Global Mapper v25.0

Use DEM to create a mask?

I'm working with raster image data including DEMs from a blueberry farm.  The farmer plants grass between the rows.  I would like to change the grass areas to gray so they do not show up on an NDVI.  Can I somehow create a mask using a DEM?  Since the ground isn't flat, I can't just pick an absolute elevation and get rid of everything below that.  Seems like I'll need to process the data outside of GM to calculate a local minimum elevation as a reference.