Vector fields of population travel demand

I just finished reading an interesting paper by Xintao Liu and colleagues entitled Time-geographic relationships between vector fields of activity patterns and transport systems, in the Journal of Transport Geography.

I don't really have much to say about it, but I'll post a link to it anyway so I don't forget about it: I've briefly explored this technique in the past, and hope to revisit it in the future. It's pretty damn hard.

Do you know of any similar examples?