make a map.md

First, install svgis and breaks. They require some hefty prerequisites. sorry. The last step requires ImageMagick.

brew install gdal numpy scipy # os x
apt-get install python-numpy python-scipy gdal-bin # linux
pip install svgis breaks

Download get-tiger. Optional: edit it to include the Census data fields you desire.

# this will download a counties SHP file and add in some default data fields
make COUNTY YEAR=2014

We can find some metadata what we just downloaded in get-tiger/data.json. Here's something cool: B25035 MEDIAN YEAR STRUCTURE BUILT.

Let's make a chloropleth of that!

# Create a new SHP file with data bins for Year Built
breaks 2014/county/tl_2014_us_county.shp B25035_001 county_yearbuilt.shp

OK, great, now let's turn that into an SVG.

First we'll need some CSS. Let's turn to colorbrewer and grab a scheme with five classes:

# save as style.css
.bin_0{fill:rgb(255,255,212)}
.bin_1{fill:rgb(254,217,142)}
.bin_2{fill:rgb(254,153,41)}
.bin_3{fill:rgb(217,95,14)}
.bin_4{fill:rgb(153,52,4)}
.bin_None{fill:rgb(225,225,225)}

(The None value will show up if any of the rows have missing data)

And let's pick a map projection. This one looks good: USA Contiguous Albers Equal Area Conic

Now draw a map!

svgis draw county_yearbuilt.shp --style style.css --crs epsg:102003 --scale 8000 --bounds -125 24 -66 50 -o county_yearbuilt.svg

Taking that step-by-step:

--style style.css # use the CSS file with the bins
--crs epsg:102003 # project in USA Contiguous Albers Equal Area Conic
--scale 8000 # 8 KM to one svg unit. An svg unit might be represented as a single pixel.
--bounds -125 24 -66 50 # The bounds for our map, minx miny maxy maxy (in other words West, South, East, North)
-o county_yearbuilt.svg # save the file

Now you can open up the svg in an editor and play with it!

Or, convert to a png:

convert -density 1800 county_yearbuilt.svg -trim +repage county_yearbuilt.png