Additional funding from the Heritage Council at the end of 2007 enabled the Discovery Programme to continue its research at the Hill of Tara. In order to understand the wider landscape context an area of 70km2 was selected centred on Tara, for which high resolution stereo photography was commissioned from BKS/Fugro.
Image Capture
As with similar landscape projects recently undertaken, such as the North Roscommon Landscape Project, a photo scale of 1:7500 was selected. However since our last project BKS had invested in a new digital camera, an Intergraph Z/I imaging DMC, which meant that this would be our first project processing directly from digital images. The camera pixel resolution is 12 microns (0.012mm), which equates to a ground pixel resolution of 13.5cm. The flight took place on the 13th May 2008, in sunny conditions.
Ground Control 
BKS were commissioned to complete aerial triangulation in preparation for the photogrammetric processing. A control plan was supplied identifying the distribution of 13 ground points for which Irish Grid positions were required. These were surveyed by GPS using the Trimble VRSNOW network correction service.
The survey was undertaken in Irish Transverse Mercator (ITM) system and converted to Irish Grid using the Quest Grid Inquest converter software. BKS used these coordinates to generate x,y,z, omega, phi, kappa values, the exterior orientation, for each of the 162 images in the project.
Processing Stages
The orthoengine module of Geomatica 10 photogrammetric software was used to process the project. This is a labour intensive process requiring repetitive processing steps to be executed for each of the overlapping pairs of images:-
- Input and set project information and parameters
- Import the exterior orientation values
- Generate epipolar pairs
- Extract the DEM automatically
- Manually edit each DEM
- Geocode the Extracted DEM
- Merge and Edit the DEM
- Generate Orthoimagery
The Discovery Programme has compiled a user guide for processing digital Imagery in Geomatica 10 software, and this is available as a pdf document.
Output
These processing stages result in two products which are excellent resources for landscape analysis in GIS:-
- The DEM for the whole survey area has been generated initially with a 5m ground cell size. This provides an overview of the landscape, and can be easily and rapidly manipulated in GIS. One of the limitations of this technique in generating DEM’s is the fact that the model includes tress, hedgerows and buildings. Unlike LiDAR there is no method for removing these features and modelling the ground surface. Future work may involve re-sampling the DEM at a finer resolution if areas are identified which may benefit from more detailed investigation.
- The orthoimagery – 1km2 geo-referenced images with distortions removed – provide a mapping resource, showing relict landscape and current land-use features. The pixel resolution, 13.5cm on the ground allow features of approx 50cm or greater to be identified.
Archaeological Interpretation
With these resources now available in GIS the next stage is to examine the data and add to our understanding of the Hill of Tara in its wider landscape context.
