Mapping the Historic Landscape
Using a stereoscope to view a pair of vertical photographs
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The aim of the modern aerial archaeologist is to create a map of the prehistoric and historic landscape by mapping all archaeological features visible on aerial photographs. So how do we do it?
All available photographs, both oblique and vertical, are examined and any archaeological features are identified. We would normally use a stereoscope to do this. Pairs of photographs are taken with an overlap in which a point on the ground is visible on both images, each taken a small distance apart. This is true of all vertical photographs and on some oblique images as well. When the pair of photographs is looked at stereoscopically, a three-dimensional image can be seen. In a stereoscopic view the landscape appears in exaggerated relief which makes it easier to see slight earthwork features.
Once an archaeological site has been identified, the photograph is
scanned and then digitally manipulated in order to remove all the
distortions due to height and camera angle. This process is known as
rectification or transformation and requires a number of control points to
be chosen from the aerial photograph (for example corners of fields or
buildings) each of which need to be marked on the corresponding map of the
area.
These control points are then used to transform the image into a corrected
plan view using specialist computer software. The archaeological features
can then be digitally plotted from the rectified images in the computer
using a drawing package (we use AutoCAD). As each successive photograph is
scanned, rectified and plotted, so a map of the historic and prehistoric
landscape gradually emerges.
