Tuesday, March 1, 2011

Crusta (from UCD KeckCAVES) and Arc GRID Raster Projection Issues

It may be a long shot, but in case someone is out there wondering why their elevation data is offset from their drape imagery while using crusta, I have a little hint.

GDAL, to my knowledge, has of yet to learn how to find the projection of Arc GRID data.  So you need to specifically point to the prj.adf file.  Then it is smart enough to find the files that contain the image data and process acccording to the projection found in the prj.adf file that is part of the GRID data structure.  Otherwise, you'll be mislead into thinking Construo found projection information when it begins processing data... it defaults to WGS 84. A keen eye, and awareness of your data's projection, would be the tip off that something may be wrong. So, using construo:

construo -dem ./topo.globeFile ./lidargrid/prj.adf

The Crusta wiki instructs users to point to the GRID's container directory/folder, but GDAL won't find the GRID's projection and so Construo may create a globeFile with the wrong projection.

Construo uses its default projection... and transforms/registers the elevation data incorrectly.

Elevation data offset solved by using the prj.adf file within the GRID data structure.

This example is a good reason to understand what a projection is.   They can cause a lot of trouble.

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