NC Census Map

MY goal in making this map of North Carolina was to gain more familiarity with the Census Reporter API, and start to dig down past the county level, to the zip code, area code, and census tract levels.

Starting Point

State and County Data

I started out with some code I wrote for my Census Map, which grabbed California county information, except this time I wrote a code that would turn state names and abbreviations into FIPS codes (here: http://www.census.gov/geo/reference/ansi_statetables.html), necessary to obtain geographic information from the Census Bureau or from Census Reporter, whose lovely API I was using.

Further Sub-Levels

Via the Census Bureau's own technical documentation, http://www.census.gov/prod/cen2010/doc/sf1.pdf (it is very extensive), we can obtain the following geographical entity hierarchy:

Next, I headed over to the Census Reporter API documentation: https://github.com/censusreporter/census-api/blob/master/API.md

This didn't say anything specific about geographies below the county level, so I assumed they were there, I just needed to request them correctly.

Sure enough, I grabbed the geoid for Dare County, NC, which was 05000US37055.

I then fed that geoid to the Census Reporter API, and it gave me information about the county:

$ curl "http://api.censusreporter.org/1.0/geo/tiger2013/05000US37055"
{"geometry": null, "type": "Feature", "properties": {"awater": 3054395108, "display_name": "Dare County, NC", "simple_name": "Dare County", "sumlevel": "050", "population": 34289, "full_geoid": "05000US37055", "aland": 992627006}}

and geometry information:

$ curl "http://api.censusreporter.org/1.0/geo/tiger2013/05000US37055?geom=True"
{"geometry": {"type": "MultiPolygon", "coordinates": [[[[-75.76384399999999, 35.233886], [-75.868684, 35.579451999999996], [-75.879179, 35.604172], [-75.87683299999999, 35.628116999999996], [-75.88395299999999, 35.632784], [-75.891953, 35.643328], [-75.890496, 35.652259], [-75.89834499999999, 35.661057], [-75.901164, 35.668997999999995], [-76.027445, 35.669094], [-76.03084299999999, 35.663227], [-76.035266, 35.662022], [-76.045987, 35.666117], [-76.029265, 35.682038999999996], [-76.022328, 35.709224999999996], [-76.025088, 35.721156], [-76.01927599999999, 35.752763], [-76.02239, 35.81066], [-76.01769399999999, 35.865356], [-76.00917799999999, 35.894453], [-75.987466, 35.916453], [-75.978991, 35.964459999999995], [-75.963734, 35.984291999999996], [-75.948736, 35.994425], [-75.78040899999999, 36.047422999999995], [-75.767448, 36.055639], [-75.763783, 36.062792], [-75.763192, 36.074594], [-75.77425, 36.103245], [-75.776439, 36.135726999999996], [-75.77865899999999, 36.142323999999995], [-75.789363, 36.154436], [-75.789075, 36.172205999999996], [-75.80169599999999, 36.187985], [-75.79943, 36.202217999999995], [-75.813189, 36.223544], [-75.70907799999999, 36.244983999999995], [-75.666246, 36.148924], [-75.566047, 35.975614], [-75.48050099999999, 35.814206999999996], [-75.46623699999999, 35.801314999999995], [-75.460815, 35.784524], [-75.437617, 35.74696], [-75.41781999999999, 35.686082], [-75.406815, 35.636972], [-75.40039, 35.593512], [-75.400849, 35.558718], [-75.402422, 35.54421], [-75.415089, 35.50018], [-75.426135, 35.386922999999996], [-75.447751, 35.309794], [-75.459334, 35.246552], [-75.468477, 35.21519], [-75.484571, 35.187577], [-75.505059, 35.174426], [-75.53217599999999, 35.167148999999995], [-75.55758399999999, 35.170535], [-75.581108, 35.183783999999996], [-75.59536899999999, 35.182826999999996], [-75.63621499999999, 35.172205], [-75.70737, 35.146111], [-75.73662, 35.140564999999995], [-75.76384399999999, 35.233886]]]]}, "type": "Feature", "properties": {"awater": 3054395108, "display_name": "Dare County, NC", "simple_name": "Dare County", "sumlevel": "050", "population": 34289, "full_geoid": "05000US37055", "aland": 992627006}}

Well, nothing contained in that JSON about sub-levels...

There is code for adding a tile layer to, e.g., a Leaflet map, using this URL:

http://api.censusreporter.org/1.0/geo/tiger2013/tiles/<sumlevel>/<zoom>/<x>/<y>.geojson

But this wouldn't accomplish the goal of getting GeoJSON data falling within a given county - it would get GeoJSON data for a map tile.

The solution is to request information about a particular level of geogrpahic entites, and pass it a geo_id to restrict the search. This can be done using the following API URL:

http://api.censusreporter.org/1.0/geo/show/tiger2013?geo_ids=<geo entity level>|<geo id>

So to find information about census tracts, which were level 140 (different levels and corresponding codes are listed on this page: [1]), for Dare County, North Carolina, which has geoid 05000US37055, we would request the URL:

http://api.censusreporter.org/1.0/geo/show/tiger2013?geo_ids=140|05000US37055

Downloading Census Tract Data for Counties

As mentioned, I used the following API URL to get census tract boundaries for Dare County, North Carolina:

$ curl http://api.censusreporter.org/1.0/geo/show/tiger2013?geo_ids=140|05000US37055

Using curl, from the command line, returns the JSON to stdout. We can capture it in a JSON file by saying:

$ curl http://api.censusreporter.org/1.0/geo/show/tiger2013?geo_ids=140|05000US37055 > dare_county_nc.json

Now we can drop that JSON file as a layer onto our Leaflet map, and voila! we have our census tracts:

Map Plan

My plan for the map is to use Python to loop over a list of particular counties, request all census tracts for those counties, and map them.

Overlay of Census Tracts

There are two ways to overlay census tracts.

Download GeoJson

The first way is to download the GeoJson to a file, and serve it yourself. In this case, your Javascript will look like this:

var geoj = new L.geoJson.ajax("dare_census_tracts.json",{onEachFeature:enhanceLayer}).addTo(map);

Grab GeoJson from URL

The other, more seamless option is to get the Json directly from the API of Census Reporter. You can do this with the following code (requires jQuery):

rooturl = "http://api.censusreporter.org/1.0/geo/show/tiger2013?geo_ids=140|05000US37055";

$.ajax({
    type: "GET",
    url: rooturl,
    success: function (data) {
        var geojson = new L.geoJson(data, {
            style: {
                fillColor: '#CFC',
                fillOpacity: 0.20,
                stroke: true,
                color: '#222',
                weight: 1
            },

            onEachFeature: function(feature, layer){
                popupText = "";
                popupText += feature.properties.name;
            }

        }).addTo(map2);

    }
});