Kite Photo Rectification with High Resolution DEM

by: Stephen Holloway


Raw field data was imported into Matlab. This produced a raster contour map with breaklines, origin, and a labelled grid.

Raw field data was also imported into Arc/Info as space delimited text in x y z format. Within Arc/Info, generatetin and tinlattice were used to create a grid file which was then imported into ERDAS Imagine. This produced a grid file which retained the elevation data, but had no breaklines or labelled grid.

The origin was assumed to be the same for both images but since they had no projection information and the contouring was done differently, each image covered a different amount of area, had different pixel sizes, and had different origins. The raster file had a labelled grid and could easily be adjusted. First, find the x y location in pixels of the survey origin (labelled 1000, 1000). Second, find the x y location of the upper left hand corner in terms of survey units (meters). Calculate the unit/pixel in each the x and y projection (they should be the same, if not, an error was made). Import the *.tif into *.img format. In the Viewer use Utility->Layer Info->Edit/Change Map Info. Enter in the above information. OK. Close Image Info window. Clear the viewer and reopen. The raster image should now have the correct coordinates. Check the grid to confirm.

Photo Rectification

Imported raw field kite photos into Photoshop and save as *.tif. Sorted by high vs. low altitude, coverage, and approximate verticality. Chose a small number of photos to best cover the trench site. Rotate photos to approximate north being up. This makes it easier to recitfy once in Imagine. Import photos into *.img format. Open one photo in the left viewer. Open the raster topo map in the right viewer. In the left viewer menu, choose, Raster->Geometric Projection->Polynomial->Select 2 for order->OK->From Existing Viewer->Click on right viewer. This will launch zoom windows and a table down below. Select ground control points to evenly cover the topography maps. Rectify the photo and name the new file *rect.img. Open the rectified photo in the right viewer and check to see if it lines up over the topography correctly. If not, double check the location of the ground control points. One or two misplaced points can drastically effect the accuracy of the rectification. Also consider adding additional ground control points if the photo is not vertical or exhibits extensive distortion on the edges. If the rectification was successful, repeat with the other photos. Once all of the photos have been rectified, check their coverage and overlap by opening up all of the photos on top of the raster topography base map. If the overlaps are OK, mosaic the photos using the Data Prep button->Mosaic Images. Add each of the images, choose no histogram matching, unless the photos are vastly different. Run Mosaic. View the mosaicked image for contrast and for accuracy.

Image Drape

This procedure must be completed on a machine with Open GL. This excludes alai and tejon. Vissys5 in the computer commons GIS/Visualization lab is the best choice. Launch Imagine. In the viewer->File->Open->filename for your grid DEM->Raster Options->DEM->OK. In same viewer->File->Open->filename for mosaic. Raster->Image Drape. This will open an additional window with an eye and target. These can be moved around to view the draped image over topography at various elevations and inclinations. Sun angle can also be adjusted to enhance the image.