Laser scanners capture data by successively considering points on a discrete, regular (typically spherical) raster, and recording the respective geometric and radiometric information. There are different types of laser scanners taking into account its application: Spaceborne LS (e.g. Geoscience Laser Altimeter System - GLAS) provides global measurements of the Earth's surface with the potential on capturing additionally clouds and atmospheric aerosols. The spaceborne measurements allow to globally observe ice sheet and land elevations, approximate sea ice thickness, changes in elevation through time, vegetation coverage for biomass estimation, and height profiles of clouds and aerosols. Airborne laser scanning (ALS) systems allow a direct and illumination-independent measurement from 3d objects in a fast, remote and accurate way. Beside basic range measurements, the current commercial ALS developments allow to record the waveform of the backscattered laser pulse. Latest trends in sensor developments focus on single-photon detection. Different applications are of interest, like urban planning, forestry surveying, or power line monitoring. Further to describe the 3D scene, products like digital terrain models (DTMs), digital surface models (DSMs), or city models are provided. A terrestrial laser scanning (TLS) system is a stationary highly accurate ranging device for geodetic surveying. More specifically, TLS systems provide dense and accurate 3D point cloud data for the local environment and they may also reliably measure distances of several tens of meters. Due to these capabilities, such TLS systems are commonly used for applications such as city modeling, construction surveying, scene interpretation, urban accessibility analysis, or the digitization of cultural heritage objects. When using a TLS system, each captured TLS scan is represented in the form of a 3D point cloud consisting of a large number of scanned 3D points and, optionally, additional attributes for each 3D point such as color or intensity information. However, a TLS system represents a line-of-sight instrument and hence occlusions resulting from objects in the scene may be expected as well as a significant variation in point density between close and distant object surfaces. Thus, a single scan might not be sufficient in order to obtain a dense and (almost) complete 3D acquisition of interesting parts of a scene and, consequently, multiple scans have to be acquired from different locations. A mobile laser scanning system consists of a moving vehicle equipped with one or more usually side-looking laser scanners to capture information about the local 3D geometry. Underwater LS is applied in deep-sea as well as in shallow water regions. The ranging distance is close range and the measurement principle relies on triangulation by laser light, comparable with structured-light-projection. More recently, companies started to develop Time-of-Flight (ToF) underwater laser scanners. For Bathymetric LS the utilized green laser light with its potential penetration capabilities in water is essential. For water surface mapping the electromagnetic radiation of the laser penetrates into the topmost layer of the water column and can also be used for mapping the water surface and shallow water bathymetry. Area-wide water surface heights and depths are required for many disciplines such as hydrology, hydraulic engineering, flood risk management, ecology, climate change, etc.
Explain the laser scanner technology
Discuss different types of laser scanners
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