DTM on 1.6GB.laz: LASGround Ok but BLAS2DEM with gaps

[Rafael Walter de Albuquerque]

Hi, LASTools community.

I need to get a DTM from a 1.6GB *.laz photogrammetry file. I'm following this tutorial: https://rapidlasso.com/2014/08/25/processing-large-dense-matching-point-clouds/

With 40 meters tiles and 20 meters buffer (LASTile), I got a good result on LASGround (ground points print is attached). 

The next step performed on BLAS2DEM is not ok, once it generates no data pixels in the middle of the study area and also loses spatial projection (UTM 23S). The print of BLAS2DEM inconsistent results are also attached.

I've tried many different parameters in all the tutorial's steps, but unfortunately I can't realize what's going wrong after the LASGround step. Could anyone please share some clue on what is going wrong on the DTM procedure? Why do I get good results on LASGround but cannot convert it correctly to *.bil or *.asc files on BLAS2DEM?

If you need more information please let me know.

Regards,

Rafael Albuquerque

[Michael Stimson]

Last time I heard BLAS2DEM only supported 2^32 (4294967296) input points. The way I got around this is to tile into smaller blocks with a 1 tile overlap, convert to DEM, trim the one tile overlap off the raster and then merge the rasters.

 

Michael Stimson Senior GIS Analyst Australia Asia Pacific Level 4 HQ South 520 Wickham St, Fortitude Valley, QLD, Australia, 4006 PO Box 1559, Fortitude Valley, QLD 4006. D:    +61 7 3539 9694 T:    +61 7 3539 9500 F:    +61 7 3539 9501 E:    Michael...@rpsgroup.com.au W:    rpsgroup.com.au

 

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[Martin Isenburg]

Hello,

if you have a LASzip-compressed LAZ file of size 1.6 GB then there is a chance that you have more points that even blast2dem can handle. In general you have create a more efficient workflow with tiling (with buffers!!), multi-core rasterization, and then mosaicing the resulting DEM tiles if needed. Could you run a lasinfo report [1] on your file and report the result?

[1] http://rapidlasso.com/lasinfo

In general I discourage folks from generating such gigantic LAS / LAZ files. Keeping the data as a folder of smaller files (usually tiles) is just as efficient (from a LAStools perspective at least) but makes the data easier to handle, allows faster AOI queries, and is also safer in case in terms bit-flips or any other data corruption issues.

Regards.

Martin

[Rafael Walter de Albuquerque]

Hi, LASTools community

Thank you, Michael and Martin, for your responses.

Martin, please find attached lasinfo reports of three files: the initial LAZ obtained by laszip, the lasground result that is ok and the blas2dem result that contains gaps.

Meanwhile I'll try some workflow with lasnoise and lasthin. I'm afraid I cannot work with tiles smaller than 40 meters with 20 meters of buffer because lasground results are not ok when I reduce these values.

Best regards,

Rafael Albuquerque

[Michael Stimson]

If you’re trying to make a DEM there is only 241421410 class 2 records reported in LasGround_Ok.txt, if you use LAS2LAS with -keep_class 2 to generate a ground only file which would be suitable fodder for BLAS2DEM (< 2^32 records).

Michael Stimson Senior GIS Analyst Australia Asia Pacific Level 4 HQ South 520 Wickham St, Fortitude Valley, QLD, Australia, 4006 PO Box 1559, Fortitude Valley, QLD 4006. D:    +61 7 3539 9694 T:    +61 7 3539 9500 F:    +61 7 3539 9501 E:    Michael...@rpsgroup.com.au W:    rpsgroup.com.au

 

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[Rafael Walter de Albuquerque]

Thanks, Michael.

Now it was possible to conclude DTM process. When using LAS2LAS  with keep_class 2 and then BLAS2DEM with merged command things were ok!

Best regards,

Rafael Albuquerque