[GIS] Clipping Census tract shapefiles to boundaries of City Council shapefiles using ArcGIS Desktop

arcgis-desktopcensuskml

I am currently working on a project where I am using Census tract data to figure out the population characteristics of the city council districts of several major US cities. For the cities that I have completed so far, I have been simply uploading the KML files for both the city council districts and the corresponding Census tracts (clipped to the city boundaries when I can find them) to Google Maps. I use Google Earth when the files exceed the 5MB limit on Google Maps. Then, I go through the tracts one by one and in a spreadsheet I record which city council district(s) they are in.

As you can guess, it takes an excruciatingly long time to do this manually.

I was wondering if there was a way to run some sort of script in ArcGIS Desktop that could do this automatically, recording the percent of the area of each Census tract contained within each district?

Best Answer

If you have access to ArcGIS, you could use Select By Location to select census tracts that are within the city council districts. You will need ArcGIS friendly files for this (ArcGIS is not super KML-friendly, though you can convert from KML with this tool). Census Tracts are available on the the U.S. Census website. City boundary files should not be difficult to find.

You can export selected features as a dBASE table (.dbf), which can be opened in Excel and saved as a .csv or .xslx file. In the attribute table in ArcGIS, use the drop-down in the top left and 'Export...'

For calculating the percentage of one polygon (i.e. census tract) within another polygon (i.e. district boundary), look at the overlay analysis tools in ArcGIS. If I understand your question correctly, you might only need to use the Tabulate Intersection tool.

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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.

[GIS] Python problems with select by attributes and user input

The Select Layer by Attribute tool applies a selection to a layer. However, your Copy Features tool is copying the entire "plano_tract" dataset, and not the "plano_tract_lyr" layer which has the selection applied. Here's the revised code:

#user selects tract, Select by Attribute tool runs
arcpy.MakeFeatureLayer_management("plano_tract", "plano_tract_lyr")

arcpy.SelectLayerByAttribute_management("plano_tract_lyr", "NEW_SELECTION", "\"Tract_Number\" = "+ tract)

arcpy.CopyFeatures_management("plano_tract_lyr", "plano_tract_feat")

To zoom to selected features and export to JPEG:

#Current map document
mxd = arcpy.mapping.MapDocument("CURRENT")

#Assumes only one data frame
arcpy.mapping.ListDataFrames(mxd)[0].zoomToSelectedFeatures()

#Export 
arcpy.mapping.ExportToJPEG(mxd, "Output.jpg")

Best Answer

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[GIS] How to combine data from overlapping polygons

[GIS] Python problems with select by attributes and user input

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