Lidar is a rapidly evolving surveying technology for collecting 3D point-clouds both from airborne platforms and in land-survey settings. This magazine regularly devotes in-depth features to the principles and applications of this important emergent technology and, with the inclusion of a Product Survey, Technology in Focus and no fewer than four articles on terrestrial Lidar, it is the main theme of this issue. Terrestrial Lidar makes possible the swift measurement of millions of points by automatically scanning the scene at high speed. In the resulting dense point-cloud objects can be easily identified, allowing the creation of 3D-models with a level of detail impossible to achieve within a reasonable space of time using traditional technologies. Terrestrial Lidar thus opens up new dimensions in surveying.

Laser scanners are active sensors that emit laser beams for measuring the distances to objects without human/object contact. The principle is based on either time-of-flight or phase-shift. In the time-of-flight technology the sensor emits a laser pulse in the direction of the object; the time taken by the part of the pulse reflected back to reach the instrument is measured. Distance is calculated by multiplying this travel time by the speed of light and dividing the result by two. In phase-shift technology the sensor emits laser beams which are modulated as sine waves. The phase of the reflected part of the laser beam is measured and compared to the phase of the outgoing one, and distance then calculated from the difference in phase (phase-shift). Point density is usually so high that by recording the strength of the reflected signal quasi images can be created.

Phase-based scanners, on the market for about fifteen years, were initially aimed at close-range, high-accuracy industrial applications. These scanners are characterised by a precision ranging from sub-millimetre to sub-centimetre level, and high scan rates of up to half a million points per second; they are thus able to capture objects at very high density. However, these favourable precision and scan rate numbers come at the cost of maximum achievable range, which is less than 100m. In contrast, time-of-flight systems may measure distances up to 1km and even more, but their precision is usually limited from sub-centimetre to centimetre level, while scan rates are 10kHz.

Laser scanners are often compared to reflectorless total stations and, as far as the measurement principle is concerned, this is fine. Both instruments measure distances using pulsed laser light or phase shifts. Total stations can achieve higher precision because many measurements, even up to thousands, to the same point are taken and averaged, while laser scanners measure each distance only a few times, sometimes just once. To determine the X,Y,Z coordinate, the coordinates of the position of the instrument have to be known as well as the horizontal and vertical angles of each outgoing laser beam. The 3D coordinates of each point are calculated in a local or national reference system, from the laser distance, the known X,Y,Z coordinates of the instrument, and horizontal and vertical angles of each outgoing laser beam. The coordinates of the position of total stations are usually determined by centring the instrument above a known point. The position of a laser scanner is usually determined indirectly by placing special targets in the scenes the three coordinates of which are measured using traditional survey instruments such as total stations. The similarity in measurement technology inspired manufacturers of traditional survey instruments to modify their total stations into a quasi laser scanner able automatically to scan areas of interest at predefined intervals, despite scanning rate being significantly lower than that of laser scanners.

At product level, similarity bounces. A land surveyor is used to interpreting a scene as a collection of characteristic points each of which has to be measured individually; connecting the characteristic points correctly allows reconstruction of the boundary of the object. Using a laser scanner, no selection of individual points takes place during scanning; it is a matter of chance which points are hit by the laser beam. As a result, unwanted objects, such as crossing pedestrians, are also captured. The actual measuring is done in the office by fitting geometric primitives such as lines and planes through parts of the point-cloud. Boundaries and characteristic points are computed from intersecting neighbouring geometric primitives. Laser scanners can collect points for hours on end without human intervention, making them particularly suitable for capturing hostile environments such as nuclear plants, where placing the instrument needs to be done fast.

Laser scanners and total stations are also compared at time-efficiency level. Indeed, surveying with a total station is only feasible when the object can be modelled by a limited number of characteristic points. Laser scanning enables capturing scenes consisting of objects of complex shape, such as chemical plants, cultural-heritage and traffic-accident sites. Some vendors of laser scanners posit the idea that if a laser scanner acquires 8,000 points per second while it takes a survey team ten seconds to measure a single point, using the scanner is equivalent to working with not one but 80,000 teams; a faulty and misleading comparison. Surveyors select their points intelligibly, while laser scanners take points blindly, without identification, interpretation and selection. These activities have later to be carried out in the office. As such, laser scanning bears more comparison with photogrammetry than with surveying. Laser scanning may thus bring land surveyors and photogrammetrists closer together. Or even result in the merging of the two professions.