Friday, 15 January 2016

How to calculate a percentage of an area covered by polygons ArcGIS 10.1


I created an Aspect Layer with 9 Zones on which I want to use the Tabulate Area Tool with another polygon layer namely Woods. So what I actually need to know is the percentage, covered by woods for each of these areas. I tried it with tabulate area but without success, the intersect tool might be useful here but I can choose the polygon layer there.



Answer



There are two ways of calculating, raster on raster or vector on vector. radouxju has given an instruction on raster on raster but I believe the vector on vector might be more what you're after.


Convert your raster into polygons using Raster to Polygon, don't simplify the polygons for more accurate results.


Percentage of cover for the overlaying polygons is Area in Zone / Total original area, to preserve this add a field for original area (let's call this OArea) as the shape areas will be updated during the overlay.



Overlay with the zones using Intersect, Identity or union... whichever one looks best to you;Union or Intersect on two layers can be done with a standard license where Identity is an Advanced tool. Then summary statistics to compile your output table: Use your original FID/OBJECTID, OArea and Zone as the case fields and shape_area as a sum, this will give the areas for each ID and Zone combination, then divide the ID/Zone area sum by the OArea to give the percentage of each polygon covered by the zone type.


If you want an overall value and not to identify each input polygon then sum the OArea as well as the Shape_Area and leave only Zone as case field, then divide Shape_Area sum by OArea mean for percentage of each zone type over all input polygons.


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