Lab 6 - Geometric Correction

GOAL AND BACKGROUND

The goal of this lab was to learn how to use geometric corrections to correct distortion. This was done by using polynomial transformations to adjust an image with a reference image.


METHODS

First an image of Chicago was corrected using a first order polynomial. First order polynomials require three Ground Control Points (GCPs) to correct the image. Four were used here, as more GCPs increase accuracy. The GCPs were placed on the study image, then placed on the same feature on the reference image. The GCPs had to be spread out across the image to minimize potential distortion. They were adjusted to maximize accuracy and minimize the RMS error. The goal was to have the RMS errors below 2. Lastly, the corrected image was extracted as a .img file.

Second, an image of Sierra Leone was used corrected with a third order polynomial. Third order polynomials require ten GCPs. Twelve were used to maximize accuracy. Once again, RMS error was brought well below 2, and the image was extracted as a .img file.


RESULTS

Shown below is the RMS error table from the GCPs in the Chicago image. Notice they are all well below the goal of 2.

Shown below is a screen capture of the final GCP locations. Notice how they are spread out.

Shown below is the RMS error table from the GCPs in the Sierra Leone image. These are also well below the goal of 2.

Shown below is the final screen capture of the GCP tool. Once again, they are spread out across the study area.

SOURCES

Earth Resources Observation and Science Center, United States Geological Survey.

Illinois Geospatial Data Clearing House.