Marching Camps

[AureliusFalco]

I was reading a"The Roman Empire" by Colin Wells. In particular I was reading about Septimius Severus' marching camps in southern Scotland. He says that through aerial photography, camps have been identified up the east coast of Scotland almost to the Moray Firth. Now I know that camps are visible from the air, but is there really a way to identify from what time period they come from? Can you tell from the air that a camp is Severus' Campaign and not, say, Agricola's? I'm not trying to be funny, I'm just curious if there are any tell-tale signs.
Thanks!!