The key to understanding the archaeology of the Vale, and through that how people in the remote past lived in and exploited this lowland area, is the relationship between the rivers, ancient human settlement, and the possible survival of archaeological evidence beneath river-deposited ‘alluvial’ sediments.
The project aims to bring together archaeological, geological, geomorphological and hydrological datasets to build a broad picture of past landscape development and human settlement in the Vale. This information is being stored and will be studied within a computer-based Geographical Information System (GIS).
Aiborne Laser Altimetry
Understanding the geomorphological development of river floodplains and terraces can assist in determining areas of maximum preservation potential for cultural archaeology, and can identify areas where river channel migration is likely to have caused damage or destruction to archaeological deposits. Within the alluvial landscape palaeochannels are of particular significance as their fill often contains peat and other material of palaeoecological significance and channel margins are often the setting for cultural activity, particularly in earlier prehistory.
A number of sources have been considered for floodplain mapping as part of the Vale of York project. Two-dimensional maps of palaeochannels, which often show as distinctive variations in vegetation, crop or soil cover, have been generated from mosaics of rectified aerial photographs. Many of the finer features of floodplain topography are, however, rarely evident on air-photographs and their identification requires a three-dimensional view of floodplain topography. Ordnance Survey topographical mapping at 10m or 5m contour intervals is of insufficient resolution to define significant floodplain features. As a result, maps of floodplain topography have traditionally been produced photogrammetrically, a time-consuming and costly process. Results are usually focused on generating terrain models for flood impact mapping; topographical and geomorphological features are not always recorded.
The Vale of York project has investigated airborne laser altimetry as a potential source for detailed topographical mapping of river floodplain and terraces as a supplement to traditional data sources. Airborne scanning laser altimetry, often called LiDAR (Light Detection and Ranging), collects high-density elevation data using an aircraft-mounted laser altimeter, typically generating 10-15k laser pulses/second. Elevation data are recorded at a typical spatial resolution of 1m with a vertical accuracy of +/- 0.2m. The scanning laser produces reflections from most opaque and semi-transparent media. Typically some laser pulses are able to penetrate semi-opaque ground cover such vegetation. Distinguishing between a first-pulse reflection (for example from the top of a woodland canopy) and last-pulse (from the underlying ground surface) allows mapping of the earth surface even in areas with dense vegetation cover. LiDAR has found wide-ranging applications including bathymetry, ocean wave dynamics, forest mapping, dune morphology, crop height mapping and river flood modelling.
Initial data processing is required to take account of aircraft location, altitude, roll, pitch and yaw, and to produce a point-cloud of xyz coordinate data corrected to a local map datum. Additional processing is required to separate last-pulse (ground) from first-pulse (above-ground) reflections. Point-cloud data are usually surfaced to produce a Digital Elevation Model (DEM) comprising a regular grid of points, usually discriminating last-pulse (ie ground surface excluding semi-opaque objects such as trees and light vegetation) and first-pulse (DEM with all landscape clutter). Extraction of opaque landscape clutter such as buildings requires the application of additional filtering techniques and interpolation of the ground surface beneath the opaque obstacle.
Analysis of LiDAR data for a variety of lowland river valleys has shown that in general the technique is particularly effective for mapping features of mature, middle reach floodplains dominated by lateral channel movement and desiccating peat dominated wetlands/floodplains.
It is less effective in upper and lower river reaches, where channel movement is confined to a narrow steep-sided valleys and where accretion is the dominant process, such as in the rivers of the Yorkshire Ouse system.
However, even in these landscapes LiDAR is able to detect and allow mapping of archaeological earthworks and other significant features of the natural landscape such as the dune formation laid down by early post-glacial wind-blown sand and forming extensive, though often poorly mapped, deposits in the north-eastern part of the Vale of York as well as the Vale of Pickering and in north Lincolnshire.
