Wednesday, December 15, 2010

Updated TIGER map

For a full history of this post you may want to read these three posts first: 12 and 3.

When I first presented my nationwide TIGER map at wherecamp5280, Samat mentioned that he was surprised that his county wasn't more heavily edited since he has done a lot of work there. A few days ago we were chatting on IRC (#osm on and he asked a question that made me realize that the map did indeed have a serious flaw in it. Earlier today a discussion came up on the talk-us mailing list that made me notice that the same error exists in the TIGER edited map that MapQuest (re)set up. So I decided to look at correcting it.

The error is that I was only looking at the latest version of the ways. This means that any edits between the original import and the mass edit that expanded street names were not taken into account. Both the import and the abbreviation expansion happened well before I started mapping so I didn't even consider this fact. As Antony Pegg says later in the mailing list thread, it is too expensive to go back and look at all previous versions. Especially since you would have to use a FULL planet file since the regular ones only contain the current version of objects, not a full history.

But we don't really care about the contents of the edits or who performed them... we just care that they happened. The initial import obviously created version 1 of the ways and if nothing else changed, then version 2 was created by balrog-kun in the name expansion edit. If any edits happened between the import and the name edit, then the ways will be on version 3 or higher. Thus any TIGER way with a version higher than 2 must have been edited by someone other than these users, even if one of them was the last to touch a way.

So here is the updated map which takes this into consideration: