dialog box opens, close it (you will deal with it later). Another dialog box, GCP Tool Reference Setup will appear. Just keep the default and click OK. A Viewer Selection Instructions dialog box will open asking which image to take the reference points from. You want to left click on the second viewer (which contains the georeferenced image). A Reference Map Information box will appear. Click OK.

At the bottom of the screen, there should be a GCP Tool window with a spreadsheet. To start georeferencing the images, identify common points between the two images (road and stream intersections are good). There will be a box on each image which corresponds to a small Viewer window at the top of the screen. Viewer #1 to Viewer #3; Viewer #2 to Viewer #4. Move the boxes onto a common point of each image. Boxes are moved by left clicking and draging them, but the select tool must be selected in the GCP Tool dialog box (looks like arrow pointing to NW). The box size can be changed by clicking and dragging the corner. When both points are in the small viewer windows at the top of the screen, click on the Create button in GCP Tools next to the arrow tool (looks like a circle with a sight inside it). To zoom in any viewer, right click hold->zoom->zoom in by 2. It is useful to get both zoom windows at the same scale. Then click on the point in Viewer #3 that you want to georeference. Click on the Create button again and click on the matching point in Viewer #4. Repeat this for the first four points you enter.

By the fifth point, Imagine will start to guess where the point should be on the second image after you enter a point on Viewer #3. For the rest of the points you enter, you will only have to hit the Create tool before entering the point on Viewer #3 and then click and drag into place the point that Imagine places on your second image. A typical aerial photograph needs about 50 to 100 points, depending on quality and scale.

Once you have entered more points, you can change to a higher polynomial order. To do this, in Geo Correction Tools (at the top of the screen) press the button with the picture of a mini-dialog box. Change the polynomial order number to 3 (for normal air photos) or 4 (for highly distorted air photos) and then click on Apply.

6. Rectification.

When you are ready to rectify the image, in the Geo Correction Tools box (top of the screen) click on the button that looks like a tilted window. Enter the filename you want for your rectified image. To save the points you used to rectify your image, go to the GCP Tool dialog box and (under File) Save Input File as ...(this saves the locations of the points on the unrectified image) and Save Reference File as...(this saves the locations of the corresponding points on the georeferenced image). Use the Save as... and choose unique names because multiple air photos will be rectified to the same topo quad and you don't want to misplace or write over previous ground control points.

7. View rectified image.

Open a new viewer (click on Viewer icon). Go to File->Open->Raster Layer and open the georeferenced image.

If you want to compare it to the georeferenced image you were using (and the georeferenced image was larger than the rectified image), open a new viewer. Go to File->Open->Raster Layer and open the georeferenced image. Then, go to FileOpenRaster Layer again. Go to the options tab and toggle off the Clear Viewer option and toggle on the Transparent Background option (and make sure the Fit to Screen option is off). Go back to the main tab and enter the filename of the rectified image and click OK. The rectified image should open on top of the georeferenced image so you can see how the edges match.