Interpreting aerial photographs
Taking pictures from the air rather than from the ground gives several advantages. It allows you to see a large area at a time and therefore pick up patterns that may not be so obvious from the ground, like patterns of fields, for instance. It also allows a much wider area to be covered in one afternoon than would be possible travelling by road or footpath.
Most archaeological aerial photographs are taken in black-and-white, which heightens any contrasts. Usually pictures are taken obliquely, that is at an angle, as this way shadows are easier to pick up. There are four different types of archaeological feature that can be seen from the air.
The kinds of features that are definitely man-made have very definite edges and appear to form geometric shapes. Sometimes it is difficult to know when something has been made by human action in the past and when something is due to modern agriculture or even caused by natural and geological events, such as silted up river channels or frost cracking of the underlying bedrock.
One error, for instance, is to mistake an “envelope” arrangement in a field as an archaeological feature, when it is really down to modern agricultural techniques. Modern agricultural marks can make it difficult to see what’s underneath. Have a look at the English Heritage website's aerial photography section for more information.
What may look like a confused jumble of humps and bumps in grassland may make more sense when seen from above. The best time to see earthworks, especially low ones that have been or are in the process of being eroded flat (for instance by ploughing) is either early in the morning, in the evening or in the winter when the sun is low. At these times of day the shadows are longest, so even slight rises and dips can be detected.
When a field is under crop (like wheat or barley), any archaeological features may have already been ploughed flat and so would not be visible, even when the crop has been harvested. What you can look for are crop-marks. In the early part of the growing season the crop growing over buried ditches grows taller and ripens more slowly, because the soil in ditches is looser and holds more moisture. On the other hand, if the crop is over buried walls or roads the crop grows stunted and ripens quicker, so the crop over walls and roads is shorter and yellower than surrounding plants. You can see crop-marks in the morning or evening, similarly to earthworks.
You can sometimes see the remains of walls, roads and other hard surfaces in parch-marks in grass, if the summer has been hot or dry enough. Just like with crop-marks, buried stone blocks grass roots and they can’t get enough moisture to survive. You have to look for these during a prolonged dry period.
If you fly over a field between harvest and planting, in the late autumn and winter, when there is no crop you may be able to see soil-marks. Buried ditches not only tend to have looser soil, but it is often a different colour. This is because if ditches are left open for many years they eventually silt up with material which is not the same soil that has been dug out to construct the ditches. Sometimes the remains of ploughed out mounds or banks can also be detected, especially if the underlying rock is chalk. You will see a spread, a line or a ring of white in the dark soil.
Where in the picture is the archaeological feature?
Is the feature a soil-mark or a crop-mark?
What modern agricultural patterns obscure the features?
Is the feature an earthwork or a crop-mark?
What do you think the features show?
Are the features in any imminent danger of being damaged?
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