# Creating a raster surface of overlaps from vector polygon shapefiles?

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!

-

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'.

-
Good idea, but you need a different encoding. After all, 3 = 1+2 cannot distinguish between the presence of polygon type "3" and an overlap of "1" and "2". Use powers of 2 to assure uniqueness. You will be limited to at most 32 distinct polygon types, which rather limits this approach as a general solution, but it works fine for three layers--provided there are no overlaps within each layer! – whuber Feb 15 '11 at 19:24
True - I realized that as I wrote the question, but was not aware of that approach (which is brilliant!). I'll edit my answer to incorporate that. – djq Feb 15 '11 at 19:33
+1 for the edits, and additional kudos for pointing out how to treat the NoData values. – whuber Feb 15 '11 at 23:19

It sound like it could be simply done using spatial analyst "weight sum" feature. The function overlays several rasters multiplying each by their given weight and summing them together.

Please refer to this help page: for further information on the topic:

http://webhelp.esri.com/arcgiSDEsktop/9.3/index.cfm?TopicName=Weighted_Sum

Please, also take a look at the Weighted_Overlay feature:

http://webhelp.esri.com/arcgiSDEsktop/9.3/index.cfm?TopicName=Weighted_Overlay

-

Use the compare with your vector polygons then use:

Clip Management +clipping_geometry

http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//00170000009n000000.htm

Clipping Geometry —The raster data set is clipped based on the perimeter of the polygon shape.

-