Multiple Channel Lidar

Several new lidars use multiple channels, or frequencies.   There are separate lasers, and while they are closely aligned, they each follow a different path, have different locations for the returns, and may have different numbers of returns for the same pulse.  Two of the laser channels will be 532 and 1064 nm, those commonly used for bathymetric (532, green) and topographic surveys (1064, NIR), and the third could be 1550 nm.

Water penetration is one of the big reasons for using three channels.  In this survey in Florida across a coastal barrier island, there are two areas of open water, seen in the green lines on the left side of the profile.  This is the green lidar (532 nm), and is the only one of the three to penetrate into the water.  In the left channel, there are some reflections from the surface with the two NIR lasers, but many fewer in the right channel where the water may have acted as a specular reflector for non-nadir returns.

 

Profiles from two lidar channels:

  • Top is the green channel, 532 nm.  There are reflections from both the surface, and the bottom of the water.
  • Bottom is one NIR channle, 1064 nm.  There are only reflections from the water surface.

In both cases, this region is probably near nadir, to allow reflections to the instrument directly overhead. 

  
Returns colored by channel in the lidar.

Note the linear tracks for each channel, and the separation between them on the ground.  This is an area in coastal swamps in Florida, and in much of this small area, there is only one return per channel for each pulse.

Returns colored by the return number, all three channels intermingled.

  • Green-first
  • Red and Blue subsequent

 

Compare the pattern of returns from the three channel lidar, with that from repeat surveys with conventional lidar.

Three surveys at different times in New Mexico.

Note the almost random pattern, and that the surveys are clearly not sampling the same location. 
View of a single survey from those show above, showing the linear scanning pattern.

The lines of points are not parallel, and the density varies on the very fine scale.

The area on the center left has vegetation, and note the resulting pattern hides the linear trace of the laser because returns come from multiple positions with the canopy, and becasue the scan was away from nadir, this shifts the projections of the returns on the ground.

 

 

Two surveys in Pennsylvania, two months apart.

They are roughly parallel,  but have different patterns in the density over small regions.

The western portion of the area has water, and the survey shown in red had essentially no returns, while the survey shown in green had returns, but a lower density than on the region to the right.  This probably represents different scan angles, and slightly different flight paths.

 


last revision 1/20/2018