Map algebra - subtract cell value from neighbour cells

Discussion created by johnb91 on Aug 2, 2010
Latest reply on Aug 4, 2010 by johnb91
Hi all,

This may have been addressed in the past but unfortunately I didn't even really know what to search for in the forums and so found nothing.

I would like to modify a grid such that the 'centre' cell value is subtracted from each cell in each 3x3 block of cells. That makes pretty much zero sense, but I'm having trouble firing synapses today and so if you would please refer to the attached pic, hopefully things will become clear. Looking at the image, I'd like to subtract 'A' from all yellow cells, 'B' from all orange cells, and so on. Obviously this is a simplification of my actual grid...

Is there a way to create this type of grid of differences with raster calculator? (I have a spatial analyst license and ArcGIS 9.3).

For my first attempt, I created a regular point sample with points coinciding with the centre (A, B, C, D, etc) cells, and intersected this with the original grid. I thought I might then be able to convert it to a raster which could be subtracted from the original raster, but couldn't work out a way to do it.

Any suggestions appreciated!