Basically what I'm trying to do (in ArcGIS 10) is to compare three vector files in order to create a 'comparison' raster layer. I tried using Data Comparison (under Data Management tools) but it didn't output what I was looking for. I basically wanted to create a raster surface that shows where each type of polygon overlaps. So for example if layer a and b overlap at some point, the raster pixel value will reflect that (and if a, b and c overlap, etc.). I can't figure it out and I've been browsing ArcMap's help for ages.
Thank you!
Best Answer
Agreeing with relima, but just adding a little more detail to the first step. You need to convert your polygons into rasters (make sure that you set the raster extents to be sufficiently large that they include all of the polygons). When converting poygons to rasters, you can choose what the raster value is. One easy way to keep track of the various intersections could be to set the raster value to 1, 2, 4 (which is 2^0, 2^1, 2^2; Whuber pointed out in a comment that using powers of 2 is necessary, as my previous example of using 1,2,3 could have had an ambiguous solution). Then when you add raster 1 and 4 together (using spatial data) the intersection will have a value of 5 which is only due to combining raster 1 and raster 4.
Something to watch out for in this process is the 'NoData' values. By default ArcGIS will make any area that do not contain data to the value of 'NoData'. 'NoData' when added to '0' results in 'NoData'. You may need to reclassify your values of 'NoData' to a value of '0'.