Numerous techniques are available to map fluvial geomorphology. Outside the Trent Valley these have included the use of borehole data (Brown and Keogh 1992) or borehole data and geophysical investigation (Bates and Bates 2000) to map subsurface stratigraphy in areas of deep alluvium, Multi-Spectral Scanner images to identify relict channels (Powlesland et al. 1997), geomorphological field mapping (Moores et al. 1999), photogrammetric and tacheometric survey (Lane and Richards 1997), cartographic sources to analyse historical river channel change (Hooke and Redmond 1989; Bravard and Bethemont 1989; Braga and Gervasoni 1989), airborne LiDAR (Charlton et al. 2003) and aerial photography (French et al. 1992). In the Trent Valley, use has been made of gravel pit surveillance records, river walking, archival evidence, aerial photographs and LiDAR (Salisbury et al. 1984; Salisbury 1992; Large and Petts 1996; Collcutt 1996; Challis in prep.).
Many of these techniques require significant capital investment in time-consuming primary field survey or the acquisition of expensive datasets (Multi-Spectral Scanning, LiDAR), and hence the majority of the examples cited above have been carried out over restricted spatial extents, in general at the reach scale, or have focused on major channel change only (Salisbury 1992; Large and Petts 1996). The catchment-scale focus of the Trent Valley Survey discounted all but aerial photographs and cartographic sources, on the grounds of time and budget. The requirement to devise a methodology with generic application, and the presence within county archives of comprehensive aerial photograph sets, led to the adoption of aerial photographs as the primary data source. While cartographic evidence provides good data relating to major channel change in the historical period (Large and Petts 1996), it was considered unlikely to record earlier or smaller-scale channel change, and was therefore rejected as a major data source. A single, large-scale epoch of historic mapping (the Ordnance Survey One Inch First Edition originally surveyed during the 1820s and 1830s) was, however, adopted as a secondary source relating to recent palaeochannels. Large and Petts (1996) noted the correspondence between abandoned channels and parish boundaries at Barton-in-Fabis and around the Dove confluence; digital parish boundary data was therefore used to assess this correspondence more widely.