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.