Lab 6: Geometric Correction

Introduction

In this lab, students will expand on their knowledge of geometric correction by reforming image-to-map and image-to-image rectification on a spatially distorted image. These processes are commonly done to satellite imagery before data extraction or visual analysis is preformed. First, a USGS 7.5 minute digital raster graphic (DRG) of Chicago, IL will be used as a reference image for a slightly distorted satellite image of the metropolitan area. Second, a previously rectified satellite image of Sierra Leone, Africa will be used as a reference image for a heavily distorted satellite image of the same area.

Methods

Image-to-Map Rectification
This method uses a map with an established coordinate system as a reference to modify the spatial location of features in a distorted image to match that of the map.

Figure 1: With the distorted image to be rectified in an active window, navigate to Multispectral > Control Points.

Figure 2: The Set Geometric Model window will appear. For this lab, Select Geometric Model is set to polynomial. Click OK.

Figure 3: Next, the GCP Tool Reference Setup window will appear. For this lab, the default value of Image Layer (New Viewer) is kept. Click OK.

Figure 4: Next, the Reference Image Layer window will appear. Here the reference image will be chosen. For the first part of the lab, the Chicago USGS 7.5 minute DRG is selected. After clicking OK, a window will appear indicating the coordinate reference system of the image. Click OK. Then the Polynomial Model Properties (No File) window will appear. For the fist part of this lab, the Polynomial Order is set to 1, all other default values were accepted and the window was closed.

Figure 5: The Multipoint Geometric Correction window will now appear with both the distorted image and the reference image located within. Each image is portrayed in three different scales. In the upper right, the image is at full scale. In the upper left, the image is zoomed to the extent of the inquire box. Below the two smaller upper windows is a larger window where the image can be manually zoomed and panned. This larger window is where the ground control points (GCPs) will be added.

Figure 6: To add a GCP, select the Create GCP tool on the Geometric Correction toolbar and click an area on the distorted image. Then select the Create GCP tool again and click the same area in the reference image. After a desired number of GCPs have been added, zoom in and reposition the pairs of points to spatially match as close as possible. Continue with this process for all GCPs until the RMS error, read in the bottom right corner of the window, is less than 2 (requirement for lab). For the first part of this lab, four GCPs were collected and the RMS value was reduced to 0.124.

Figure 7: Once all GCPs are added and the RMS error is low enough, select the Multipoint Geometric Correction tool on the Geometric Correction toolbar. The Resample window will appear. For the first part of the lab, Nearest Neighbor is chosen as the Resample Method and all other default values were accepted. A rectified output image is generated.




Image-to-Image Rectification
This method uses a previously rectified image as a reference to modify the spatial location of features in a distorted image to match that of the rectified image.

Figure 8: The process to execute an image-to-image rectification is the same as image-to-map rectification. Because the distorted image is so heavily distorted a third degree polynomial will be used instead of a first degree polynomial like the first part of the lab. In the Polynomial Model Properties, change the Polynomial Order to 3.

Figure 9: Because a third degree polynomial was used, the number of minimum controls points increased from 3 (for first degree) to 9. For the second part of this lab, 10 GCPs were added and the RMS error was reduced to 0.0916.

Figure 10: After selecting the Multipoint Geometric Correction tool again, the Resample Method was set to Bilinear Interpolation. Click OK and a rectified output image is generated.




Results

Figure 11: The resulting image over the distorted image with the swipe tool activated enabling both images to be viewed and compared. The section of river highlighted within the yellow circle showcases the difference between the two images.

Figure 12: The resulting image is the lighter colored image. On the left is the output image over the distorted image, the difference between the two is evident. On the right is the output image over the reference image, the two seem to be spatially identical.



Data Sources
UWEC Department of Geography and Anthropology