topic Re: Problem on calculate area in clipped layers in Geoprocessing Questions

Problem on calculate area in clipped layers

GiacomoAssandri — Thu, 16 Aug 2012 14:09:21 GMT

Hi all,
I have a problem with arcgis 9.3.
I clipped a layer using the tool of arctool box. The problem is that clipped layer (output feature class) area is bigger than clip feature area.
The output differ from the clip feature because is made of many poligons.
The difference is 0,06 ha out of a layer of 12,5 ha.
Is possibile that the difference between the two is dued to the perimeters of poligons?

thank you
Giacomo

Re: Problem on calculate area in clipped layers

DanLee — Thu, 16 Aug 2012 15:15:04 GMT

Are the input features and the clip feature in the same coordinate system? If possible, you can share the data so that people can have a look.

Re: Problem on calculate area in clipped layers

GiacomoAssandri — Thu, 16 Aug 2012 15:37:04 GMT

Yes they are.
Before i didn't say that the area of the ouput is calcolated by the sum of the single polygons.
Do you need the impute feature, the clipper and the clipped features?

TY

Re: Problem on calculate area in clipped layers

DanLee — Thu, 16 Aug 2012 15:47:00 GMT

Sure, just a sample area that would reproduce the issue, please.

Re: Problem on calculate area in clipped layers

GiacomoAssandri — Thu, 16 Aug 2012 16:18:18 GMT

Ok. There are 4 shape files. The imput, the clip and the output and also the output with the polygon areas calculated.
In my opinion the sum of the areas should be equal to the clip area...

TY

Re: Problem on calculate area in clipped layers

DanLee — Thu, 16 Aug 2012 20:12:56 GMT

Your input features have many overlaps. Here is how you can analyze your input:
1. Use Dissolve tool to make one polygon from the entire input (uncheck "create multipart features"). The area of the output polygon is 30.9 ha.
2. Add a field in the input features table and then use Calculate Geometry to get ha values for all polygons. Check the sum of the all polygon areas - it is 31.03 ha. It is bigger than the area of the dissolved polygon.
3. Run Feature To Polygon tool on the input features. Then add a field and calculate areas. Sort ascendingly the calculated area field. You will see many small values; zoom to these small polygons - they are mostly overlaps or gaps. The larger sum of area comes from these overlaps.

You would need to get the input features cleaned up before doing any spatial processing.

Hope that helps.

Re: Problem on calculate area in clipped layers

GiacomoAssandri — Sat, 18 Aug 2012 08:23:09 GMT

Thank you! Unfortunatly I'm a very basic user so I need some more details.
1. Point 1 and 2 are necessary for point 3 or are just to understand my problem?
2. Where can I find the "feature to polygon" tool? In toolbox I was not be able to find it.
3. To calculate the area can i use the tool in the toolbox?

TY

Your input features have many overlaps. Here is how you can analyze your input:
1. Use Dissolve tool to make one polygon from the entire input (uncheck "create multipart features"). The area of the output polygon is 30.9 ha.
2. Add a field in the input features table and then use Calculate Geometry to get ha values for all polygons. Check the sum of the all polygon areas - it is 31.03 ha. It is bigger than the area of the dissolved polygon.
3. Run Feature To Polygon tool on the input features. Then add a field and calculate areas. Sort ascendingly the calculated area field. You will see many small values; zoom to these small polygons - they are mostly overlaps or gaps. The larger sum of area comes from these overlaps.

You would need to get the input features cleaned up before doing any spatial processing.

Hope that helps.

Re: Problem on calculate area in clipped layers

DanLee — Sat, 18 Aug 2012 17:13:52 GMT

No problem.

My points 1 and 2 are just to analyze the areas (your issue): the dissolved area vs. the sum of all individual areas. Since the sum area is greater than the dissolved area, there must be overlaps among the individual polygons that contribute to the sum.

Where are the overlaps? There are different ways to find out; using the Feature To Polygon tool (my point 3) is just one of them. You can find Feature To Polygon tool (requring ArcInfo license) in Data Management toolbox - Features toolset.

Yes, you can use Add Field tool followed by Calculate Field tool (both in Data Management toolbox - Fields toolset) to add a field (choose the Double field type) to your shape file and to calculate the geometry. Have a look of this help topic:
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#//00170000004m000000
And examples here:
http://help.arcgis.com/en/arcgisdesktop/10.0/help/index.html#/Calculate_Field_examples/00170000004s000000/

Re: Problem on calculate area in clipped layers

GiacomoAssandri — Sun, 19 Aug 2012 18:35:20 GMT

Thank you for your politeness!
Unfortunatly I haven't got arcinfo. Could you please explain me another way to correct the problem?
Thank you,
Giacomo

Re: Problem on calculate area in clipped layers

DanLee — Sun, 19 Aug 2012 22:52:09 GMT

If you have ArcGIS 9.3 - ArcView license, you can use the Union Tool (Analysis toolbox - Overlay toolset); you can uncheck the "Gaps allowed" box. The output contains a FID_inputname field; where this field has a value of -1, the polygon would be slivers (overlaps or gaps). You would have to be in an Edit session to modify the polygon geometry.

