Overlay Operation Problems

One common problem in GIS is combining layers from different sources such as the boundary between two piece of property or even two countries. These are tyipcally combined together with "Merge" and then you can find "slivers" or areas of overlap between the polygons and "gaps" or areas where there spaces between the features.

 

These sliver polygons can cause problems both with analysis and with creating maps. You can prevent these from occuring by setting an "X,Y tolerance" on your overlay operations.

• Remedies
  • Set an X,Y tolerance when you perform your overlay operation.
    • X,Y tolerance = a definition for the minimum tolerated distance between vertices.
    • Forces nodes or lines to be coincident if they are within the specified X,Y tolerance.
  • Manually go through and remove sliver polygons.
  • Dissolve
  • The "Eliminate" geoprocessing tool*
    • Merges selected polygons with neighboring polygons with the largest shared border or area.

Also, see managing topology in future GIS classes.

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