Polygons with arcs do not have the correct area

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01-08-2016 08:59 AM
DonnyVelazquez
Occasional Contributor

It appears there's a bug in the fgdapi when inserting polygons with arcs. It doesn't calculate the area correctly. It only calculates the straight sides and does not include the area the arc is creating. Once you run it through repair geometry, it fixes the calculation.

Anyone else run across this? Vince Angelo

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LanceShipman
Esri Regular Contributor

Area calculation in the file geodatabase API does not include curves in the calculation. We will consider it in a later release.

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VinceAngelo
Esri Esteemed Contributor

I have yet to have CAD objects in a project, so I don't have the experience to respond.  Generally speaking, geometry area is a *calculated* object, so the value returned would be the one calculated by Simpson's Rule​, which wouldn't consider perfect curves. 

To which "repair geometry" do you refer?  ArcObjects is a different code base, and has CAD support code, for which the area function would be different.

- V

DonnyVelazquez
Occasional Contributor

It doesn't have anything to do with CAD. We're just giving it an array of points with the arc info.

Which gets added to the file geodatabase and looks great in ArcMap. It's not slightly off with the

area, the entire area of the arc is not included.

We're using the repair geometry tool in ArcMap. I feel like the calculation function inside the API

is not including the arc and is just cutting it off.

Do you know of someone else I could ask about this?

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DanPatterson_Retired
MVP Esteemed Contributor

About calculating COGO area—Help | ArcGIS for Desktop is the only reference regarding this topic.  I highly doubt that they use anything more than n-gon representations of arcs, circles or the like in normal calculations of lengths or area.

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RebeccaStrauch__GISP
MVP Esteemed Contributor

Have your tried converting your lines to a polygon?

Feature To Polygon—Help | ArcGIS for Desktop

Area is a product of a polygon....arcs are just lines even if they do close.  Fixing geometry may be doing the same thing?

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DanPatterson_Retired
MVP Esteemed Contributor

Rebecca, If you buffer a point to make a circle, it is represented by an n-gon (360 sides) and the area is close to that of a circle, but not exact

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RebeccaStrauch__GISP
MVP Esteemed Contributor

Hi Dan,....sorry, old ArcInfo coverage brain....  arc = line.  Not thinking about arc = curve.    I was reading the question as creating a line from coordinates and not calculating areas. 

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VinceAngelo
Esri Esteemed Contributor

Actually, depending on how you got to the circle area, the value might be a perfect "pi R squared" value, as I had found over in GIS SE.

- V

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DanPatterson_Retired
MVP Esteemed Contributor

Actually, If I am concerned about area, I just make the radius of the triangles forming the ngon a tad bigger to compensate for the area differential...so that area is exact (ie Pi r-ish squared)

If I need diameter or radius, I just use the defined radius

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VinceAngelo
Esri Esteemed Contributor

Please post a code example of the issue you feel is a defect.  I don't see any area functions in the File Geodatabase API, so I don't see how this could be a FileGDBAPI.dll issue.

ArcObjects does of course have such functions, but this GeoNet "place" is only about the File Geodatabase API

- V

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