LiDAR Technology
LIDAR (Light Detection and Ranging) is an optical remote sensing technology that measures properties of scattered light to find range and/or other information of a distant target. The prevalent method to determine distance to an object or surface is to use laser pulses. The system, mounted under airplane or helicopter uses a combination of DGPS and inertial sensors to record its position and is therefore able to return a high density of accurate 3D ground points – a point cloud. The laser senor receives multiple returns from the ground, and by processing the data models can be created which have the vegetation removed – bare earth models.
With the advent of fixed-wing airplane LiDAR the ability to aerially record the topography of vast swathes of landscape remotely has become available to the archaeological surveyor. Many research projects have powerfully illustrated the potential for this technology to record the archaeological landscape in 3-dimensions, including those areas of terrain beneath tree cover. Another factor that has made the application of this technique advantageous is the relatively short time period between the commissioning of a survey to the creation of the final functioning Digital Terrain Models (DTM).
Usually the horizontal and vertical accuracies of the final data models are 0.6m and 0.15m respectively. These values are suitable for topographic modelling of archaeological landscapes. However, the ability of this technology to successfully extract all the subtle features of an individual monument is questionable. As height point resolution is based upon the frequency of the LiDAR sensor and the air speed of the sensor platform, fixed wing LiDAR have the disadvantage that the speed of the plane can only be reduced to such a point before stalling occurs.
In 2006 The Discovery Programme was made aware of a new aerial LiDAR system to rival that of fixed wing. Operated by BKS, Coleraine and Fugro, Netherlands the Fli-MAP 400 LiDAR system utilises a twin engine helicopter as the operating aerial platform beneath which three laser scanners and various imaging devices are suspended. This technology was devised for the 3-dimensional recording of infrastructural assets such as train lines and electrical distribution cables. To be able to capture these features effectively, the accuracy must be approaching the levels achieved by GPS and be of a sufficiently high resolution to define the objects.
Most suppliers of LiDAR also capture imagery and video from the flight, and can provide ortho-imagery, similar to that produced from aerial photogrammetry.