Time of Flight (TOF)
Time-Of-Flight (TOF) - also known as terrestrial laser scanning - is an active method commonly used for the 3D digitisation of architectural heritage (e.g. an urban area of cultural importance, a monument, an excavation, etc). The method relies on a laser rangefinder which is used to detect the distance of a surface by timing the round-trip time of a light pulse. By rotating the laser and sensor (usually via a mirror), the scanner can scan up to a full 360 degrees around itself.
The accuracy of such systems is related to the precision of its timer. For longer distances (modern systems allow the measurement of ranges up to 6km), TOF systems provide excellent results. An alternative approach to TOF scanning is Phase-Shift (PS), also an active acquisition method, used in closer range distance measurements systems. Again they are based on the round trip of the laser pulse but instead of timing the trip they measure the wavelength phase difference between the outgoing and return laser pulse to provide a more precise measurement.
Airborne Laser Scanning
In addition to ground based systems Airborne lidar (light detection and ranging), also known as Airborne Laser Scanning (ALS) has also been used as a data capture technique.
A lidar system is based on sending out thousands of laser pulses per second. The laser pulses are reflected by vegetation or buildings or soil. By measuring the time that passes while the light travels from the airplane to the ground and back, the distance between airplane and ground can be measured. The current position of the airplane and the direction into which each laser pulse was sent are measured using GPS and inertial navigation units. By combining millions of such measurements, a very detailed three-dimensional model of the earth surface can be computed.
However the true power of lidar mapping systems is that some laser light is reflected from the vegetation canopy, but some reaches the ground – the scanner onboard the airplane receives multiple reflected signals. To visualise the ground surface beneath the canopy the data needs to be filtered. For every pulse that was sent out, only the last reflected signal - the one that has travelled the longest way - is chosen. Some laser pulses do not reach the ground at all. They can be identified and discarded by comparing neighbouring points. The result is a three-dimensional model of the ground surface from which the bushes, hedges and trees have disappeared. Barrows, hollow ways, mining pits or former field structures become visible in this virtually de-forested landscape and can be mapped by archaeologists.
Schematic principles of data acquisition in Airborne Laser Scanning.