In order to provide meaningful data in a monitoring scenario, repeated measurements of the same set of points is necessary. In another widely used monitoring system, prisms are used to provide a fixed point in the area of suspected movement that can be re-measured over time. The disadvantage of this method is that the prisms are the only points that can be measured, and there are always gaps in the coverage that can be achieved. To overcome this problem, a large number of prisms must be installed, which can be expensive and dangerous (or even impossible where access to the unstable area is prohibited).
SiteMonitor overcomes this problem by using a different approach, made possible by the fact that the scanner works in a "reflectorless" manner. Rather than measuring to a fixed number of discrete points, a grid of points completely covering the area of interest is defined. These points, or nodes, do not need a reflector or prisms, or any other kind of target - they are theoretical locations, defined only in the co-ordinate system of the scanner. Each time the area is measured, the scanner returns to each of the nodes and takes a range measurement.
Mitigation of atmospherics
Laser scanner range measurements are based on the time-of-flight (TOF) principle, in which a pulse of light is emitted by the laser scanner, and the time taken for it to be returned is recorded. Using the known constant speed of light, the range to the target can be calculated.
The range measurement recorded by the scanner can be influenced by atmospheric conditions - the temperature and pressure of the air through which the laser beam passes. Rather than take measurements of the atmospheric conditions at one or two locations (for example, next to the scanner and adjacent to the area being monitored), which would provide insufficient data to accurately compensate for variations in range measurements acros the area of interest, SiteMonitor calculates a Range Correction Factor (RCF) by measuring to a number of control points with known coordinates, and comparing the true range with the measured range, before applying this correction to the data. These control points would be situated around the area of interest on stable ground, and, in a highly robust system, would be monitored continuously by GPS. This provides a much more realistic approximation of the actual effect that atmospheric conditions are having on range measurements at the time of measurement.
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