Terrestrial & Close Range Laser Scanning

181

Laser Scanning is a form of indirect data capture that typically involves the use of a laser beam to measure distances to a series of three dimensional points on the surface of an object.. The term indirect indicates that the user does not choose which individual points to record, instead the resolution and area of interest are specified and the scanner fires enough laser beams to indiscriminately measure however many points are needed. This results in what is called a point cloud, a mass of 3D measurements.
 
Laser scanning is an ideal technique for a range of purposes from detailed recording of objects and architectural features to recording of buildings, complexes of buildings or entire landscapes or cityscapes. A number of types of laser scanner are available to suit different needs. The technique is particularly suited to archaeological work where there may not be obvious lines or points for a surveyor to record and what is really needed is a spread of measurements across a surface, for example the walls of a ruined castle or an earthwork.
 

Buildings, structures and landscapes

Terrestrial Laser Scanning (TLS) employs either a time of flight or a phase-offset method to establish distances and is often used to survey entire buildings and large engineered structures but can also be used to survey the ground surface ie for topographic survey. Data captured this way can be imported into Geographic Infomration Systems for detailed analysis to produce Digital Surface Models (DSMs) and contour plans. This makes it possible to draw profiles across surface features, to examine the slope or aspect of the ground surface and to conduct spatial analysis such as visual impact assessment.
 
Airborne Laser Scanning (ALS) techniques, including LiDAR, use similar methods but from an aerial viewpoint; more information on such techniques can be found here.
 

Objects and Artefacts

Close Range Laser Scanning (CRLS) uses very high resolution scanners to capture much smaller areas of data at very  high resolutions; right down to tens of microns. This is particularly useful for recording smaller objects or in-situ structures and features requiring very detailed surface analysis.
 

182 Applications of technology

Terrestrial laser scanning of buildings, structures and earthworks typically reduces time spent on site compared with more traditional direct survey methods., The big difference however is in the post-fieldwork stage. Having a detailed point cloud and other supporting information allows our archaeologists to make better informed decisions in order to produce a range of informative products such as elevation drawings or orthographic images. We are also able to return to a scan dataset many times in order to extract measurements for purposes which may not have been foreseen during fieldwork.
 
TLS is also suitable for monitoring purposes; to investigate and measure change over time. This may be to quantify the effects of natural erosion, damage to buildings and earthworks caused by people, animals or vehicles or it could be to assess the deformation of a historic structure over time.

183

Guidance for metric survey projects including TLS is provided by the Metric Survey team at English Heritage and all our work is undertaken to their standards. Wessex Archaeology are also members of the Survey Association, a professional body for organisations undertaking survey work.
 
Contact us on 01722 326867 or
email p.baggaley@wessexarch.co.uk for further information.
 

Case Studies