TopoJSON files are handy for creating customizable maps in web browsers. Feel free to grab ca_map.json to use as a base for your own projects.
Especially well suited for building data visualizations with D3!
Total credit to the awesome tutorial published by @mbostock, which explains how to create a map of the UK for free with open source tools. Here's a summary of how I followed these steps to prepare a similar map for California.
For those of you who are new to this stuff, you can find snapshots of what the files look like after each step of the process here.
You'll need to install two command line tools before you can transform the files. Super explanation (again from @mbostock) here.
To get the data for California, we'll need to start with the file containing the counties for all 50 states. It comes as a ZIP folder that can be downloaded here.
From the command line, navigate to the directory that contains the 5 files downloaded from census.gov. Type this code to filter the non-California states and create your California geoJSON:
ogr2ogr -f GeoJSON -where "STATEFP = '06'" ca_afterOgr2Ogr.json tl_2013_us_county.shp
more info on this step coming soon!
more into on this to come as well..
need to add notes on some tools for rendering topojson, ie github! and tutorial links... such as bostock posts