LiDAR: Working of LiDAR and Measurement of Relief, Effect of Surfaces (Especially for GATE-Geospatial 2022)

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Lidar is a method for measuring distances (ranging) by illuminating the target with laser light and measuring the reflection with a sensor. Differences in laser return times and wavelengths can then be used to make digital 3-D representations of the target.

In Photogrammetry, LiDAR (Light Detection and Ranging) is necessary when a DSM with greater accuracy is going to be created; either because we want to make a True Orthophoto or because we need a more accurate DEM or DSM.

LiDAR makes reference to a system that uses electromagnetic radiation at optical frequencies of the visible and near infrared, being able to perform distance measurements.

Electromagnetic Spectrum

Working of LiDAR

The airborne LiDAR is an active system based on a sensor that transmits and receives a laser pulse. It can be installed on aeroplanes or helicopters. Nowadays, this technology is replacing classical photogrammetric methods for high-precision DSM, because it gets lower costs and shorter lead times, while accuracies of about 15 cm in elevation and higher density of point measurements are obtained.

The operation of the LiDAR system consists of a scanning sensor that emits laser pulses and measures the time these pulses take to reach the Earth՚s surface and to return up to the sensor. It has the same running than the distance measurement of a total topographic station.

In the plane, a GPS system is installed, so that position coordinates are calculated to increase the positioning accuracy of the instrument, a GPS device is also installed on the ground. After the flight, GPS data of the airline is combined with GPS on the ground, and accuracies of 5 cm are obtained in the instrument position. The inertial navigation system is also connected, as in a conventional photogrammetric plane.

Lidar Flight Disposition

All laser emissions are sent out in a unique direction, but there is a rotating mirror that deflects them to both sides of the plane with a configurable opening angle. In this way, a scan of a particular land strip is made, and its width depends on the flying height.

By combining all this information (distance measured with the laser, angle of the mirror that performs the scanning, exact position of the instrument with GPS and inertial navigation system) , we can obtain the measurement of the land՚s height with an accuracy of 15 cm in Z coordinate.

The different performances of the earth field depending on its characteristics can be distinguished. When the laser beam hits the ground, it acts differently.

Lidar Performance in Different Surface

In a solid surface (buildings, land, etc.) , the beam is reflected without any problem and then it goes back to the plane. On the water, the laser beam undergoes a specular reflection and it does not return to the plane so that no information is obtained, appearing a gap that must be interpolated from the heights of the surrounding surfaces or other available bathymetric data.

In the case of vegetation, the beam hits firstly with the top of the tree. At this point, part of the beam is reflected and goes back to the plane, but as it is a non-solid surface, another part of the beam passes through the vegetation until it reaches the ground and returns to the plane. The system saves the coordinates and heights of all the received pulses.

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