Friday, August 14, 2015

Landsat image pre-processing in ArcGIS - tools for topographic correction

Correcting Landsat imagery for topographic effects is challenging at best and ignoring the differential illumination resulting from topography can lead to some pretty misleading results, especially when it comes to change analysis. Topographic efffects arise not just from shadowing, but also (more importantly) from different angles of the ground relative to the sun angle. When the sun is directly overhead pixels will be much brighter than when the sun angle is at a much lower angle. In our lab we've used a variant of the empirical-line method for removing topographic effects on Landsat image. In the Landsat Toolbox for ArcGIS 10.1. First, a Landsat metadata on sun angle and azimuth is used to generate a hillshade (illumination) raster that mimics illumination at the time of satellite overpass. Then each band is extracted and a linear regression is used to predict the reflectance as if each pixel were illuminated the same. We've found this method to be simple and quite successful at reducing the differential illumination effects in an image.

The images below illustrate some mountainous terrain before (left) and after (right) topographic correction. You can see that the terrain appears flat aster topographic correction.

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