Calculating Topographic Exposure with Raster Calculator

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03-14-2014 06:13 AM
MaryLynn_Tobiasz
New Contributor
I need you help trying to calculate Topographic Exposure (TOPEX) on a DEM in ArcGIS.  I am trying to figure out wind exposure on a DEM with 25m cells.
There is a great blog post where someone explains how to do it but he uses GRASS. http://jamiepopkin.blogspot.ca/2011/01/calculating-togographic-exposure-with.htmlI thought I would be able to put the same equation into ArcGIS Raster Calculator but it says the equation is invalid. I've tried playing with it as much as I can but I can't seem to make it work.

His first equation is:    r.mapcalc incl4north = "atan(((dem - dem[4,0])) / ((25 * 4)))"
And then he goes on to string a bunch of them together
[ATTACH=CONFIG]32197[/ATTACH]
Does anyone know how the equation should appear in Raster Calculator in either ArcGIS or QGIS?
I am super grateful for any assistant that anyone could provide
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DuncanHornby
MVP Frequent Contributor
The reason why your equation is failing is that you are using the syntax that grass uses.

dem[4,0] means nothing in ArcMap and is an invalid syntax.

Reading the web page you linked to it seems that this syntax is all to do with calculating some off-set.

I'm not sure if this is a red herring as my raster processing skills is not as good as my vector skills but I know you can shift a grid using the Shift tool. Or maybe someone can suggest some whizzy way of using focal stats?

Duncan
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