Problem on calculate area in clipped layers

[GiacomoAssandri]

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

[GiacomoAssandri]

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...

[DanLee]

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.

[GiacomoAssandri]

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.

[DanLee]

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.

[GiacomoAssandri]

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...

[DanLee]

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.

[GiacomoAssandri]

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!!

[DanLee]

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!