7 replies to this topic

[NeverEasy]

Hello,

 

I have a shape file with lots of overlapping polygons that i wish to sum in a raster. So if a cell in the output raster happens to intersect with, say, 5 polygons; the cell value would be 5. I'm having a difficult time figuring out how to do this

 

Thanks!

[frax]

What software do you have at hand?

[NeverEasy]

ArcView/Basic  9.3 and 10.1. Spatial analyst, geostatistical analyst.

[David Medeiros]

You can Union the polygon layer to itself (that is only add it to the Union feature list). Then dissolve the resulting polygon stacks so that all overlaps become a single polygon with a count of the overlaps involved in the dissolve (this may require setting a merge field rule).

 

In order to dossolve the stacked polygons you'll need to give them an ID that all identically overlapped polygons will share. You can do this bycreating the X and Y coordinates for the Unioned polygons and then concatentating into a new field, XY_id, before dissolving on that field. If the DIssolve does not  directly give you a count field you can creat your own by adding a field "count", setting all feature values to 1, then settign the file merge rul to "sum" when dissolving.

 

Then convert to raster using the count field as the raster value.

 

Does any of that make sense? I can try to explain it better if not.

[NeverEasy]

Yeah, that did the trick - thank you very much!

[François Goulet]

You could have use the feature to raster tool for each layer. A value of 1 would mean that there's a polygon and 0, none. Then, using the raster calculator, you could sum all the raster (raster1 + raster2 + raster3, etc.). The result would have the number of polygon overlapping, if any.

[David Medeiros]

You could have use the feature to raster tool for each layer. A value of 1 would mean that there's a polygon and 0, none. Then, using the raster calculator, you could sum all the raster (raster1 + raster2 + raster3, etc.). The result would have the number of polygon overlapping, if any.

 

I think he had a single feature class with overlapping polygons, not multiple overlapping feature classes. Not sure but I think feature to raster will not honor the overlaps? He could split the single layer up so each polygon is it's own layer then do feature to raster etc... Both good methods to know as this comes up failry often in geoprocessing.

[François Goulet]

You could have use the feature to raster tool for each layer. A value of 1 would mean that there's a polygon and 0, none. Then, using the raster calculator, you could sum all the raster (raster1 + raster2 + raster3, etc.). The result would have the number of polygon overlapping, if any.

 

I think he had a single feature class with overlapping polygons, not multiple overlapping feature classes. Not sure but I think feature to raster will not honor the overlaps? He could split the single layer up so each polygon is it's own layer then do feature to raster etc... Both good methods to know as this comes up failry often in geoprocessing.

 

You're right... I wouldn't work with a simple feature class with overlapping polygon. 

 

Nevermind what I said!