By Mathias Lemmens, senior editor, GIM International

Most systems in the present overview are based on the ‘time-of-flight' principle; the distance to a surface point determined by first measuring the time taken for an emitted laser pulse to travel to it, then dividing the outcome by two. In contrast to phase-shift systems, time-of-flight systems can bridge distances up to several kilometres; for example, the maximum ranges of Leica HDS4400, Riegl LMS Z420i and Optech ILRIS-HD are 700m, 2000m and 2000m, respectively. In comparison, the maximum range of phase-shift-based system DeltaSphere-3000IR from 3rdTech is no more than 15m, but the accuracy at this distance is 7mm. Phase-shift systems also have a very high measurement rate (points per seconds emitted), e.g. Leica HDS6100 emits 500,000 points per seconds. Also listed here is one hybrid system, the Callidus CPW 8000, which uses both pulse and phase measurements, combining a maximum range of 80m with an accuracy of 2mm at 30m while measuring 50,000 points per second. Interestingly, with Leica's HDS6100 and HDS4000, and Trimble's FX and GX, two of the main TLS manufacturers now have both a phase-shift and a time-of-flight-based scanner on the market.

The features of the systems of Maptek (I-Site 4400LR and I-Site 4400CR), both introduced in 2006, are very similar to the Leica HDS4400 system introduced in February 2009. With a measurement rate of 15 points per second, the Trimble VX is actually a total-station enhanced for scanning parts of the scene in a land survey setting.

The best choice of a TLS largely depends on how good the features match the needs of the application area. In general, phase-shift systems are well suited for high precision and detailed measurement of very nearby scenes, such as industrial objects (e.g. cars), heritage sites and crime scenes, while pulse systems are well fitted for 3D reconstruction of scenes farther away from the scanner, for example to create 3D-models of plants, or even entire cities.