DARPA projects from more than a decade ago (VSAM/WAMI for arial platforms like Gorgon Stare) used arial imagery to capture ground shadows for gait tracking purposes.

From chatting with some of the researchers many years ago my understanding is that it usually wasn't accurate enough for unique identification and the gait shadow was dependent on shoe type and clothing, so a persistent gait shadow database wouldn't have been useful. But it could be correlated with ground-based surveillance for identification, for example person A and B were identified on a ground-based security camera entering a building, then gait tracking could be used to monitor where they went after they left the building even if they avoided ground-based security cameras after that point.