M is for Merging ("Dissolving") Several Geographic Shapes (Shallow Thoughts)

Akkana's Musings on Open Source Computing and Technology, Science, and Nature.

Fri, 29 May 2020

M is for Merging ("Dissolving") Several Geographic Shapes

[San Juan County Council Districts]
San Juan County Council districts
[San Juan County voting precincts]
San Juan County Council voting precincts

For this year's LWV NM Voter Guides at VOTE411.org, I've been doing a lot of GIS fiddling, since the system needs to know the voting districts for each race.

You would think it would be easy to find GIS for voting districts — surely that's public information? — but counties and the state are remarkably resistant to giving out any sort of data (they're happy to give you a PDF or a JPG), so finding the district data takes a lot of searching.

Often, when we finally manage to get GIS info, it isn't for what we want. For instance, for San Juan County, there's a file that claims to be County Commission districts (which would look like the image above left), but the shapes in the file are actually voting precincts (above right). A district is made up of multiple precincts; in San Juan, there are 77 precincts making up five districts.

In a case like that, you need some way of combining several shapes (a bunch of precincts) into one (a district).

GIS "Dissolving"

It turns out that the process of coalescing lots of small shapes into a smaller number of larger shapes is unintuitively called "dissolving".

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[ 17:43 May 29, 2020    More mapping | permalink to this entry | comments ]