Ok, first of all, PLEASE don't laugh at my improper use of flowcharting symbols! :s I can't remember what the correct ones are and I just can't be arsed to look it up. Now, what I am doing is this... I am analyzing a binary raster row by row. everything on the left side of a wall (or river in this case) I want to be set to -999.99. So this works fine unless the river meanders. so I figured that out. the problem comes when the river runs straight left to right (adjacent cells in the same row). I have the logic worked out for it but only if there is no more than 2 adjacent cells in a row... after that it falls apart.... any ideas or suggestions?