Most books dealing with the uses of aerial photographs in the natural sciences show a staged progression from photo reading to photo interpretation. They also stress that the latter has to include mapping. This book omits discussion and description of the stages of photo examination but asserts most strongly that photo interpretation must include mapping.
A photo interpreter will be working with collections of material that record disparate fragments of past sites and landscapes.
Mapping allows these fragments to be combined into a single picture (Figs 6.1, 7.1 and 7.3). Without mapping, the best that can be done is to make a detailed description of features seen in the photographs. By making a map we are able to indicate exactly which features we think are archaeological, which natural and which recent, so as to produce a considered interpretation of what are otherwise just marks on a photograph. Mapping a
single site enables features to be located accurately on the
ground. Plans of different sites can be measured to provide data that facilitate their comparison. As well as recording detail of individual sites, aerial photographs also provide a superb
resource for the examination of extensive landscapes from the past, yet this is not in any real sense possible until they have been interpreted and mapped.
Mapping allows the photo interpreter to combine, locate,
measure, and build up information that has often been collected on a number of different flights over a time-span of many years.
It should be obvious from the first part of this book that the occurrence of archaeological features is irregular and
unpredictable. Information on vertical photographs and from satellites will be particularly dependent on the time of year and day when they were taken, even though they have the capability to record everything that was visible at the time of exposure.
Oblique photographs tend to be targeted on single modern fields, or parts of them, in which archaeological features have been identified by the airborne observer. Vast numbers of new discoveries are made each year in Britain – often on each flight – despite more than fifty years of reconnaissance by Cambridge University, various official bodies and local fliers.
With both single sites and wider-spreading landscapes, the preparation of a map that also includes natural features can aid archaeological understanding. The relevant information may be contours, to show the topography, or other features such as watercourses and palaeochannels that may have determined or affected the location of settlements and the uses made of the land. An example can be seen in Fig 7.2 which makes clear that this simple addition contributes significantly to any
archaeological interpretations that we may make.
Photo A shows archaeological features in the central field. There are hints of their continuation in the upper field but parts of the crop there have become beaten down by rain or wind, making any archaeological traces unreadable.
Photo B, taken nine weeks earlier, shows the upper field in exceptional detail. By combining interpreted information from
these and other photographs maps can be produced that depict different levels of detail and have different uses.
Map C is an extract from a survey of 1400 sq km of the low-lying
Cambridgeshire Fenlands in eastern England. Photo interpretation was carried out to produce mapping at 1:10,000 scale as a rapid summary of Fenland archaeology. At this scale sites can be seen in context,
relationships with other features can be identified and analysed, and most archaeological features can be depicted with reasonable clarity and
accuracy. Interest in the Fenlands increased as new aerial information became available and as considerable field evidence was collected for Roman-period settlement and salt-making.
Map D. After the collection of this new evidence further mapping was undertaken at a scale of 1:2500. This focused on the settlement /salt-making sites but also showed some of the peat-cutting areas (shaded) which provided fuel for the salt-making process. The larger scale allows more detail to be shown. The interpretation adds information from photos taken after the 1:10,000 mapping had been completed.
However complete one phase of mapping appears to be, examination of new or different photographs can add detail and confirm or disprove features previously mapped.
Fig 7. 1 Combining information at different map scales
It may be necessary to combine information from two or more photographs so as to complete the plan of a single site. It becomes essential to examine as many photographs as are readily available if the environs of that site, or larger landscape, are to be studied. It is the nature of oblique aerial survey to pick out and photograph those fragments of the past that are visible on the day of photography, and vertical photographs are similarly restricted to recording only what is detectable at the time of their exposure. Therefore a complete archaeological landscape is unlikely ever to be photographed on a single date and the interpreter needs to combine the evidence from a number of flights, often spread over a considerable period of time. This is definitely the case in lowland areas where archaeological
visibility is dependent on crop growth or farming routines, and is likely to be similar in upland areas where shadows and
directional lighting may ‘hide’ upstanding features on steep slopes facing away from the sun. Modern computer software allows a photo mosaic or ortho-photo to be made of all the
relevant pieces, but a more intelligible result will be produced by a map that shows and distinguishes between different features interpreted from the photographs (Fig 6.1).
The recoverable landscapes will vary in different terrain and in different parts of the world. For the cultural past to become
visible on aerial photographs requires there to have been either earth-moving (usually by digging holes or ditches) or structuring of stones or other material (to form, for example, banks, walls
and buildings). Aerial photographs are unlikely to record
evidence of past sites which consisted of small fences, tents, or temporary enclosures, like those made of thorn and other
branches in parts of Africa. Even within hole-digging or stone-moving communities the use of ‘civil engineering’ will vary from place to place. Examples are illustrated in Figs 6.1, 9.1 and 9.8 from two survey projects that mapped and analysed three
different topographical locations: a river valley, a piece of chalk downland and an area of hill country (Palmer 1984; Whimster 1989). These and other examples are discussed in more detail in Chapter 9.
Mapping, and analyses of the resulting maps, are beginning to raise new questions that require a fresh type of field investigation to help provide answers. Landscape study – aided so much by air photo interpretation and mapping – is desperate for
approximate dates that can be provided by small-scale excavation at crucial points or intersections. Unfortunately,
‘minimalist excavation’ of this kind is not currently fashionable in Britain although some useful data have come from projects of field-walking survey.
When compiling maps of past landscapes it is important to try to take account of contemporary features that will not be visible from the air. Crucial among these are areas of past woodland that must have been of considerable importance, and value, in all times up to the relatively recent past but of which there
remains little trace in the landscape of the present day. Air
photographs may record the former boundaries of now-shrunken areas of woodland, though many of these changes may have taken place in relatively recent times. In Britain, some of these features may have Saxon and medieval origins, but earlier woods are almost impossible to identify definitively on the ground or from the air.
Finally, maps showing levelled archaeological features also need to indicate those areas where no information can be, or has
been, recorded from the air. These so-called ‘negative zones’
include roads and railways, built-up areas (beneath which no aerial information will be visible), woodland (which sometimes can mask earthwork features), and bands of deeper soil such as alluvium (which is often too deep for roots to penetrate and
enable crop growth to indicate subsoil differences). Knowledge of these negative zones helps archaeological interpretation, which may otherwise read false significance into apparently
‘empty’ spaces on maps (see for instance Figs 6.1, 9.1 and 9.4).