[GIS] Find and list all polygons that overlap with another polygon

analysisoverlapping-featurespolygonqgis

I am working with two polygon layers which overlap but are unrelated (one is congressional district boundaries, the other is MPO area boundaries).

I would like to generate a list of all the polygons in one layer which overlap with each polygon in the second layer. I.e. I want to generate a list of all the Congressional districts which are at least partially within each MPO area.

Which vector tools or plugins can I use to get there?

I'm using QGIS 1.8 on OSX 10.6.

Best Answer

You can use the intersect tool in the Vector menu under Geoprocessing Tools. You select the two layers you are interested and an output location. This will create a new shapefile with the attributes of the overlapping features. In the attributes you should then have the MPO and Congressional District numbers.

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[GIS] How to combine data from overlapping polygons

To answer your question 3, reverse the spatial join - select the tracts and join with districts. This will give you a table with a row per tract with an attribute for the district. You can then add a field that computes 1 or 0 depending on whether the tract is majority Hispanic. You can then use an Excel pivot table to get your sums.

The remaining problem (at least using the ESRI data I have) is that the tract and districts are drawn at different precisions, so tracts that really fall completely within one district may appear to lie in more than one. I haven't found a way to do the join to select the district that is the dominant one. It appears you've already converted your tracts to points (centroids?). Just do the spatial join using the tract centroids with the districts and you will avoid the overlap issue. Since very few districts actually overlap multiple districts (and since you already ignored this concern in answering questions 1 and 2), this join plus a new field plus Excel pivot tables will answer question 3.

I have no idea what shape index is (and Google isn't helping me learn what it is), I have no suggestions there.

Shapely – Quickly Find All Polygons That Overlap with Two or More Polygons in Python

The solution ended up being to change the if-statement to:

   if 1:
      for k,item in enumerate(result):
         print(len(result),k)
         touch=tree.query(item.centroid)
         inside=[]
         for x in touch:
            if x.contains(item.centroid):
               inside.append(item)
         if len(inside)>=2:
            final.append(item)

The reason for the original error was that the tree.query(<>) only returns the polygons that are likely to interact with the query <> object based on the bounds (top, bottom, left, right) of the objects inside of the tree. The solution was to check whether or not the centroid of the polygon was contained within each member of the list generated by the query. In some cases, the objects returned by the query were in the vicinity of the query object but did not in fact contain it.

Best Answer

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[GIS] How to combine data from overlapping polygons

Shapely – Quickly Find All Polygons That Overlap with Two or More Polygons in Python

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