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Percentage of polygon overlap using Area on Area overlayer & Expression Evaluator totals 101% when it should be 100%

helenwhiteley
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I am using an Area on Area Overlayer and Expression Evaluator to calculate the percentage of overlap of one polygon dataset with another -the scenario is percentage ownership. I have noticed that those polygons that should have 100% overlap / ownership because they are completely within the other, do not show as a nice round 100%, always 99.something up to 101.something. I'm guessing this is something to do with geometry and /or tolerance, but how to solve it? 101% ownership will raise questions when the data is presented!

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Best answer by ebygomm 28 April 2021, 10:00

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david_r
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The obvious, but perhaps somewhat cheeky solution would be to simply set the value to 100.0 if it's > 100.

caracadrian
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Try setting the Tolerance in AreaOnAreaOverlayer to 0. It can slow down the workspace a little but it should fix the isue.

helenwhiteley
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The obvious, but perhaps somewhat cheeky solution would be to simply set the value to 100.0 if it's > 100.

Thanks David. I suppose I could! I guess, it makes me concerned for the relative accuracy of all the other percentages though...

helenwhiteley
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Try setting the Tolerance in AreaOnAreaOverlayer to 0. It can slow down the workspace a little but it should fix the isue.

Thanks. Unfortunately, the issue remains with tolerance set to 0.

caracadrian
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Thanks. Unfortunately, the issue remains with tolerance set to 0.

Run a topology or an AreGapAndOverlapCleaner.

AoAO uses all the features, maybe you have gaps or unwanted overlaps.

Also run a GeometryValidator and fix any issues regarding geometry validity.

ebygomm
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Are you able to share the geometry of two polygons you are comparing that you think should show 100% overlap but don't?

helenwhiteley
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Are you able to share the geometry of two polygons you are comparing that you think should show 100% overlap but don't?

Have attempted to attach... Thanks

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helenwhiteley
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Thanks. Unfortunately, the issue remains with tolerance set to 0.

Thanks, that definitely identified a lot of 'donuts', but, they are legitimate donuts. However, even with a non-donutty subset of data (see below) I don't get 100%

ebygomm
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Have attempted to attach... Thanks

The figure held in the Shape_Area field of the smaller polygon does not match the area of the polygon as calculated by FME (before doing any area on area overlaying). So if you are measuring the area after the area on area overlayer and then using this with the Shape_Area to calculate the percentage you are going to see strange results.

 

My suspicion is the differing area values are something to do with the fact that this small polygon is made up of arc segments.

helenwhiteley
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Have attempted to attach... Thanks

Amazing, thank you! I have used the Area Calculator to calculate an FMEArea field and used that in my percentage calculations and now have nice round 100%s. Why would the 2 area calculations (Shape_area and FMEArea) be different?

ebygomm
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Have attempted to attach... Thanks

Looks like that input polygon is actually not made up of arcs, but Bezier curves. These are stroked when read into FME which is probably introducing the area difference

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