Lab 3

Lab 6

Goal:

The goal of this lab was to demonstrate geometric correction, which is a very important preprocessing activity so that the extracted information of that image is reliable. Geometric correcting eliminates most geometric distortion in an image so that individual pixels are in their proper planimetric position. 

Methods:

Part One:

Part one consisted of geocorrecting an image of the Chicago region to a contour map of the same area in Erdas Imagine by placing ground control points (GCPs) in the same places on the image and the map. A first order polynomial equation was used, so four GCPs were placed (an extra was added for better accuracy). When placing the GCPS, one is first placed on the image and another is placed on the same exact spot on the map. In order to make sure the GCPs are in the exact same spot, one must observe the Root Mean Square (RMS) error located to the right of each GCP on the bottom panel and the control point error (CPE) in the bottom right corner of the screen. These errors shows how close the GCPs are to matching exactly. An ideal RMS error is 0.5 or lower, but in this section 2.0 or lower was acceptable. By zooming into each GCP they could each be altered until the CPE was below 2.0. The result is a geometrically corrected image.

Part Two:

In this part a slightly distorted image of Sierra Leone was rectified with another reference image of Sierra Leone. This image was geocorrected using the same methods as in part one, but it used a 3^rd order polynomial instead, so 9 GCPs are required, but 12 total were added to prevent wrap tool error. In completion of the georectification the bilinear interpolation resampling method was used. The result is a nearly identifcal geometrically corrected image of Sierra Leone to that of the reference image of Sierra Leone.

Results:

Below is the geometrically corrected image of the Chicago region in part one. For unknown reasons the control point error would not appear in the lower right-hand corner of the screen, but the individual RMS error of each GCP is displayed and each one is below 2.0.

Below is the geometric correction of the Sierra Leone region. The corrected image is on the left and the reference image is on the right. By looking at the bottom right corner of the screen the CPE is displayed and is less .5, meaning that the image is geometrically accurate.