You'll need a couple of software packages installed in order to run this pipeline: GDAL (for manipulating the geodata), Cura (recommended for rendering GCode), and Blender, for tweaking the 3D model, e.g. to turn it from a surface into a solid.
Also unzip and wget.
To generate a topographical model of San Francisco, run at the command line:
git clone http://github.com/justinmanley/topoSF
cd topoSF
make
You will have to complete the last two steps manually - there are instructions in the Makefile.
Here's what you'll get once you've finished those last two steps:
When it's printed, the model will be about 2 1/2 by 3 1/2 inches. If you want it to be bigger or smaller, just change the parameters passed to phstl on the command line. Here's what the final print looks like:
Have fun!