topic Re: Difference between !shape.area! and Calculate Geometry > area in Python Questions

Difference between !shape.area! and Calculate Geometry > area

ΠαναγιώτηςΠαπάζογλου — Wed, 18 May 2016 12:46:53 GMT

I have a shapefile with 62.000 polygons.

I have created two fields (Double) AREA1 and AREA2 with identical properties.

On AREA1 I have calculated the area of polygons with Calculate Geometry > area process.

On AREA2 I have calculated the area of polygons with Field calculator and the command: !shape.area!

When I compare the difference between the two fields ([AREA1] - [AREA2]), I find 30 polygons with no zero difference less than 0,01 square meters.

Is there a different algorithm for these processes?

Does anyone know why is this happening?

Re: Difference between !shape.area! and Calculate Geometry > area

DanPatterson_Retired — Wed, 18 May 2016 13:03:31 GMT

does the file have a defined coordinate system? problems in calculating shape geometry arise when there is no know coordinate system. This is well documented on this site.

Re: Difference between !shape.area! and Calculate Geometry > area

ΠαναγιώτηςΠαπάζογλου — Thu, 19 May 2016 06:07:40 GMT

The shapefile has a coordinate system defined (Greek Grid).

I tried yesterday after publishing my question to calculate with and without defined Projection and the result was the same.

Re: Difference between !shape.area! and Calculate Geometry > area

DanPatterson_Retired — Thu, 19 May 2016 10:09:16 GMT

Two things to check

the dataframe and file are in the same coordinate system
run a repair geometry on the file to correct any unseen geometry errors

Re: Difference between !shape.area! and Calculate Geometry > area

ΠαναγιώτηςΠαπάζογλου — Thu, 19 May 2016 12:12:33 GMT

The check geometry found no errors but I run Repair Geometry.

The dataframe in on the same coordinate system.

The difference remains!

Re: Difference between !shape.area! and Calculate Geometry > area

DanPatterson_Retired — Thu, 19 May 2016 12:17:27 GMT

Pagagiotis.... export and post a sample, I would be interested in seeing this vagrant shape set since I am interested in computational geometry issues.

Re: Difference between !shape.area! and Calculate Geometry > area

JoshuaBixby — Thu, 19 May 2016 13:47:44 GMT

What version of ArcMap are you running? It always helps to know the versions of software we are discussing.

Re: Difference between !shape.area! and Calculate Geometry > area

ΠαναγιώτηςΠαπάζογλου — Thu, 19 May 2016 19:11:01 GMT

I am using ArcMap 10.3.1 on Windows 10 Pro.

Re: Difference between !shape.area! and Calculate Geometry > area

DanPatterson_Retired — Thu, 19 May 2016 19:24:11 GMT

any chance on the sample geometry?

Re: Difference between !shape.area! and Calculate Geometry > area

ΠαναγιώτηςΠαπάζογλου — Thu, 19 May 2016 20:04:56 GMT

I will provide it tomorrow.

Re: Difference between !shape.area! and Calculate Geometry > area

DanPatterson_Retired — Thu, 19 May 2016 20:14:42 GMT

great

Re: Difference between !shape.area! and Calculate Geometry > area

ΠαναγιώτηςΠαπάζογλου — Thu, 19 May 2016 21:00:04 GMT

Well I had a small change on the parameters of the question.

First the problematic polygons are 31.

The differences are on the 7th decimal (aka 0,0000001).

I know that this difference is quite small but the quality tests of the project that I work on does not want this kind of differences.

I also noticed that the calculation for AREA2 lasts from 3 to 4 times more than AREA1.

I attached the 31 polygons that have the difference, as requested.

https://onedrive.live.com/redir?resid=AF5E4C8DEC50543E!457058&authkey=!AEIBlooHmQ1Fu-o&ithint=file%2czip

Re: Difference between !shape.area! and Calculate Geometry > area

JoshuaBixby — Thu, 19 May 2016 23:51:57 GMT

I am running ArcMap 10.4 on Windows 7 x64 Enterprise Edition.

When I preview your shapefile in ArcCatalog, I don't see any differences at all:

Re: Difference between !shape.area! and Calculate Geometry > area

DanPatterson_Retired — Fri, 20 May 2016 00:42:54 GMT

No errors... your problem is a display issue, your geometry is correct.

Re: Difference between !shape.area! and Calculate Geometry > area

ΠαναγιώτηςΠαπάζογλου — Fri, 20 May 2016 05:26:31 GMT

Thanks for your replies but....

As you can see on the following screanshot there errors and they are visible and countable.

Also if you try to Select by attributes with AREA1 <> AREA2, all polygons are selected.

And with ArcCatalog, on the view of the table, it uses the default size for decimals (6 digits), that's why you see only zeros.

As you can see there are a few polygons that have 0,000000000 as difference. I assume that the difference is even smaller (probably 0,0000000001). If you change the number format of the DIFF field in scientific, you will see the exact difference, as follows:

The main question for this is WHY is it happening?

When we use Python programming, we want to automate the EXACT actions we do by hand.

If the Calculate Geometry tool, uses a different algorithm to calculate the area of a polygon related to the Pythonic syntax !shape.area!, then the least ESRI should do is to document it and explain WHY it happens.

The second step for all this would be to solve this problem.

Re: Difference between !shape.area! and Calculate Geometry > area

NeilAyres — Fri, 20 May 2016 07:55:12 GMT

Perhaps there is a slight difference ( very small) and in area terms is extremely tiny % of 1m2.

So, to be realistic, why not use whichever is quicker to calculate and round the result to 2 decimals.

Re: Difference between !shape.area! and Calculate Geometry > area

ΠαναγιώτηςΠαπάζογλου — Fri, 20 May 2016 08:44:44 GMT

You are correct.

In real world, on the field of environmental and landuse surfaces, differences in area less than 1 square millimeter are negligible.

On the other hand, the fact that there is a difference is a problem.

When you calculate areas on the same polygons, on the same format (shapefile) and with the same program, the results must be the same.

At the same time, which of the two areas is the correct one, as long as they can't be both?

And when I have more than 100 shapefiles to calculate the area, I expected to be able to simulate the Calculate Geometry tool with the Calculate Field tool and the command !shape.area!.

Apparently, what I was expecting was not the case, and I will have to make the calculation manually but I think it should be documented by ESRI that the two algorithms are different and produce slightly different results.

Re: Difference between !shape.area! and Calculate Geometry > area

NeilAyres — Fri, 20 May 2016 10:13:45 GMT

I think you are being unnecessarily pedantic. And in the real world, no one is mapping anything especially environmental or landuse variables to a resolution of 1 square millimeter. And your difference is a billion times smaller than that.

Re: Difference between !shape.area! and Calculate Geometry > area

DanPatterson_Retired — Fri, 20 May 2016 10:20:28 GMT

I calculated it several ways and the results were still the same... no difference ... I don't know what makes your installation different that all the rest of us, but I would investigate that as well. PS, side the areas were in m^2, that area represents less than the area of a skin cell.

Re: Difference between !shape.area! and Calculate Geometry > area

ΠαναγιώτηςΠαπάζογλου — Fri, 20 May 2016 10:39:03 GMT

I don't think I am pedantic.

If the produced shapefiles cannot pass the quality testing of our clients, we won't get paid.

In order to avoid this, I will have to explain why this is happening.

The options are two:

Wait ESRI to fix it.
Wait ESRI documented in detail, so that I can support the result.

P.S.: Some differences are 0.0000001m² that equals to 0,1mm²