A Resolution Measure for Terrestrial Laser Scanners

Terrestrial laser scanners are increasingly being used for cultural heritage recording and engineering applications that demand high spatial resolution. Knowledge of an instrument’s spatial resolution is necessary in order to prevent aliasing and estimate the level of detail that can be resolved from a scanned point cloud. In the context of laser scanners, spatial resolution can be decoupled into range and angular resolution. The latter is the focus of this paper and is governed primarily by angular sampling interval and laser beamwidth. Both factors give rise to uncertainty in the angular position of a range measurement, though in terms of reporting scanner resolution, it has become a common practise to emphasise one of these factors—typically sampling interval—as an indicator of resolution. Since both affect the resolution of a scanned point cloud, consideration of only one can lead to a misunderstanding of a system’s capabilities. The ramification of this is that the actual resolution may be much lower than that perceived when visually inspecting a scan cloud. It will be demonstrated that consideration of only one factor independent of the other is inappropriate except under very specific conditions. A new, more appropriate resolution measure for terrestrial laser scanners is therefore necessary and one is proposed in this paper. The effective instantaneous field of view (EIFOV) is derived by modelling the inherent uncertainties in equal angular increment sampling and laser beamwidth with ensemble average modulation transfer functions (AMTFs). The practical outcome of this approach is a scientifically sound method of quantifying laser scanner resolution for users of the technology. Four commercially available terrestrial laser scanner systems are modelled with AMTFs and analysed in terms of their angular resolution as measured by the EIFOV. It is demonstrated that point cloud resolution as indicated by the EIFOV is much more coarse (by up to 21 times) than the sampling interval.