LAS overlap problem

[Aija Zans]

Hi,

I am very new to using LAS datasets in ArcMap 10.4.1. so I apologize for stupid questions/problems.

My overall goal is to produce a DEM of relatively high precision with 3 given LAS files, to use in ArcMap 10.4.1, so that "you can click on any area and get precise x,y,z info for that area" and to then be able to combine that with other data, such as bathymetry. The area is a sandbar that is usually below sea level, except during low tide conditions.

I am having problems merging the slightly overlapping LAS tiles. The coordinates are correct (or at least they show up in Arc in the correct area - Coordinate system OSGB1936) for all of the tiles, but the classification and z values seem to be way off. (attached images 1 and 2).
How can I fix this? How can I check whether the original data is correct for the coordinates for example?

Like I said, I am very new to this. I don't know what other information needs saying for anyone to help at all.

[Tobias K Kohoutek]

Seems like your LiDAR files aren't adjusted. You could use free available CloudCompare, if you do not have any other software, to adjust the files. 

Once adjusted the tool to use is then las2dem.

Cheers,

Tobias

[Martin Isenburg]

Hello Aija,

as Tobias pointed out it seems that your different LiDAR files are not matching each other. What is the source of the three different files? Is each file from a different flight line / trajectory? In this case you could use lasoverlap to create a visualization "from above" how bad the miss-alignment is:

http://rapidlasso.com/lasoverlap

http://lastools.org/download/lasoverlap_README.txt

This is the call when each file is a separate flightline / trajectory:

lasoverlap -i flightlines\*.laz -faf -merged ^

                 -step 2 -min_diff 0.1 -max_diff 0.4 ^

                 -odir quality -o overlap.png

This is the call when each file is a tile (containing points from multiple flight lines) *with* flightline numbers stored in the point source ID field of each point:

lasoverlap -i flightlines\*.laz ^

                 -step 2 -min_diff 0.1 -max_diff 0.4 ^

                 -odir quality -opng ^

                 -cores 4

This is the call when each file is a tile (containing points from multiple flight lines) *without* flightline numbers stored in the point source ID field of each point but with proper GPS time stamps

lasoverlap -i flightlines\*.laz ^

                 -step 2 -min_diff 0.1 -max_diff 0.4 ^

                 -recover_flightlines ^

                 -odir quality -opng ^

                 -cores 4

Each time '-step 2' means that the calculation of miss-alignment is done per 2 by 2 meter area and that elevation differences until 0.1 meter are okay (colored white) but elevation differences above 0.4 meter colored deep red / blue and elevation differences between 0.1 and 0.4 receive a shading from white to red/blue.

Regards,

Martin

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