DTM creation

[Mauro Assis]

Hi,

I'm trying to create a DTM for a dense forested area using LASTools. The points density is around 6 points/m^2.

The lasinfo result for the original cloud is:

lasinfo (160921) report for ..\..\clouds\NP_T-142.las
reporting all LAS header entries:
  file signature:             'LASF'
  file source ID:             142
  global_encoding:            0
  project ID GUID data 1-4:   6BF19841-3551-4AA3-D3B2-DEEC04E72183
  version major.minor:        1.2
  system identifier:          'DISCRETE'
  generating software:        'Global Mapper'
  file creation day/year:     160/2016
  header size:                227
  offset to point data:       447
  number var. length records: 2
  point data format:          1
  point data record length:   28
  number of point records:    50186154
  number of points by return: 30384079 13968723 4576441 1055718 201193
  scale factor x y z:         0.001 0.001 0.001
  offset x y z:               772377.0419896394 9459698.481987901 0
  min x y z:                  772380.118 9459698.593 189.868
  max x y z:                  778600.473 9471143.958 441.255
number of first returns:        30384079
number of intermediate returns: 5852723
number of last returns:         30307272
number of single returns:       16357920

I ran:

lasground -i ..\..\clouds\np_t-412.las -o .\classground_np_t-412.las

blast2dem -keep_class 2 -gray  -i classground_NP_T-412.las -o MDT_NP_T-412.asc

When I open ASC MDT in QGIS it shows in legend that the minimun height value is -126.98 and the maximum is 125.75. You can see in lasinfo output abouve that the min z and maz z are 189.868, 441.255. I checked the ASC file and the -126 values are there.

Also, the image that appears looks has to black regions that seems to me to indicate a deep declivity variation, what I can't see if I filter the class 2 points using las2las and see the cloud using a LAS viewer (I put an image attached showing what I'm seeing in QGIS).

What am I doing wrong?

Tks in advance,

Mauro Assis

[Martin Isenburg]

Hello Mauro,

you are using the option '-gray' and that turns the range of elevations into a grey value between 0 and 255 which - in your case - seems to get interpreted as a value from -128 to 127. Simply try running this one without using the option '-gray' instead:

blast2dem -i classground_NP_T-412.las ^

                 -keep_class 2 ^
                 -o MDT_NP_T-412.asc

Regards,

Martin @rapidlasso

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