Boundary effects redux

[dwtkns]

Just resurfacing #13 - it looks like this is still happening in several metro areas. So far I've seen the tile edge artifacts in Denver, San Francisco, Los Angeles, Houston, and New York.

I pasted some snapshots on the previous issue.

[jharpster]

Thanks @dwtkns. If you could post some bounding boxes or approximate X,Y's that would help us in another iteration of the model.