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I am currently working with multiple shape files that I have clipped to the exact same extent, but when I calculate area (acres) within each polygon.
I am getting different results between the different shape files.
Each file represents a different variable, ie: I am working with a slope polygon, aspect, vegetation, etc.
The acreage results (from calculate geometry) are similar, but not the same (9015, 10294, 8999).
There are multiple geometries, each different for each shapefile, and I am comparing the acreage sums across each shapefile that I have clipped to the same size.
The plotted clipped extents are mostly just squares with some irregular shaped polygons.

Fixing the topology errors did not make the acreage's equal. They are all on the same projection. Fixing geometry errors did not work either.

Angela
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  • Welcome to GIS.SE! Please edit your question to include what software you are using. – Paul Jul 10 '14 at 19:10
  • "Clipped to the same exact extent" is confusing me. Is there just one geometry, or are there multiple geometries, in which case, are you comparing the acreage sums? Is there overlap within the shapefiles, or are they "mutually exclusive and exhaustively complete"? What do the plotted clipped extents look like? Please update the question. – Vince Jul 10 '14 at 19:24
  • There are multiple geometries, each different for each shapefile, and I am comparing the acreage sums across each shapefile that I have clipped to the same size. As far as overlap goes, I'm not sure how to find that out? Again, I've got only pretty basic GIS skills. The plotted clipped extents are mostly just squares with some irregular shaped polygons. – Angela Jul 10 '14 at 19:32
  • Is the polygon used for clipping smaller than all of the original data polygons? – Erica Jul 10 '14 at 19:39
  • Yes it's much smaller than the original polygons – Angela Jul 10 '14 at 19:42
  • are these 3d polygon faces (you mention slope, aspect) a 3d polygon would have a different area than a 2d polygon of the same x,y corners. – Brad Nesom Jul 10 '14 at 20:02
  • You could be very right about that, but the polygons that are 2d still don't have the same area. Will look into converting the 3d to 2d right now! Thanks! – Angela Jul 10 '14 at 20:29
  • As clarifications are sought via comments it is best if you can respond to them by using the edit button to revise your question and make it clearer. Potential answerers will sometimes only have the time or desire to look at the question body rather than the trail of comments that go with it so making it standalone is advised. – PolyGeo Jul 11 '14 at 01:17

3 Answers3

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My money is on overlapping polygons. You don't say what software you have access to, but in ArcEditor or ArcInfo (I may never get used to saying ArcGIS for Desktop Advanced, yuck) you could create topology with a "must not overlap" rule and find out that way. (That would also be how to fix it.)

Or perhaps easier, fill them with different colors and make them partially transparent, and then you'd see overlap.

ArcView (ArcGIS basic) doesn't do real topology, but map topology is there, kind of a weak substitute.

Jersey Andy
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  • Sounds like the likely culprit, I will definitely try this. Thank you! – Angela Jul 10 '14 at 21:24
  • Aaand I don't have either software. Of course :( – Angela Jul 10 '14 at 21:36
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    You should edit your question to say what software you do have, and add tags for it too. You'll get better answers that way. – Jersey Andy Jul 10 '14 at 21:38
  • Gotcha. And I was wrong, I did find a way to edit it. Now I have a whole new problem that I cannot even explain clearly enough to be fixed :) – Angela Jul 10 '14 at 22:13
  • I fixed the topology errors on two different layers and the acreage sum is still different, after I fixed all gaps. – Angela Jul 10 '14 at 22:31
  • It is very difficult to manually fix topology errors in shapefiles with many features and expect to get all the gaps and overlaps. If you only have ArcGIS Basic you might try the Integrate tool on copies (it changes the input) and see if larger tolerances fix things without changing your data too much. If not you could copy and paste your polygons into a line layer and use it to build polygons (see http://gis.stackexchange.com/questions/19077/how-to-create-polygons-from-lines), then recalculate your area. – John Jul 11 '14 at 20:05
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Try checking if they are all on the same projection! if they are on different projections their areas may come up as different. You can do this by right clicking your shapefile and going to preferences, then open the tab 'source' and scroll down to Projected Coordinate System.

Fred
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I would check the actual geometry for errors as well. Self intersects can cause differences in reported areas. Run the "repair geometry tool" to find and fix them?

GISI
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