Do you know about LIDAR?

Nov 11, 2022

LIDAR is a radar system that detects the position, velocity and other characteristic quantities of a target by emitting a laser beam. It works by transmitting a detection signal (laser beam) to the target, and then comparing the received signal reflected back from the target (target echo) with the transmitted signal and making appropriate processing to obtain relevant information about the target, such as target distance, bearing, altitude, speed, attitude, even shape and other parameters, so as to detect, track and identify targets such as aircraft and missiles. It consists of laser transmitter, optical receiver, turntable and information processing system, etc. The laser turns electrical pulses into optical pulses and transmits them, and the optical receiver then reduces the optical pulses reflected back from the target into electrical pulses and sends them to the display.

Why is LIDAR Important?

Compared to traditional measurements, LiDAR sensors collect more detailed and highly accurate elevation points - faster. In addition to speed and accuracy, other factors that make LiDAR important include:

It is an active sensor, which means data can be collected during the day or night.

In dense forests, unlike photogrammetry, LiDAR pulses can travel through tight spaces to reach the ground. Therefore, it is important to map the forest floor.

Compared to conventional measurements, LiDAR sensors measure surface points continuously, resulting in a more uniform height model.

Applications of LiDAR digital terrain models include canopy modeling, hydrology, coastal engineering, and building deformation monitoring.

For LiDAR, there are two accuracy specifications: absolute accuracy and relative accuracy. 

Absolute LiDAR Accuracy 

Absolute LiDAR accuracy refers to the horizontal and vertical accuracy of LiDAR data. Absolute accuracy is evaluated by comparing LiDAR data with ground measurement checkpoints.

Horizontal checkpoints are well-defined points/features that are visible on the ground. Their horizontal position is measured precisely with respect to a reference geodetic datum.

On the other hand, vertical checkpoints do not need to be visible points. They are points that are measured on flat, open terrain. This minimizes interpolation errors when comparing checkpoint elevations with elevations interpolated from the dataset. For the LiDAR dataset, the vertical accuracy achieved on flat and open terrain is referred to as the "base" vertical accuracy.

While there is no specific method for determining the appropriate checkpoint distribution, it usually depends on the land cover type and terrain. The new ASPRS 2014 standard provides specific recommendations on the density and distribution of checkpoints.

Relative LiDAR Accuracy

Relative LiDAR accuracy refers to the internal quality of LiDAR elevation data without using measured ground control points. Relative accuracy is a measure of local differences between points in a point cloud. It is influenced by the calibration of the LiDAR system. Relative accuracy is assessed in two ways.

Intra-band accuracy assessment: Data collected within the same band or flight path are evaluated. It indicates the stability of the LiDAR system.

Inter-strip accuracy assessment: assesses data collected between strips/adjacent routes. It involves comparing the overlap in adjacent strips.

A "good" relative accuracy means that the individual points in your point cloud correlate with the entire point cloud. Relative accuracy is especially important for applications such as slope and grade direction based on the elevation of points adjacent to each other.

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