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).

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.

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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