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I am new to GRASS and am trying to use it because I can't afford the Spatial Analyst extension in ArcGIS.

I am trying to remove all of the values in a NDVI raster less than some value to exclude shadows and bare soil before I calculate averages. I have tried

NDVI_b = if(NDVI_a <= 0.1,null(),NDVI_a)

My understanding is that this statement means that a new map (NDVI_b) should be created where cell values <= 0.1 are replaced with null otherwise the value remains the same as in NDVI_a. However all I'm getting is null value for all cells in my output raster. Is there something fundamental that I'm missing? I have tried using both the GUI Raster Map Calculator as well as the command console.

  • Have you set your region to match the NDVI_a raster? (This is FAQ #0 for GRASS users. Always set the region first) – Micha May 26 '16 at 9:13
  • Yes, I did set my region to match my raster and I opened it and it projected correctly. I think that it is a problem with my Geotiff. I was able to do something similar with a sample raster that I downloaded, but I can't get it to work with a geotiff that was outputted by Pix4D. I compared the gdalinfo for my file and the downloaded file but did not notice substantial differences. I thought that a geotiff was a goetiff. – eapedaile May 26 '16 at 18:00
  • When I run r.mapcalc on my geotiff, the output raster is basically empty: – eapedaile May 26 '16 at 18:58
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Figured it out - the problem was that my original geotiff was not properly projected. I fixed the projection and the raster calculation works.

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