LasOverlap

[Anahita Khosravipour]

Hi Martin,

I used the lasOverlap.exe for quality checking of my LiDAR data.

but I can not interpret the result. it has only 2 colors (random white and blue).

maybe something wrong on my commend line:

lasoverlap -lof file_list.txt -merged -files_are_flightlines -step 1 -no_diff -o

dir "d:\LiDARprocessing\......\......." -o "me

rged.png"

I would appreciate if you could guide me.

Thanks

Anahita

[Martin Isenburg]

Hello Anahita,

an example use of lasoverlap.exe is described in this tutorial

http://rapidlasso.com/2013/04/20/tutorial-quality-checking/

it seems you are using LiDAR tiles as input and LiDAR tiles are not
LiDAR flightlines, hence the command '-files_are_flightlines' makes no
sense. What this command line argument does is that it marks the
points of each file as being a different flightline so that the
overlap can be computed where they overlap.

But you do not have flightlines but non-overlapping tiles and
therefore your overlap raster is mainly. It should really be
completely blue as there should not be any overlap between tiles.
However, it seems that that in your case there are points along the
tile boundaries that fall into the same 1 by 1 meter pixel giving you
this "aqua-colored" raster pattern. This should not happen for tilings
produced with lastile (unless you run an unlicensed version for some
LAStools operation where tiny xyz noise is added).

The only way you can use lasoverlap with tiles is *iff* your tiles -
that are derived from tiling the original flightlines - maintain the
flight line information. This can be achieved with LAStools by adding
the '-files_are_flightlines' option to lastile.exe as described in
this tutorial

http://rapidlasso.com/2013/10/13/tutorial-lidar-preparation/

You can check if you have flight line information by checking if the
point source ID of the points specifies the flightlines by running:

lasinfo -i tiles_final\fitch_271920_4714320.laz ^
-nh -nv -nmm ^
-histo point_source 1
point source id histogram with bin size 1
bin 1 has 566013
bin 2 has 395391
bin 3 has 122710
bin 4 has 274899
bin 5 has 845614
bin 7 has 366913
average point source id 3.74133

If this is the case then you can either run all into one output PNG with

lasoverlap -i tiles_final\fitch*.laz -merged ^
-step 1 ^
-no_diff ^
-o coverage.png

Or you can run each tile individually (possibly on multiple cores) with

mkdir quality
lasoverlap -i tiles_final\fitch*.laz -merged ^
-step 1 ^
-no_diff ^
-odir quality -opng ^
-cores 4

If your tiles do not have the point source id set to capture the
flightline information then you still may be able to recover it with
lassplit.exe assuming you have GPS time stamps. See the README of
lasplit.exe on how to split the LiDAR points back into the original
flight strips by binning them to different files based on carefully
chosen GPS time intervals ...

Regards,

Maritn @rapidlasso

--
http://rapidlasso.com - fast tools to cover check your LiDARs

[Martin Isenburg]

Hello,

let me summarize my long explanaition below with a simple rule: "Never
use the '-files_are_flightlines' option with LiDAR tiles."

Regards,

Martin