Terrestrial Laser Scanning
Terrestrial laser scanning (also known as time of flight) is an active method commonly used for the 3D digitisation of archaeological monuments and historical structures and buildings, survey methods are required which can effectively record the complexity and structure of the whole site whilst being able to capture its components and features. The method relies on recording the position and timing of a laser pulse round trip from the scanner to a heritage structure and back, and carrying this out hundreds of thousands if not millions of times a second. By rotating the laser and sensor (usually via a mirror), the scanner can scan up to a full 360 degrees around itself and capture the full environment. The resulting datasets produced is called a point cloud and consists of a series of millions of data points in space with each representing an individual measurement of the structure with resulting sub-centimeter accuracies and resolutions. The resulting point clouds can be coloured by superimposing onboard or external imagery onto the data sets which provides a photo-realistic geometric model of the survey site. In addition, laser scanners will record the reflective intensity of each pulse return which will be dependant upon the surface material and texture characteristics and can be coloured to provide an alternative point cloud visulisation method.
An alternative method to time of flight scanners is phase-shift scanners which use wavelength difference between the outgoing and return laser pulse to provide a distance measurement. Normally the range of such devices is shorter than that of time of flight scanners and are therefore used on smaller or less expansive structures but can be smaller and easier to use in awkward environments. Both systems are active methods and can carry out surveys in dark conditions, however, the additional information created by the imagery will not be captured effectively.
The Discovery Programme operates two terrestrial laser scanners for a range of surveys:
Trimble TX8 – Time of flight scanner which provides up to 340m scanning range at 1 million pts/sec with an effective accuracy and resolution at 120m of <2mm and 5.7mm respectively. The TX8 includes onboard HDR imaging and however, this is often supplemented with additional spherical imagery from an external SLR camera. Excellent for the recording of historic buildings, structures and larger monuments
Faro Focus 3D – Phased based terrestrial laser scanner which is compact and portable for those difficult survey areas such as souterrains and remote areas. Effective accuracy of ±2mm at 10m with up to 700 million points recorded per scan.
