Aerial photography and ground obscuration

Examination of aerial photography is one of the primary methods by which archaeologists have surveyed the landscape of England for new sites and for new information about known sites, in a process that continues to this day.  However, it is only possible to find buried archaeological features by this method under certain conditions.  One particular adverse condition that halts all aerial photographic survey work is the obscuration of the ground surface by human and natural features.  Woodlands / forests (LiDAR can see through these to some extent, but photography cannot), lakes, buildings, roads, railways, etc. can hide the ground surface and make the detection of surface and subsurface features impossible.

As a result, distributions of archaeological sites discovered through aerial prospection will inevitably be biased towards areas of open country, particularly arable and pasture lands.  If we wish to make quantitative statements about such distributions, we need a methodology by which to quantify the obscuration of the ground surface, in order to demonstrate which areas of apparent blankness on such a distribution map are, in fact, only blank due to the impossibility of aerial prospection.

When the Ordnance Survey made available some of its data under its OpenData initiative, it became possible to undertake this quantification of obscuration using some quite simple (albeit intensive) computational methods.  This is because the Vector Map product produced by the OS is organised thematically, making it quite simple to download and join together thematic map layers for the whole of the UK (as the current project is only concerned with England, the method discussed below has only been undertaken for England, however).  This forms a series of data layers that would have been very difficult to pull together prior to the OpenData initiative.

To build up a map of ground obscuration for England, the following OS OpenData layers were downloaded and joined together for several regions (European parliamentary constituencies) that together spanned the whole country*: buildings, water areas, forested areas (all polygons), roads, and railways (line data).  It would have been possible to include other layers (such as glasshouses), but it was decided that those listed above were sufficient to produce a good generalised map.  The spatial precision of these layers actually appears very good, especially for the resolution of analysis undertaken (see below).  Buffers were generated for the roads and railway lines, of varying width depending on the type of entity (based on a quick survey of a few entities of each type on Google Earth): 10m for most types of road and for narrow gauge railways; 15m for A roads and single track railways; 20m for trunk roads; and 25m for motorways and multi track railways.

The buffer layers, buildings, water areas and forest layers were then joined together using the union tool in ArcGIS to create a polygon map of ground obscuration for each region.  A 1km by 1km polygon grid square layer was generated using Geospatial Modelling Environment and then reduced down to the outline of England via a spatial overlap query.  The identity tool in ArcGIS was then used to calculate how the polygons in the obscuration layers overlapped with the grid polygons, and the area of each resulting overlap polygon was then calculated.  The attribute tables were exported for these output layers and joined together in Excel into one big table listing the ID number (CELLID) for the related grid square and the area of each obscuration polygon within that square.  A python script was written which went through this table, adding together the total area of obscuration for each CELLID (this took around seven hours to process), and outputting a new table listing CELLID and total area of obscuration.

This output table was joined to the 1km by 1km grid square layer in ArcGIS based upon the CELLID.  We now knew the total area of obscuration for each kilometre grid square of England.  The percentage obscuration was calculated and this percentage figure was then used to create a 1km resolution raster layer showing what percentage of each cell’s ground surface area was obscured by buildings, woodland, water, roads and railways:

% obscuration of ground cover for 1km grid squares in England
% obscuration of ground cover for 1km grid squares in England

Obviously, as with all models, this is not a perfect or perfected result, but I do believe that it provides a very useful quantification of the extent to which the ground surface of England is obscured to any aerial visual observer (the picture would be somewhat different for LiDAR prospection, as then I would not have included trees as a form of obscuration).  There are undoubtedly other types of obscuration feature that could also have been included (areas of alluvium or peat, perhaps) and there may be some types of included feature that can, in certain circumstances, be seen through.  It does, however, provide a good basis for quantifying the extent to which gaps in aerial prospection results for England have resulted from the impossibility of achieving results through that method.  In the context of this project, this is particularly relevant when dealing with English Heritage’s National Mapping Program data, as this was constructed on the basis of aerial survey.

– Chris Green

* This division into regions was purely to ease the processing burden on ArcGIS and my computer.

Visual blog now on-line!

Also involved with the EngLaId project is Miranda Creswell, project artist. In addition to developing more specific landscape-based projects, Miranda also records the team’s working methods through drawings and images that focus on the theme of connecting ideas. We are pleased to announce that Miranda’s visual blog is now active: http://visualenglaid.wordpress.com/.

