Tag Archives: Geocoding

Wisconsin Voters Banished to NULL Island

The top headline in my local paper this morning was “Glitch puts some Wisconsin voters in Africa” … an interesting thing to ponder over a bowl of Quaker Oatmeal Squares. I suppose this problem merits at least some attention given the heated political climate surrounding the state’s voter redistricting process. But headline news? Above the fold? Sounds like a slow news day to me.

Online, of course, the debate has already devolved into the standard round of mudslinging and name-calling so good luck trying to find out what’s going on from that crowd. The reporters themselves focused on the political fallout of the issue rather than an explanation so no help there either. I guess it’s up to the humble folks at Ideas Illustrated to offer up some insight!

The first clue to the problem can be found in the article’s pullout quote, which describes the voter’s location as the “coast of Africa” and not a specific country in Africa. The second clue can be found deep within the article when it is mentioned that clerks have recently made changes to the way voters are being entered into the voter registration database:

” … voters are [now] being entered into different districts by the physical location of their address in computerized maps. Previously, they were entered into different districts in the state voter database according to where their address fell in certain address ranges.”

These two hints point to a very common problem associated with geocoding, which is the process of converting a postal address to a set of map coordinates. Let’s backtrack. An online mapping tool like Google Maps uses specific geographic coordinates (latitude and longitude) to place a location on a map. However, because none of these physical locations are actually stored in a database anywhere, the tool needs to interpolate the coordinates from a vector database of the road network (i.e. a mathematically represented set of lines).

For example, if you look up the address for Trump Tower, you find that it is located at 725 Fifth Avenue in Midtown Manhattan. When you enter this address into Google Maps, the tool finds 5th Avenue on the underlying road grid and then uses an algorithm to determine that the “725” address is somewhere between 56th and 57th streets. It will also determine which side of the street the address is located based on stored knowledge of the “odd” and “even” numbering pattern. In other words, it’s guessing.

Google Map detail of the area around Trump Tower

 

TIGER/Line® Shapefile detail of the same area

These guesstimates work pretty well in dense urban environments where there are a lot of cross streets to serve as reference points. In rural areas, the curvilinear streets and widely-spaced buildings make things a little more difficult. When the situation gets really muddled, some mapping tools essentially “punt” and enter a default set of coordinates. In the case of the Wisconsin voter addresses, these default coordinates are 0.00 degrees latitude and 0.00 degrees longitude. Where is this exactly? It is the intersection of the Prime Meridian and the Equator … which occurs just off the coast of Africa.

Geographers have actually given this place a rather fanciful name called NULL island (it is not, in fact, a real island). It even has its own web site and unofficial flag (below right).

So there are no nefarious schemes behind this situation … just normal, everyday data problems. The state clerks need to tell their IT guys to flag the errant voter addresses and then they can assign them to the appropriate districts by hand. Problem solved. However, they should be aware that interpolation is an imperfect process and, in addition to assigning blocks of voters to NULL island, the geocoding process may also assign voters to the wrong districts. This could be particularly true for people who live close to a district boundary. It might actually make sense to keep the old method around for backup.