The ridge of Offa's Dyke ©
Picture taking
Before going further, it is important to understand that there are two basic types of aerial photograph used by aerial archaeologists; verticals and obliques.
A vertical aerial photograph offers a plan view and is captured by a specially calibrated camera pointing straight down at the ground.
Vertical sorties are flown at predetermined altitude and speed in order to achieve a specific nominal scale (eg 1:10,000); the photographs are taken at set intervals to ensure that each frame overlaps the next by 60 per cent. The overlap ensures that each part of the ground appears on three frames (more about this later).
Verticals are rarely taken specifically for archaeological reasons, but are nonetheless a rich source of information. Most parts of the country have been photographed several times since the 1940s, so verticals offer chronological depth, often capturing changes in the landscape that are not reflected in contemporary mapping.
'...all you need is a good camera with shutter-speed priority to counter the vibration of the aircraft.'
A Roman villa:
the light lines reveal buildings and walls ©
Oblique photographs are quite simply all those that are not verticals. To take obliques does not
require any special equipment; all you need is a good camera, with shutter-speed priority to counter
the vibration of the aircraft, and high quality lenses.
A head for heights and a strong stomach are also handy.
Digital cameras are not yet standard kit, but with rapidly improving technology and lowering prices,
it will not be long before they are.
Oblique aerial photographs usually offer a more familiar view than the straight down perspective of
a vertical, but the scale changes continuously from foreground to background, making measurement from the image difficult.
