This will not work, I am afraid.
The laser beam bends when refracted at a (e.g. water) surface. The first order “wave walker” described in the LAS 1.4 specification handles this case (i.e. – the wave walker is local).
Regards,
Lewis
Lewis Graham
AirGon LLC, small UAS Solutions
GeoCue Group
9668 Madison Blvd., Suite 202
Madison, AL USA 35758
01-256-461-8289
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This is the new paragraph I plan to propose:
dx, dy, dz: A local vector that specifies for each return the direction from where the laser light originates and the distance it traveled in picoseconds. Multiplying these local vectors with p picoseconds and adding them to the position x,y,z of the corresponding return allows traversing the sampled waveform through space. The position P of the waveform p picoseconds before its return is given by:
Px = x + p * dx
Py = y + p * dy
Pz = z + p * dz
The local vectors of all returns of a pulse are identical if the laser light travels a straight line, which is a valid assumption for most topographic surveys. The illustration in Figure OLD (not attached) gives a simple example of how three different returns of a laser pulse relate to their digitized waveform.
However, in bathymetric surveys the laser beam gets refracted as it hits the water surface. Here the traversal of the sampled waveform needs to be done in segments. Traverse only in direction of the local vector from the first return to the first waveform samples, traverse in direction of the local vector from the second to the first return and no farther, traverse only in direction of the local vector from the third to the second return and no farther, ... etc. Only the traversal from the last return to the last waveform sample can go in the opposite direction of the local vector as illustrated in Figure NEW (attached).
The units of dx, dy, and dz are per picosecond and identical to the units of the x, y, and z coordinates of the return.
dx, dy, dz: A delta vector that specifies for the associated return the direction from where the laser light originated and the distance it traveled in picoseconds. Multiplying these delta vectors with p picoseconds and adding them to the position x,y,z of their corresponding return allows traversing the sampled waveform through space. The position P of the waveform p picoseconds before its return is given by:
Px = x + p * dx
Py = y + p * dy
Pz = z + p * dz
The delta vectors of all returns of one pulse remain constant if the laser light travels a straight line, which is a valid assumption for most topographic surveys. The illustration in Figure CONSTANT (attached) gives a simple example of how three different returns of a laser pulse relate to their digitized waveform.
In a bathymetric survey the laser beam may get refracted when it hits the water surface and change both direction and speed. By storing changes in the delta vector with each return it is possible to store the result of a refraction calculation to the LAS file. In this case the traversal of the sampled waveform needs to be done in segments as illustrated in Figure CHANGING (attached):
* use the delta vector of the first return to traverse from the first return to the the start of the waveform sampling,
* use the delta vector of the second return to traverse from the second to the first return,
* use the delta vector of the third return to traverse from the third to the second return,
* ...
* only the reversed delta vector of the last return to traverse from the last return to the end of the waveform sampling.
The units of dx, dy, and dz are per picosecond and identical to the units of the x, y, and z coordinates of the return.
That said, the graphics and language surprised me with the implied sign of dx/dy/dz. I would have expected the dx/dy/dz values to be positive along-pulse – i.e., from the origin (sensor) and toward the points. Is that not the case?