[GIS] How to combine data from overlapping polygons

arcgis-desktopoverlapping-featurespolygon

Here's what I'm working with:

  • Shapefile of the current 32 Congressional districts for all states
  • Shapefile of the tract boundaries for Texas
  • A DBF format table of Hispanic counts and proportions for Texas census tracts (joined to the tract boundaries)

Using this data set, I'm trying to determine the following based upon the centers of the census tracts:

  1. Total Hispanic population for each Texas Congressional district
  2. Hispanic proportion of total population for each Texas Congressional district
  3. Count of majority Hispanic census tracts for each Texas Congressional district
  4. The shape index value for Texas Congressional district

I'm also trying to create a map applying shading symbology to display the number of majority Hispanic census tracts for each Texas Congressional district.

My problem arises when I try to combine the attribute data from the census tracts with the congressional districts. I need to combine all of the tracts within individual congressional districts and sum up their Hispanic populations. Dissolve isn't working, because the congressional districts and census tracts don’t share any common attributes. When I join the shape files spatially, most of the attribute data gets lost.

Best Answer

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.

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