Introduction to Discrete Return LiDAR

Compiled by:
Jeffrey S. Evans & Andrew T. Hudak (USDA Forest Service)
Alistair M.S. Smith (university of Idaho)

 Part I - Intro and Sampling
 

LiDAR Basics:

Light detection and Ranging (LiDAR), sometimes referred to as scanning laser altimetry, is a widespread technology that allows the relative height of surfaces to be measured from an aircraft or satellite sensor.         

The concept of LiDAR is simple, as it uses the simple relation that:

distance to an object = speed x time.

Essentially, the time for the light to travel to and from the target is then used to determine the distance.

The distance to the target and the position of the airplace is then used to determine the elevation and location.

The most important property of LiDAR is that unlike other remote sensing methods, which rely on inference and correlation (e.g. NDVI and LAI), LiDAR provides DIRECT measurements of relative height (or elevation). 

Return Density:

In LiDAR the footprint size decreases with increasing post-spacing and importantly the last return from a discrete return system is not always the ground.

LiDAR sensor systems vary in the number of returns from a surface. Below is an example of how many of the 1st, 2nd, and 3rd returns were obtained from within a pulse:

The LiDAR Intensity Image:

This is a commonly unused bi-product of a LiDAR acqusition and is the intensity of object that the laser pulse is striking. This is an uncalibrated 8-bit (0-255) image that is orthorectified as therfore can be used as an orthophoto. 

These images are not typically used in quantative analysis as the image gains are always set to 'the adaptive gain' setting when the images are acquired.

»»» Part 2 - Getting Heights »»»