Buffer areas not the same

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06-10-2015 05:14 PM
LorrieAgnew
New Contributor

I have a grid of points that I need to apply a buffer to, in order to compare the total area of the resulting circles to other polygons. I'm using the Buffer tool, in the Analysis toolbox, to put an 7 mile buffer around each point. When I open the attribute table of the new feature class and look at the areas of each buffer circle, they are not the same. I would assume they should be the same, since it is the same buffer being applied to each point. Is there a reason this happens? Am I missing something? Thanks.

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5 Replies
DanPatterson_Retired
MVP Esteemed Contributor

you are using prjected data I hope? and can you provide a list of input parameters to ensure you arent using values from a field

ScottMoyer
Occasional Contributor

Another thing to consider, if you select Dissolve All polygons, overlapping polygons will be combined to create larger areas.

Scott

MelitaKennedy
Esri Notable Contributor

What version of the software? And as Dan asked, what coordinate system is the point feature class using?

Also, buffer is distance-based, not area-based, so I wouldn't expect to see identical areas. If you used an equal area projection, then there would be some distance distortion to deal with. How big are the differences you're seeing?

Melita

ChrisSergent
Regular Contributor III

Melita Kennedy I thought you needed an envelope to create a buffer.

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VinceAngelo
Esri Esteemed Contributor

Still another issue is the coordinate reference precision, with respect to the buffer distance (1mm precision on a 10cm buffer would result in visibly shifted boundaries).  And then there's the number of vertices in the resulting buffer (e.g., 9 vs 19 vs 37 vs 181 vs 361), which influence the best possible approximation to true circle area.

But the issue is most likely either overlap between neighboring buffers or vagaries introduced by a geodetic buffer. You should certainly be able to zoom to the extent of the features which are most unlike your expected area.  If there's no overlap, then you need to compare the difference of area values as a fraction of the coordinate reference precision.

- V