Done by pointing the camera straight down at the ground with the aircraft flying along gridlines.
- good desktop study source for initial study of landscape
- you can view in 3D by overlapping the photographs and viewing them though a stereoscope
- acurate plans can be drawn of the site (photograsmmetric mapping)
- can miss features
taken from an low flying aircraft with the picture taken at a right angel to the ground..
- the most widely used in archaeology
- good for locating sites and features
- can miss features
in low light either at the beginning or end of the day shadows are at their longest.
areal photographs taken from a low flying aircraft and recorded with a camera pointed into the sun have a distorted perspective which emphisises the shadows .
- best used for illustrating existing sites and locating detail within them
- shaddows are created when crops are at different heights.
- winter is the best time to do this as the sun is low and vegetation which could mask possible sites has died down.
- snowing and flooding can accentuate appearance of hollows and earthworks
The growth rate of crops is related to the amount of moisture that their roots can access. plants with better acess to moisture grow taller and can also turn a different colour than plants around them. if archaeological features are burried under a field it it can either produce a negative or positive cropmark.
E.g. a burried ditch with its infill of humus and topsoil will often hold moisture creating a dark green line in the crop above. This will look like a 'positive' crop mark that is visible from the air.
The opposite happen with plants over a buried wall, the plants will most likely be stunted and produce a yellow negative cropmark.
on soil where there is a marked contrast between the colour of top soil and sub soil evidenc eof ploughed out monuments can occur as soil marks.
e.g. flag fen a Roman Road appeared as a orangey stripe against the black peat soil.
soil marks are sharpest in the winter when vegetation is low.
used to distunguise between the imaging techniques