Problem on calculate area in clipped layers

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08-16-2012 07:09 AM
GiacomoAssandri
New Contributor
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
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1 Solution

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DanLee
by Esri Regular Contributor
Esri Regular Contributor
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 Top...
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 t...

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.

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DanLee
by Esri Regular Contributor
Esri Regular Contributor
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.
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GiacomoAssandri
New Contributor
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
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DanLee
by Esri Regular Contributor
Esri Regular Contributor
Sure, just a sample area that would reproduce the issue, please.
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GiacomoAssandri
New Contributor
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
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DanLee
by Esri Regular Contributor
Esri Regular Contributor
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.
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GiacomoAssandri
New Contributor
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.
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DanLee
by Esri Regular Contributor
Esri Regular Contributor
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/00170000004s0...
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GiacomoAssandri
New Contributor
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
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DanLee
by Esri Regular Contributor
Esri Regular Contributor
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 Top...
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 t...

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.
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