Free Lidar PDF manual book for overall idea on Lidar

Lidar can be defined as  the integration of three technologies into a single  system capable of acquiring data to produce  accurate digital elevation models (DEMs).  Discussion over Lidar and photogrammetry. These  technologies are lasers, the Global Positioning  System (GPS), and inertial navigation systems  (INS). A complete Lidar manual available for free download. Combined, they allow the positioning of  the footprint of a laser beam as it hits an object,  to a high degree of accuracy.  Lasers themselves are very accurate in their ranging capabilities, and can provide distances accurate to a few centimeters.  The accuracy limitations of LiDAR systems are due primarily to the  GPS and IMU (Inertial Measurement Unit) components.  As advancements in commercially available GPS and IMUs occur, it is becoming possible  to obtain a high degree of accuracy using LiDAR  from moving platforms such as aircraft.

A LiDAR system combines a single narrow-beam  laser with a receiver system.  The laser produces an optical pulse that is transmitted, reflected off  an object, and returned to the receiver.  The receiver accurately measures the travel time of the  pulse from its start to its return.  With the pulse travelling at the speed of light, the receiver senses  the return pulse before the next pulse is sent out.   Since the speed of light is known, the travel time  can be converted to a range measurement.   Combining the laser range, laser scan angle, laser position from GPS, and laser orientation from  INS, accurate x, y, z ground coordinates can be  calculated for each laser pulse.  Laser emission  rates can be anywhere from a few pulses per second to tens of thousands of pulses per second.  Thus, large volumes of points are collected.   For example, a laser emitting pulses at 10,000  times per second will record 600,000 points every  minute.   Typical raw laser point spacing on the  ground ranges from 2 to 4 meters.

Some LiDAR systems can record “multiple returns” from the same pulse.   In such systems the  beam may hit leaves at the top of tree canopy,  while part of the beam travels further and may  hit more leaves or branches.  Some of the beam  is then likely to hit the ground and be reflected  back, ending up with a set of recorded “multiple  returns” each having an x, y, z position.  This feature can be advantageous when the application  calls for elevations for not only the ground, but for  tree or building heights.   As surface types and characteristics vary and  change the laser beam’s reflectivity, then the  ability of the LiDAR to record the return signals  changes.  For example, a laser used for topographic applications will not penetrate water,  and in fact records very little data even for the  surface of the body of water.  Where the application calls for a laser to penetrate water to determine x, y, z positions of undersea features, then  a slightly different variation of LiDAR technology  is used.