Aerial photography and ground obscuration (part 3)

I have previously written twice about factors affecting the possibility of discovering archaeological features using aerial photography (1) (2).  The original version of this took into account human factors (see post 1), based upon OS OpenData, specifically buildings, roads, railways, and woodlands / rivers / lakes (I count woodland as a human factor, as many of them are plantations and others have been left / managed by humans, rivers / lakes are obviously less human influenced).  Here is a reworked example, showing the percentage of the ground surface obscured by such factors:

Ground obscuration taking into account woodland and human structures. Relevant to aerial photography generally.

To add to this previous work, a few months back at our Project Advisory Board meeting, Jeremy Taylor of the University of Leicester told us about a paper which classified the different soil types of England and Wales according to their prospects for showing buried features (whether geological or archaeological) as crop marks (Evans 1990).  This paper grouped the different soil types defined by the 1983 Soil Survey of England and Wales into five categories:

  • Soils that show extensive crop marks;
  • Soils that show extensive crop marks in dry conditions;
  • Soils that show frequent crop marks over small areas;
  • Soils that rarely show crop marks;
  • Soils that never show crop marks.

Usefully, the 1983 soil survey maps have been digitised and are made available via the National Soil Resources Institute (NSRI) at Cranfield University.  As a bona fide researcher, this is available on payment of a processing fee (i.e. without paying royalties), so we obtained this data to try to create a map of Evans 1990 classification.  On receipt, I found that there were five soil types (924a, 924b, 952, 961, 962) in the NSRI data that did not show up in the Evans 1990 classification.  However, these were all types of industrial spoil heap or reconstructed ground surface, so I assumed that they would fall within the category of soils that never show crop marks.  As a result, I was able to reclassify the NSRI soils data into Evans’ five types:

Soil types of England and Wales graded according to possibility of showing crop marks. After Evans 1990.

In respect of ground obscuration, I decided to only take the “never” category as being a factor.  I therefore combined this with the “human” factors and the peat / alluvial sub-soils from the British Geological Survey data (see post 2) to create a new map of ground obscuration:

Ground obscuration taking into account soil type, some sub-surface geology (peat / alluvium), woodland, and human structures. Relevant to crop marks.

This latest map is only really relevant to crop marks, as presumably earthworks would still show even in the more unsuitable soils (under the right lighting / weather conditions).  We can see, however, that large parts of England are not at all suitable for aerial prospection for crop marks.  I am not sure that the large peaty areas seen in the east would necessarily mask all crop marks, but then all models are imperfect.  It might be useful to include the “rare” crop mark soils in this map too, perhaps with a lower weighting to reflect their partial effect, but I decided to work for now on a stricter basis.

The point of all this is to assess the effect of ground obscuration on the patterning of archaeological discoveries.  As an example, we can compare the distribution of NMP projects against this map of ground obscuration:

NMP projects as of 2012 compared against ground obscuration (re. crop marks).

This shows a reasonably good correlation between areas in which NMP projects have been (or are being) undertaken and areas suitable for aerial prospection (for crop mark features).  Where the NMP projects cover areas unsuitable for crop mark discovery, these tend to be upland areas where thin soils ought to mean earthworks are fairly prominent (I think).  It also shows that although the NMP covers around 50% of England, much of the remaining 50% is not necessarily suitable for NMP type projects (as they are reliant on aerial photography), with the particular exception of the rural West Midlands and parts of East Anglia.  However, many of these less suitable areas might be suitable for LiDAR survey.

Further down the line, I will be comparing distributions of archaeological sites against these obscuration maps to see if I can discover any intrinsic biases towards areas suitable for aerial prospection within the data.

Chris Green


Evans, R.  1990.  “Crop patterns recorded on aerial photographs of England and Wales: their type, extent and agricultural implications.”  Journal of Agricultural Science, Cambridge 115, 369-382.