TopoJSON notes, watershed boundaries and HUCs

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Nelson's log

I played around with Mike Bostock’s TopoJSON today, a file format + Javascript library for efficiently encoding maps. It basically does the same thing as geojson, but gets amazing compression in part by identifying common line segments between polygons and only encoding them once (ie: draw the border just once) and in part by quantizing and simplifying all the data. It’s quite clever!

My test data is the Watershed Boundaries from NHDPlusV2; the California-only NHDPlusV21_CA_18_WBDSnapshot_01.7z (89MB) and the national NHDPlusV21_NationalData_WBDSnapshot_Shapefile_01.7z (1.7GB).

Converting

Converting the smaller file to TopoJSON was a snap, just

The flags are to add some GeoJSON properties to the resulting data file.

Simplifying

Taking the default arguments, TopoJSON reduced my 89MB SHP file to a 9.3MB TopoJSON file. Not bad, particularly since the straight GeoJSON is 257MB. A lot of compression thanks to shared boundaries. But also quantization; by default the topojson tool quantizes data to 10,000 values…

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