##California Counties in TopoJSON Open sourcing this mostly because I couldn't find a good simple TopoJSON file of California counties suitable for mapping with D3.

caCountiesTopoSimple.json is your best bet. It's smaller and its lines are smoother than ca-counties-topo.json which is the same data before simplification.

Here's how the map looks with a Mercator projection and some styling and a tooltip. Live version here.

##How It's Made I made this using Census TIGER files and converted to geoJSON with ogr2ogr. Then converted to TopoJSON with Mike Bostock's topojson command line tool.

Bostock shows how it's done.

Also included in the topojson is some random stuff that the census included which I decided to leave in. areaLand and areaWater properties should give the respective land and water areas in square meters (if I'm reading this correctly).

With so few subunits in this file, it doesn't really add to the filesize.

##Projection I found a nice "projected" TopoJSON from Bostock's examples, which means you can set the projection to null.

But I wanted to get a pure TopoJSON for future-proofing purposes. I couldn't actually figure out the Albers projection without using AlbersUSA and transforming to find California (seemed like a copout!) so I just used Mercator:

var svg = d3.select("#map").append("svg")
    .attr("width", width)
    .attr("height", height);