The majority of obliques taken by or for archaeologists are targeted on features thought to be archaeological. By circling each site a photographer is able to record it from the best
viewpoint and produce a clear photograph of its visible features.
It is difficult to improve on a good oblique record of a site as it will generally offer a closer view than a vertical. If obliques have been taken with interpretation and mapping in mind – that is, as stereo pairs from as near vertical as possible and with ample and well-spaced control points – they can be a pleasure to work with.
Obliques are excellent for recording individual sites or for photographing targets on a field-by-field basis. They are less
able to cope with continuous linear features and they are a cumbersome way of recording, for instance, a wide-spreading system of centuriation or the open fields of a medieval
settlement. If the density of individual sites is high or if there are
‘continuous’ archaeological features, it is easy for even an experienced air photographer to ‘lose’ some parts during the repeated circling to take the photographs (COWLEY 2002). In such cases verticals taken at an appropriate time may provide a more complete record.
One advantage that verticals have over obliques is that they record all of the ground within the survey area. If they have been taken at times of year when crops, bare soil or lighting are
appropriate they can thus offer more information for
interpretation than the obliques of inevitably more limited areas taken by an archaeological photographer. For example, in
Austria on a survey flight to take oblique air photograps it was apparent that crop conditions were exceptional and the number of sites was too high to be sure of recording them all using
conventional oblique tactics. The solution was to commission a vertical survey of a block of land covering 160 sq km at a scale of 1:10,000 to provide photographs that were later examined stereoscopically. It was recognised that lower-altitude
archaeological survey might have detected more sites, as they appeared and then faded over a period of weeks, but this was not possible, the only qualified archaeological surveyor being fully occupied at that time with other work (DONEUS 2000).
On verticals it is often possible to follow linear ditches from field to field as colour or tonal differences. Occasionally these can be extended because they are visible as height differences in the crop when viewed stereoscopically. In such cases there may have been nothing for an oblique photographer to observe (or believe in) unless the sun was low enough to produce a shadow of the taller crop (Fig 2.9, bottom right). The use of verticals allows an interpreter to more closely examine and question features than is usually possible from the air within the
constraints of flying costs. A disadvantage of using verticals, especially in the hands of an inexperienced interpreter, is the ease with which the mind can ‘invent’ features which are not really there. This is especially so with prints which record only a few genuine features and the interpreter may become desperate to find at least something. As Irwin Scollar noted: ‘Prolonged examination … usually leads to unwarranted fantasy which often cannot be confirmed by later excavation’ (SCOLLAR et alii1990, p.
27).
In general, a photo interpreter will not be concerned whether the principal source of information is an oblique or a vertical
photograph. More important will be the spread of control points and the availability of stereo pairs. Magnification can be used as necessary to reveal archaeological and other features and
specialist transformation software can handle either type of photograph with equal ease.
Stereoscopy
A single photograph, be it vertical or oblique, provides only a two-dimensional view of reality. This may sometimes be
enhanced by the lighting or by viewing verticals with shadows falling towards the observer. But this does not provide the detailed – and slightly exaggerated – view that comes from three-dimensional viewing of stereoscopic pairs of photographs.
A person’s eyes, spaced some 65mm apart, allow depth and distance to be perceived because a view is seen from two slightly different positions. Stereoscopic pairs of photographs provide similar information by showing the same scene from two different points so that it may be recreated in three dimensions by use of a stereoscope. In it simplest form a stereoscope comprises two lenses in a frame that holds them at a suitable height for focusing on the pair of photographs placed below them. The lenses direct each eye to see one photograph and, if these are correctly placed, the mind combines the two images to create a three-dimensional view. These so-called ‘pocket’
stereoscopes, are favoured by most archaeological interpreters in Britain (Fig 7.8, top). They are available with lenses that give either 2x or 4x magnification. Both versions are small and easy to use, and give the viewer a sense of contact with the
photographed evidence. The 2x magnification model has legs that are high enough for the interpreter to use a pen to mark interpreted information on a transparent plastic overlay placed over the photographs, as discussed in Chapter 8. The 4x
models have shorter legs, making it more difficult to annotate the overlay.
Larger ‘mirror’ stereoscopes (Fig 7.8, bottom) direct light from photographs to the inerpreter’s eyes using mirrors and prisms.
Many can be used with a wide range of supplementary lenses and there is at least one model that has an excellent zoom lens.
Different magnifications have different uses and most archaeological interpretation of conventional photographs (obliques and verticals with contact scales up to 1:12,000) will not need to use more than 4x magnification. At 1x magnification it may be possible to see a complete stereoscopic view of a photo pair. This can be superb for examining topography on small-scale prints (1:20,000 and smaller) and can be used for a first-stage examination of larger-scale prints (1:12,000 and
greater). Use of 2x or 4x magnification, however, will be required for the interpretation of detail.
For an archaeological interpreter stereoscopic examination of prints should become the normal way of working. Viewing
stereoscopically aids perception of very slight features and may give an interpreter the confidence to accept them as
archaeological. Stereoscopic examination is essential for any work on upland areas where earthworks or upstanding walls of only a few centimetres in height may be perceived and mapped by an experienced interpreter. Stereoscopic perception
increases with a viewer’s experience. This is both long term –
Top. A pair of vertical photographs being examined in three dimensions using a pocket stereoscope that allows 2x magnification. Note how the top print is being curled up so that the area beneath it on the lower photograph can be seen. The interpretation is being drawn on a transparent overlay taped to the right-hand print (as viewed in this photograph).
Bottom. A pair of vertical photographs being examined in three dimensions under a mirror stereoscope that allows 1x or 4x magnification.
The prints do not need to physically overlap one another and there is plenty of room in which to use a pen to mark interpretation overlays. This instrument can be used to ‘scan’ the photographs by using controls which move prisms in x or y direction. Other stereoscopes do this by using a movable baseboard.
Fig 7. 8 Use of a pocket and mirror stereoscopes
This split view of an oblique stereo-pair, taken about a second apart from an orbiting aircraft, is here displayed for viewing in three dimensions with a pocket stereoscope with a magnification of x2, as in Fig 7.8.
On the light-toned patch can be seen at least one rectangular enclosure and many probable pits. Most of these, and the enclosure, are visible only as height differences in the crop – enhanced by the stereo view which also shows that the lighter area stands on slightly higher ground of the kind usually selected as a location for past settlement. Impressions of height in the photographs are helped by the shadows falling towards the viewer, allowing changes in the height of crops above the
enclosure ditches to be seen as highlights and shadows.
In overcast conditions or with a higher or different angle of sunlight such features might only be seen through stereo viewing.
Fig 7. 9 Advantages of stereoscopic photographs
over a period of years – and in the short term, when the eye-and-brain may need several minutes to become adjusted to the view through the lenses.
A stereoscopic view can sometimes help distinguish natural from archaeological features because their relationships to the
topography will be clearly seen. A small dry stream, for example, may have a deeper or wetter channel that affects crop growth in similar ways to an archaeological ditch and may be
indistinguishable from examination of a single print. Stereoscopic viewing usually shows the stream to be in an eroded hollow and so it can be correctly identified.