[GRASS-user] [LAStools] Computing for the Density

[Martin Isenburg]

Hello Charm,

great question. Since you are expecting around 2 pulses per square meter and since you are using a step of 5 which equals 5m by 5m = 25 m^2 and since you are using the last only (or first only) retirn filter to count only one return per pulse you should expect to get values around 50 in the grid cells.

You will get - possibly much - smaller values whenever the strip does not fully cover the grid cell. It is possible that just one pulse hits one corner of a grid cell.

You will get - possibly much - larger values when the sampling on the ground gets condensed (but also streched out) because of strong pitch variations of a small aircraft in turbulences. But the more "systematic" larger values come from hardware that use a zig-zag scanner to scan. As the oszillating mirror slows down to change direction at the scan line edges you tend to get much much denser sampling there ... somewhat unrepresentative for the average sampling so it is important for zig zag scanners not to count the density of points along the edges when computing averages. One way of "not" counting them is to use a filter on the scan angle. For example

lasinfo -i sample.laz ^
           -compute_density ^
           -drop_abs_scan_angle_above 18

or

lasgrid -i sample.laz ^
           -density -step 5 ^
           -drop_scan_angle -20 20 ^
           -o density_25sqrmtr.asc

could do the trick.

Another issue ... if you density ranges up to 255 then higher values may be cut off and you should use '-density_16bit' instead of '-density' and use wider counters (16 bit instead of 8 bit).

Regards,

Martin                                  

On Jun 17, 2013, at 3:39, Charmaine Cruz <charmaineasiscruz at gmail.com> wrote:

> Hi Martin and everyone,
> 
> I would like to ask a question about lastools.
> My task is to compute for the density of LiDAR points for every flight.
> From my previous report, I used lasgrid to compute for the actual values of  density  (step is 5) and the output is number of points per 25 sqm, and it ranges from 1-255. It seems that I get higher value of density than expected.
> 
> What is the importance of considering the value of scan angle in computing for the density?
> 
> Thank you very much.. :)
> 
> -- 
> Engr. Charmaine A. Cruz
> Research Associate
> Disaster Risk and Exposure Assessment for Mitigation (DREAM) Program
> National Engineering Center
> Alfredo Junio Hall
> University of the Philippines Campus
> Diliman, Quezon City 1110
> +639159383637
> 
> 
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