Aerial photography
Introduction
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. Usually pictures are taken
obliquely, that is at an angle, as this way shadows are easier to pick up.
This is a photo taken obliquely, at an angle
Whiteleaf Cross
This photo is taken vertically, straight down
West Wycombe
Mostly archaeological aerial photographs are taken in black-and-white, which
heightens any contrasts. There are four different types of archaeological
feature that can be seen from the air.
1) Earthworks
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 or in the evening
when the sun is setting. At these times of day the shadows are longest, so
even slight rises can be detected.
This shows two barrows. They are visible because
they create a shadow. Thornborough.
2) Crop-marks
When a field is in 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 cropmarks. In the early part of
the growing season the crop
growing over buried ditches
grows and ripens quicker, 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 and ripens slower, 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.
3) Parch-marks
You can sometimes see the remains of walls, roads and other hard surfaces
in parch-marks in grass, if the summer has been hot 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 at the end of the summer.
Parch-marks of Luffield Priory at Silverstone
4) Soil-marks
If you fly over a field between harvest and planting 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 windblown material from elsewhere
and 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.
Soilmarks of a circular enclosure, several rectangular enclosures and pits etc…
Parsonage Down, Wiltshire
What to look for
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.
Have a look at the English Heritage website and their aerial photography
section for more information.
A common error is to mistake an “envelope” arrangement in a field as an
archaeological feature, when it is really down to agricultural techniques. Find a
picture with the envelope feature on and put it here. Agricultural marks can
also make it difficult to see what’s underneath. Can you see a rectangular
enclosure on this photograph of a field outside Water Stratford? It is masked
by the straight lines the tractors make when they fertilise and spray pesticides
on the fields.
Now you know what to look for on aerial photographs, here are a few for you
to investigate. You get one mark for each correct answer.
Quarrendon
Is this an oblique or a vertical aerial photograph?
What kind of archaeological feature/s can you see (circle one)?
a) Earthwork b) Crop-mark c) Soil-mark d) Parch-mark
What do you think these features are the remains of?
Cheddington
Is this an oblique or a vertical aerial photograph?
What kind of feature/s can you see?
b) Earthwork b) Crop-mark c) Soil-mark d) Parch-mark
What do you think the features are remains of?
Can you see the modern features?
Haddenham
Is this an oblique or a vertical aerial photograph?
What kind of feature/s can you see?
c) Earthwork b) Crop-mark c) Soil-mark d) Parch-mark
What do you think the features are remains of?
Can you tell which features are made by natural processes?
Ludgershall
Is this an oblique or a vertical aerial photograph?
What kind of feature/s can you see?
d) Earthwork b) Crop-mark c) Soil-mark d) Parch-mark
What do you think the features are remains of?
Chearsley
Is this an oblique or a vertical aerial photograph?
What kind of feature/s can you see?
e) Earthwork b) Crop-mark c) Soil-mark d) Parch-mark
What do you think the features are remains of?
Can you see the modern features?
Once you have completed this exercise your teacher will give you a
photocopy or a printout of an aerial photograph. You must decide what type of
feature you can see and also decide what it might have been. Once you have
done that you have to work out where the feature is on a map and draw it on.
To do this you need a good picture with several field boundaries on so that
you can match up the field boundaries to the map. Put stuff in here about how
to plot from an aerial photograph.