HER Forum 7 December 2011

On 7 December 2011, Chris Gosden gave a talk to about 70 local authority Historic Environment Record (HER) officers from all over England about the EngLaId project to delegates of the HER Forum Winter Meeting, held in the Birmingham and Midland Institute in Birmingham. The aim was to present the project to HER officers and obtain feedback from the audience. It was emphasised that the EngLaId project hopes to establish a two-way conversation about the possibilities and advantages of collaboration between the EngLaId project and HERs. The EngLaId project depends to a significant degree on data held by HERs, but, where possible, aims to use these data in such a way that HERs will enjoy maximum benefit from the information exchange, for example through feeding into HER Research Frameworks.

Chris first outlined the project’s main objectives. Highlighting the enormous potential brought by the increase in archaeological data since the 1990s, he stressed the importance of long-term and countrywide syntheses that ignore traditional period and regional boundaries. Such an ambitious approach is perhaps only possible in an English context, because the high standards with which data are recorded and made available for research in England – not only through HERs, but also through EH’s National Mapping Programme (NMP) and other data repositories such as the Portable Antiquities Scheme (PAS), the Celtic Coin Index and the Early Medieval Corpus (EMC) of single coin finds – are unrivalled internationally.

Chris then explained that the project will take place on two scales. First, the project will analyse patterns on a national scale – based mainly on NMP, HER and PAS data – with the aim to identify broad patterns in land use and metalwork deposition. Second, the project’s researchers will focus on a number of case studies, initially selected on basis of NMP coverage in combination with other investigation types, although the boundaries of case study areas can be altered if HER officers feel that that will be more beneficial.

Suggested case study areas.

The output of the project, in addition to traditional scholarly publications including a monograph, will be a website that, through application of new technology derived from the Semantic Web, will provide a search function that links to existing on-line data repositories. It was stressed that it is NOT the aim of the EngLaId project to create a new data hub. The project will approach HERs for data, but only to be used for research and analysis. The eventual website will merely provide a search function to existing repositories where data are kept under their existing conditions of release. This approach also ensures that the EngLaId website will not become out-dated, and that HER data will be accessed and appreciated more widely.

After Chris’s presentation, there was sufficient time for discussion and debate.  Reactions from the floor were overwhelmingly positive, with several suggestions for slight improvements to case study area boundaries and specific possibilities for collaboration. In conclusion, Chris reiterated that his presentation to the HER forum was an invitation to establish a two-way conversation about the possibilities and advantages of collaboration. In this context, we welcome any further ideas and discussion regarding the EngLaId project, either through this blog or directly by email to the Project Administrator, Laura Morley (laura.morley@arch.ox.ac.uk).

MSRG Winter Seminar, 3 December 2011

On 3 December, the English Landscape and Identities project presented a poster at the 2011 Winter Seminar of the Medieval Settlement Research Group, held at Newcastle University. The seminar was entitled ‘Heartland to Frontier: The Tees-Forth Region in the Middle Ages’, incorporating a number of papers that presented new evidence from excavated settlements and finds in this north-eastern region. As two of the case studies that the EngLaId project will carry out are also situated in this general area, it was extremely useful to attend the Winter Seminar. The project poster was well received and generated some useful comments from MSRG members.

The MSRG Winter Seminar was coupled with the MSRG AGM, during which one of the EngLaId project members, Letty ten Harkel, was elected as committee member for the MSRG, laying the basis for continuing contact and co-operation between the English Landscape and Identities project and the MSRG which, we hope, will be mutually beneficial.

Welcome

Welcome to the English Landscape and Identities Project blog.  You can read more about the project and the team.  We intend to use this space to keep people up to date with progress on the project and events we will be attending.  There will also be some posts about more technical aspects of the project on a separate blog.

On 11 November, a project poster was presented at the international conference ‘Power and Place in Later Roman and Early Medieval Europe’, held at the Institute of Archaeology in London, which received many favourable and enthusiastic comments from conference delegates. Our next public appearance will be on 3 December at the Medieval Settlement Research Group Winter Seminar in Newcastle, where we will be presenting a poster. We will also be present on 7 December at the Historic Environment Records Forum Winter Meeting in Birmingham. See this space for further details and announcements!