Archaeological sites are increasingly threatened with destruction. The main threat is the erosion, accelerated in areas with intensive agriculture, that is removing the archaeological layers centimeter by centimeter. There is also a growing destruction of archaeological sites due to the exploitation of our resources, like gravel and sand mining, and due to the construction of railways, roads or big industrial areas. Time and again this results in the complete destruction of archaeological sites. We can assume that a quite high percentage of them was still unknown, leaving irretrievable holes in the archaeological landscape. Therefore, we have to perform what may be called "preventive prospecting", which means, that we have to try to detect and document sites before their destruction.
Data acquisition is therefore probably the most important part of aerial archaeology. To be able to provide efficient aerial archaeological support, you should have available a collection of photographs covering (at least) your working area. The more photographs, the better, especially, if they were taken throughout several years and at different seasons.
There are two kind of photographs, that can be useful for aerial archaeology: vertical and oblique photographs. As regards aerial archaeology, both kinds have their advantages and drawbacks.