Geoprocessing 1 : Intersect

The intersect operation is one of the many overlay operations. When intersecting layer A with a layer B the result will include all those parts that occur in both A and B. In this post I'm going to compare the results of the ArcGIS Intersect tool with the corresponding operation in 3 other GIS packages. The reason why did this was that I noticed that the output from ArcGIS contains more features then I expected when dealing with overlapping geometries. This is usually not a problem but it becomes one when you have lots of overlapping geometries.

The GIS tools I used to compare the result of ArcGIS with were : uDig, Quantum GIS and gvSIG. All these and some more can be found on Portable GIS. Portable GIS brings open source GIS to your usb.

To test the code I prepared 2 shapefiles. One with a long small polygon and the other one with 3 overlapping rectangles (see image below).

ImageHost.org

When intersecting with ArcGIS (version 9.2 and 9.3 tested) the output looks like below. As you can see in the attribute table the result contains 9 polygons.

ImageHost.org

To be able to do the intersect operation with uDig I had to install the Axios Spatial Operations Extension. You can install this extension by clicking Help -> Find and install from the menubar. With Quantum GIS I needed to enable the ftools plugin to enable the geoprocessing functionality. The results of the 3 used open source GIS packages where the same. Only 3 features where created in the output shapefile. For completeness I added the screenshots of the results.

uDig :

intersect_1_2_uDig.jpg (28 KB)

Quantum GIS :

intersect_1_2_QGIS.jpg (41 KB)

gvSIG :

intersect_1_2_gvSIG.jpg (19 KB)

Have any comments or questions ? Let me know !

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