Following on from my previous post on this subject, I have now produced a second version of the obscuration layer. On the suggestion of Graham Fairclough of English Heritage, this version includes the same obscuration factors as before (woodland, water, buildings / roads / railways), but also adds in areas of alluvial and peat sub-surface deposits. These types of deposit tend to obscure archaeological features that were present on the former land surface before they formed, due to their thickness. However, this is not as complete an obscuration as with the previous categories used, for several reasons:
1. Peaty soils across England are being eroded by agricultural / drainage practices, revealing their buried archaeological material.
2. Archaeological sites that were constructed after (or later on during) the formation of the deposits will not be (or will be less likely to be) obscured, i.e. the older a site is, the more likely it is to be obscured.
3. Peat / alluvium deposits may be thin enough for substantial buried archaeological features to show through the masking effect, especially if denuded by more modern intervention.
As such, this result should be viewed more critically than the previous one, in that some areas showing as highly obscured may, in fact, show some archaeological features from the air (notably the region around the Wash), especially when dealing with sites from more recent times. Also, as with all models, the result presented should not be taken as perfected. Here, then, is the map showing percentage obscuration for 1km x 1km grid squares across England (built environment, water, woodland, peat, alluvium):
The data for peat and alluvium deposits were taken from the British Geological Survey’s 1 in 625,000 geology dataset (superficial deposits) which they provide for free download and unrestricted usage (subject to providing appropriate acknowledgment) under their open data initiative. This data is provided at the perfect scale for a task such as the one undertaken (i.e. national patterning), but would be less useful for more intensive surveys. Together with the OS Open Data also used, however, it does demonstrate the excellent results that can be produced as a consequence of organisations opening up their data for free usage by researchers (and, by extension, the general public).