Trent Valley GeoArchaeology                                                                            

Airborne Laser Altimetry (lidar)

Airborne lidar is an active remote sensing system which utilises the principle that distance can be calculated through the time period between a pulse of light being emitted, reflected and received back at the source location. A lidar scanner therefore includes a laser ranging unit capable of emitting and receiving laser pulses at between 20 and 100 thousand points per second, coupled with precise kinematic positioning provided by a differential global positioning system (dGPS) and an inertial measurement unit (IMU). Raw lidar data is in the form of a three dimensional point-cloud which can then be projected to a local map datum, sorted, filtered and used to generate a regular grid of elevation values. This grid can then be interpolated to form a surface representing topographic changes across the flight area.

The lidar receiver is capable of recording multiple returns for a single laser pulse, allowing the collection of points from a semi-opaque object such as a woodland canopy (referred to as a first-pulse (FP) return) and from the opaque ground beneath the canopy (a last-pulse (LP) return). The intensity (amplitude) of the laser return can also be recorded, providing information regarding the backscattered intensity of reflection from differing surface materials.

The research into the use of lidar for geomorphological investigations as part of this project has shown that the 1m resolution lidar surface model can provide spectacular evidence for floodplain and terrace geomorphology, although with a corresponding high data volume. Lidar surface models at 2m resolution provide comparable information regarding geomorphological landforms but with some degradation in the representation of archaeological earthworks. The lidar intensity data was seen to contribute significantly to the mapping of terrace and floodplain geomorphology, with features not visible on aerial photography or the lidar surface model being visible on the intensity surface models. Intensity values appear to vary inversely in relation to soil moisture, suggesting that intensity is a reliable way of identifying wet or waterlogged features. The backscattered intensity of airborne lidar is being investigated further through another ALSF research project (for further information click here).

                  Lidar first-pulse (FP) surface model                                              Lidar last-pulse (LP) surface model                                             Lidar backscattered intensity surface model                   

Inteferometric Synthetic Aperture Radar (IFSAR)

Airborne radar uses radio waves to measure the distance between an aircraft mounted sensor and the ground surface. Interferometry relies on picking up the returned radar signal using antennas at two different locations. Each antenna collects data independently, although the information they receive is almost identical, with little separation (parallax) between the two radar images. Instead the phase difference between the signals received by each of the two antennas is used as a basis for calculation changes in elevation. The results are enhanced by using processing techniques during data collection to generate a synthetic aperture of much greater size than the physical antenna used and so enhance resolution. Combining the principals of Synthetic Aperture Radar with Interferometry, Inteferometric Synthetic Aperture Radar (IFSAR) is capable of producing both a radar image of the ground surface and calculating elevation changes to enable production of a digital surface model (DSM). IFSAR data is available in the form of a 5m spatial resolution DSM with a vertical accuracy of between 0.5 and 1.0m and a 1.25m spatial resolution radar image.

The analysis of the IFSAR DSM demonstrated that although the technique is able to distinguish broad geomorphological zones within the study area, such as the river terrace and floodplain, it provides a relatively poor record of the subtle microtopographic features that are required for a detailed mapping of alluvial geomorphology. Anthropogenic features such as ridge and furrow were not evident at all on the IFSAR DSM model. The IFSAR Orthorectified Radar Image (ORI) also failed to comprehensively detect even the significant geomorphological features and was found to be of no value for archaeological work.

IFSAR digital surface model                                                 IFSAR Orthorectified Radar Image (ORI)

Aerial Photography

The study also included the examination of vertical aerial photographs from the National Monuments Record (NMR) collection and those held at the repositories at Leicestershire County Council and Cambridge University. The analysis focused primarily on identifying and digitising the natural landscape and geomorphological details evident on the aerial photographs, rather than any cultural archaeological features. However, the extent of both earthwork and crop/soilmark ridge and furrow was mapped as this broadly reflects the different geomorphological units.

The work highlighted the value of using more than two sets of vertical air-photographs, with the analysis of images taken at different times adding considerably to the overall knowledge of the geomorphology of the study area. Photographs of the rivers Trent and Soar in flood proved to be of particular use for identifying palaeochannels and understanding the topography and structure of the Hemington terrace. Overall, it was found that some aspects of floodplain and terrace geomorphology were only evident on air-photographs and not visible to lidar or IFSAR, therefore suggesting that air-photography must remain the primary source for geoarchaeological prospection. 

Geocorrected vertical aerial photograph mosaic of the study area taken on 15th December 1954 showing extensive over bank flooding of the Rivers Trent and Soar.