topic Polygons with arcs do not have the correct area in File Geodatabase API Questions

Polygons with arcs do not have the correct area

DonnyVelazquez — Fri, 08 Jan 2016 16:59:40 GMT

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

Re: Polygons with arcs do not have the correct area

VinceAngelo — Mon, 11 Jan 2016 03:44:54 GMT

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.

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Re: Polygons with arcs do not have the correct area

DonnyVelazquez — Mon, 11 Jan 2016 14:00:49 GMT

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?

Re: Polygons with arcs do not have the correct area

DanPatterson_Retired — Mon, 11 Jan 2016 14:31:23 GMT

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.

Re: Polygons with arcs do not have the correct area

RebeccaStrauch__GISP — Mon, 11 Jan 2016 16:16:55 GMT

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?

Re: Polygons with arcs do not have the correct area

DanPatterson_Retired — Mon, 11 Jan 2016 16:19:56 GMT

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

Re: Polygons with arcs do not have the correct area

RebeccaStrauch__GISP — Mon, 11 Jan 2016 16:24:22 GMT

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.

Re: Polygons with arcs do not have the correct area

VinceAngelo — Mon, 11 Jan 2016 16:49:27 GMT

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

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Re: Polygons with arcs do not have the correct area

VinceAngelo — Mon, 11 Jan 2016 16:56:49 GMT

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.

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Re: Polygons with arcs do not have the correct area

DanPatterson_Retired — Mon, 11 Jan 2016 17:17:33 GMT

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

Re: Polygons with arcs do not have the correct area

DonnyVelazquez — Mon, 11 Jan 2016 18:31:01 GMT

There is no public method for calculating area. But my assumption is that when a feature is added using the api that the area is calculated for it internally.

Is that true? I can't imagine that when you look at the area in ArcMap that the area for all features are calculated at that time.

Re: Polygons with arcs do not have the correct area

VinceAngelo — Mon, 11 Jan 2016 18:56:11 GMT

I don't think that's a safe assumption at all! There's two competing questions:

Why waste time calculating area if it isn't needed?
Why bother storing area if it can be calculated?

These are addressed differently by different formats -- area may be stored in SDEBINARY/SDELOB "Fn" tables, for example, but for others the value is calculated on the fly (the stored value may not be of any use, anyway, since if could be based on Cartesian degreees)

The fact remains that Desktop does not use the File Geodatabase API. I suggest you take this over to the ArcObjects SDK place, posting a complete code sample to address your issue.

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Re: Polygons with arcs do not have the correct area

DanPatterson_Retired — Mon, 11 Jan 2016 19:36:17 GMT

undone

Re: Polygons with arcs do not have the correct area

DonnyVelazquez — Mon, 11 Jan 2016 20:26:09 GMT

So let me get this straight. Are you saying any data created using the File Geodatabase API is not guaranteed to work in ArcGIS Desktop correctly?

I thought the point of the API was to read and write official file geodatabase's.

Re: Polygons with arcs do not have the correct area

VinceAngelo — Mon, 11 Jan 2016 21:56:01 GMT

I didn't make any claims about software functionality.

The File Geodatabase API is an open-licensed API library for simple feature access to Esri File Geodatabase data. It is a subset of the capability of ArcObjects with respect to geodatabases, as documented. It is most certainly not the reference implementation for File Geodatabase access.

Even though steps were taken to prevent it, there still exists a possibility that data generated via FGDBAPI.dll will have issues when accessed by ArcObjects, especially with respect to true curves (aka CAD objects). This is more an issue of the arcane complexity inherent in CAD representation, where the same words mean different things in different CAD software. If geometry "repair" corrects a CAD issue, then it wasn't encoded correctly (and if the area changes significantly, it may be due to the parallel shape vertex array not being populated correctly).

Again, in order to evaluate whether a defect exists, you need to provide Esri with a reproduction case. If it's a FileGDBAPI issue, provide FileGDBAPI code; if it's an ArcObjects issue, create a new thread over in ArcObjects and provide ArcObjects code.

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Re: Polygons with arcs do not have the correct area

DanPatterson_Retired — Tue, 12 Jan 2016 02:31:42 GMT

Polygon - Wikipedia, the free encyclopedia I had hoped for a chiliagon implementation at least, to allow for the link back to René Descartes and the cartesian coordinate system (ahem)

Re: Polygons with arcs do not have the correct area

VinceAngelo — Tue, 12 Jan 2016 02:38:35 GMT

I came close, with the 720-side shape. And don't forget that C in Cartesian, professor!

Re: Polygons with arcs do not have the correct area

TimHodson — Fri, 22 Jan 2016 07:15:06 GMT

Hello Donny,

It sounds like this may be related to the code used to create the polygon. I suspect the code may not be using the simplify method prior to storing the row in the feature class? I wrote a function (adapted from old VB code) to try to reproduce your case. Though I was not seeing the specific problem you describe, I found that you can get odd results if you do not use the simplify method. In my case polylines oriented counter-clockwise, when converted to polygons, result in a negative Shape_area attribute in the stored polygon unless the Simplify method is called first.

IPolygon::SimplifyPreserveFromTo

Please see the attached code and comments for more info.

-Tim

Re: Polygons with arcs do not have the correct area

DonnyVelazquez — Fri, 22 Jan 2016 13:46:34 GMT

This problem is with the File Geodatabase API not with Arcobjects.

Re: Polygons with arcs do not have the correct area

VinceAngelo — Fri, 22 Jan 2016 17:09:17 GMT

In order to demonstrate that it's a problem with the FGDB API, you'll need to provide a code sample which exhibits the behavior. There are a number of other possible explanations, and they all key on the exact contents of the geometry. Without a code sample, there is no way to distinguish between options.