If you have ArcGIS 9.3 - ArcEditor license, you can use the proper topology tools (Data Management toolbox - Topology toolset) to find the overlaps and gaps. These tools only work with geodatabase feature set and feature classes, not shapefiles. You really need to read about how to use these tools, starting with the overview of Topology toolset:
http://webhelp.esri.com/arcgisdesktop/9.3/body.cfm?id=1970&pid=1969&topicname=An overview of the Topology toolset&tocVisable=0
and then learn how to:
- Create topology
- Add feature class to topology
- Add rules to topology (e.g. Must Not Overlap and Must Not Have Gaps)
- Validate topology

Here is another topic on polygon editing, including overlaps and gaps:
http://webhelp.esri.com/arcgisdesktop/9.3/body.cfm?id=638&pid=635&topicname=Common polygon editing tasks&tocVisable=0

Hope these help you understand your data and what need to be done. If you have more specific questions, it would be better that you start a new post for an exact question. Thanks.

Re: Problem on calculate area in clipped layers

GiacomoAssandri — Mon, 20 Aug 2012 12:24:11 GMT

I have arcgis with arcview license. I followed you advice and i corrected all the "-1" polygons. Then I used again the union tool and no "-1" polygons left. Unfortunatly there are still some difference between the 2 areas...

Re: Problem on calculate area in clipped layers

DanLee — Mon, 20 Aug 2012 15:00:11 GMT

It would be helpful that you describe in detail what areas you are comparing and how you calculate their values, and which area is bigger than which area now? If necessary, please share the data again.

Re: Problem on calculate area in clipped layers

GiacomoAssandri — Mon, 20 Aug 2012 15:34:18 GMT

Ok, I share the whole dataset. "Original layer" is my starting point. I used "dissolve tool" to make a single poligon (truly two, because they are separated). Making the sum of the poligon areas of original layer and comparing it with the area of dissolved layer, the first is bigger (of about half an ectar on a total of 113 ha)...
To calculate the areas i Used Calculate areas tool.

Re: Problem on calculate area in clipped layers

DanLee — Mon, 20 Aug 2012 17:32:56 GMT

It still incidates that there are overlaps in the original layer. Since Union doesn't produce -1 FIDs (slivers), the overlaps must be coindicent (duplicated) features.

- Use Intersect tool (Analysis toolbox - Overlay toolset) on the original layer itself. I think all the output 335 features are duplicate features. Some of them may be real features to keep one of the dups; others might be just duplicate slivers that you need to fix.

It is really hard to deal with data problems with ArcView license; you cannot take advantage of the Topology tools and other advanced tools mentioned in the help doc about overlaps and gaps to detect and solve the problems. Also new tools were added in ArcGIS 10, such as Delete Identical (e.g. identical features), to help clean up data. If you keep getting data with these problems, perhaps it is time to consider a higher license or an upgrade.

Re: Problem on calculate area in clipped layers

GiacomoAssandri — Mon, 20 Aug 2012 17:58:13 GMT

I'm not sure about the output of intersect: the 355 items are all polygons duplicated?
Ok: the small and linear ones are surely geometry error that can be fixed (one by one, sigh!!). But the bigger ones don't seem to be duplicated because are existing polygon in the original layer and if you delete them in the original layer there is nothing remaining under them...

Re: Problem on calculate area in clipped layers

DanLee — Mon, 20 Aug 2012 19:20:22 GMT

You can give me an example that is not duplicated by telling me the FID_Origin value in the intersect result.

Here is my example:
The two features with FID_Origin values 773 and 774 in the intersected output, with the calculated area of 0.071622 ha, are duplicated. Do you see them?

If you double-click one of them in the table to zoom to it, and then click the Select Features tool and click a location inside the polygon, at the lower left corner of the ArcMap window you should see "Number of features selected: 2". You have to make sure that you delete only one, not both, in an edit session.

Re: Problem on calculate area in clipped layers

GiacomoAssandri — Tue, 21 Aug 2012 10:39:02 GMT

Ok, thank you very much. Now is clear.
I have an hard and long work to do now...Hope this is the last post in this thread!!

Re: Problem on calculate area in clipped layers

DanLee — Tue, 21 Aug 2012 15:05:45 GMT

You are welcome. I am glad that you now have a better understanding about the data problems. Hope you don't have to deal with the bad data very often. Good luck to your project!