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Text author: Luděk Krtička Last update: 2022-07-11

LIDAR (Light Detection and Ranging) is a method that can be used to locate objects on the Earth’s surface with relatively high accuracy. The method is similar to radar or sonar in that the time it takes for a beam of energy to reach an object, bounce off it and return is precisely measured. While radar uses radio waves and sonar uses sound waves, LIDAR uses laser light.

A LIDAR system mounted in an aircraft or helicopter sends pulses (typically hundreds of thousands to two and a half million pulses per second) through a mirror in a pendulum pattern towards the ground. When the laser beam reaches the ground (object) it is reflected and returned to the mirror and the system records the time interval between when the beam left the device and when it was received back by the sensor.

LIDAR schema
Schema of LIDAR. Illustration by: Luděk Krtička (CC BY-SA 4.0)

The laser beam can bounce off virtually anything except problematic non-reflective surfaces such as the water surface. The first reflections where the beam does not penetrate the earth’s surface are called FR – First Returns. This can be anything – treetops, a flying bird, cars, power lines, building roofs, bare ground. In contrast, LR – Last Returns penetrate to the ground surface, or better said, to what appears to be the ground surface – a rock overhang, a bridge, very dense vegetation, etc. LRs are often in the minority compared to FRs, but it is LR points that are most important for the creation of many of the base maps used in orienteering mapping.

After the aerial data collection is completed, the data processing begins. The individual measured time intervals are converted to distance and compared with the on-board GPS data, the data from the aircraft’s central control unit (INS) and the ground-based GPS station. The GPS uniquely provides the position of the aircraft during the measurement in the form of latitude (x), longitude (y) and altitude (z). The result is a data set containing points where each point has its position precisely defined within the geographical coordinates (x, y, z). Post-processing filters out errors and several processes are used to process the data into the final products (point coordinate file, digital elevation model, etc.).

The accuracy of products obtained from airborne laser scanning data depends on the amount of points obtained from a given location and the processing method. Collection densities currently range from one to several tens of points per square metre, resulting in quite incredible height accuracy of a few centimetres. Typical basemaps generated from LIDAR data include detailed contours, shaded relief, slope gradient, shaded terrain, vegetation height or vegetation density.