Aerial Archaeology in Ireland - The Heritage Council

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Both Kelleher (1995) and Redfern (1998, 2002) have explored the use of digital image enhancement techniques to some effect.
Input of topographic data into a GIS allows the development of highly realistic 3D models of the landscape surface. The two
main forms are known as digital elevation models (DEMs) which are based on actual elevation, and digital terrain models (DTMs)
which use the data to create surfaces that can be manipulated more extensively. DTMs are extremely useful visualisation and
modelling tools. They can be integrated with rectified digital air photography to ‘drape’ a photographic image over a 3D model.
This has been explored extensively by the Discovery Programme (Doody 1993c, 1995, 1996; Newman 1997).
Redfern (1998, 2002) has gone on to develop a system that derives a digital terrain model directly from digitised photography.
This also incorporates an image enhancement facility to maximise the clarity of the features. The system further allows for the
generation of 3D horizontal and vertical measurements and a pattern recognition system allowing classification of features (see
below).
Even more recently, Shell and Roughley (2004) have demonstrated the value of lidar imagery to develop highly detailed digital
terrain models with considerable flexibility of highlighting features and their position in the landscape through manipulation
of height exaggeration, artificial low-level lighting from any angle and viewshed analyses. They have combined lidar with
photogrammetry to generate photographic terrain models allowing enhanced understanding of landscape context, a technique
also used by the Discovery Programme in the Mullaghfarna project and elsewhere.
Context Mapping
An important issue for interpretation often involves understanding the visibility of archaeology — or to be more precise, mapping
features in relation to apparently ‘blank’ areas. Such areas may not be devoid of archaeology, but may be just under different
land uses that are not susceptible to revealing earthworks and soilmarks — or cropmarks. It remains extremely rare for such
non-susceptible areas to be mapped systematically as part of the context for understanding the distribution of sites. Barrett
(1980, 1982, 1995a, 1995b) has come closest to this in analysing and mapping the distribution of cropmark sites in relation to
soils and topography. This, however, does not extend to mapping how modern land use may have constrained the development
of cropmarks.
The work on digital terrain modelling, along with soil mapping, aspect and other parameters, has a clear use in mapping and
understanding archaeology within its physical topographical context, for which GIS applications as used by the Discovery
Programme are ideal. So far, this has been developed much more for the identification and interpretation of earthworks than it
has for crop- and soilmarks.
Morphological Interpretation and Classification
Interpreting archaeological sites as revealed by aerial photography is a skill that has many pitfalls. It demands an understanding of
soils, crop growth conditions, agricultural practices, geological formations, modern and spurious features, and real archaeological
monuments of all ages (Wilson 2000). Interpretations are not always simple or clear-cut. Even those of recognised specialists
may be open to question on occasion — especially if the interpretative context is outside their specialist period or geographical
area of research. A possible example of this may be illustrated by a 1989 photograph of a rare type of cropmark site for the
area forming part of a ‘Bronze Age funerary landscape’ at Chapelstown, Co. Carlow, recently published by Gillian Barrett (2002,
11 Pl 6). It shows an oval ditched enclosure (indicated by a dark linear feature) with two large, solid circular dark marks about
20m across which ‘from the air were interpreted as pits’ (Figure 33). Subsequent archaeological assessment in advance of
development showed the large dark marks to be ‘deposits of gravel and rounded and weathered stones’ (O’Donovan and O’Neill
2003). By analogy with other barrow cemeteries, these areas could well have been ploughed-down mounds of a pair of round
barrows set within or surrounded by an elliptical enclosure similar to examples known in the Thames valley (cf Barclay et al.
1995, Figure 39, field X, site 3).