Laser Scan Principles

The two main types of laser scanner are grouped as time-of-flight and phase based systems. The time-of-flight scanner may be the most familiar in terms of function as it operates similarly to more conventional equipment such as Total Stations. A laser if fired from the scanner at a rapid rate in a single burst and distance calculated by time of return. Unlike a conventional refectories total station this process can be performed over 50,000 times a second. Phase-based systems utilize a consistent laser beam modulated over its wave length to calculate the position of reflected surfaces. In general time-offlight scanners have a far greater distance and field survey positional accuracy. Phase-based are generally slightly faster but with a shorter range.

Laser scanning produces a ‘point cloud’ comprised of many millions of individual X, Y and Z coordinate values. In addition to this each point has an associated Intensity value, a measure of the strength of reflection from any location. Bright dry objects reflect strongly while wet or damp dark organic surfaces reflect weakly. When onboard photographs are taken each point is also tagged with a red, green, and blue value taken from a matching location within the photograph. The same is achieved in post processing when using external images. It is the great abundance of captured data, sometime amounting to many hundreds of millions of points that make laser scanning so valuable and potentially more intensive to manipulate than conventional survey methods