Overthinking GIS
A roundabout way to downsampling data
Maps in the Modern Era
GIS is probably one of the best things to happen to cartography in the last couple hundred years. I say that with absolutely no knowledge of the history of map making, but GIS is wildly useful and consistent in how it is presented on publicly-accessible sites. I can go to the USGS National Map Viewer and am presented with more data and information than I could possibly ever find useful.
Even more surprising is that county-level GIS websites look similar enough to the national map and other county maps that someone just roughly familiar with GIS and layers can find more granular information about a single parcel of land. Personally I don’t have a use for a color-coded sale year plot, but this county in Georgia provides it. The breadth and depth of information to the general public is incredible.
Custom Metrics
Still though, all of this information is quantifiable because computers don’t speak human languages (LLMs are still just math) and what I wanted is something more human-centric, more qualitative: usability. I’m defining usability as “not too steep to build on” which sounds straightforward until you actually try to come up with a number that defines that. The closest thing to my usability metric is grade and while the USGS map provides elevation and topographic information it does not tell me the grade, at least I couldn’t find that option in the layers. If a GIS wizard out there wants to point me in the direction of a grade map it would make my life a lot easier.
I decided to define usability as the average grade for some area of land. Ok, great, but how do I calculate that with the data on hand? The USGS map provides topographic lines, and past experience reading topo maps has taught me what “too steep” is, but that’s the result of over a decade of hiking and reading maps. How do I tell the computer “when you see topo lines close together it’s probably too steep and has a low usability score”?.
In this section of map the grade on the Western side of the river is too steep to use, but the flat area on the Eastern side is perfect. Topo lines close to each other indicate steep terrain, and lines far apart indicate flatter terrain.
Source Data
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