Joining features across boundary lines of target feature in spatial join?

Question

I have a file geodatabase named ElectionData2012.gdb containing two feature classes:

1. Counties: The feature data for California counties

2. PrecinctVotingData: Feature data for California precincts for the 2012 election. Has two relevant attributes:

   a. total_votes: a field containing the total number of individuals who voted in the precinct

   b. proposition_37_yes_votes: a field containing the number of individuals that voted for the measure in the precinct

I want to conduct my analysis and aggregate the precinct total votes and yes vote data up to the county level. To complete this I used spatial join tool:

• target feature: county
• join feature: PrecinctVotingData

And for the merge rule I used sum for the total and the yes votes but the total votes in the original shapefile and the spatial join layer are not the same cause some of the precinct lines cross the county lines.

How can I clean this up if I want to get the total value from all of the precincts attached to the counties correct.

I use ArcGIS 10.3

Answer 1

I would not use Spatial Join, instead:

1. Make Feature Layer on Precincts, set split policy on the two vote Count fields.
2. Intersect the feature layer with Counties. The split policy will split the vote count based on change in area.
3. Dissolve by County name/id with SUM on your vote Count fields as statistics field

Answer 2

Minimizing the erroneous polygons might help you achieve this. Try converting your PrecinctVotingData to points before performing the spatial join with the summary function applied.