Aerial photographs are an extremely useful asset for fieldwork. Obviously photos that have been taken for archaeological reasons are the most useful, but other aerial photos can provide important information.
The series of photographs taken by the RAF just after the Second World War are especially valuable, as they often show areas before modern development had taken place.
There are a number of websites which offer access to aerial photographs. You can find a list of the most useful ones here…
Google Maps and Bing Maps also provide useful satellite photography of surprising detail. Even more useful is Google Earth, which is freely available to download, as it has a ‘history’ feature which allows photos taken in different years, and under different conditions, to be viewed for a given area.
There is a very useful, easy to use free program which allows you to superimpose an aerial photograph onto a map (you can find details of that here…)
There is even a certain amount of LIDAR data available on the internet.