Gridding data using user attribute

[Jo Bind]

This might be a very obvious one, but it has me stumped.

Despite having used LAStools for well over a decade, I don't work much with user attributes. But for a recent project, I calculated the height above ground, classified the point cloud into vegetation and buildings and then wanted to grid class 5 (high vegetation) using the height-above-ground values.

My workflow looked something like this:

lasheight -i *.laz -odir Classified -epsg 2193 -olaz -odix _hgt -store_as_extra_bytes

lasclassify -i Classified/*_hgt.laz  -ground_offset 0.3 -odir Classified -olaz -odix _0_3 -height_in_attribute 0
lasgrid -i Classified/*_0_3.laz  -keep_class 5 -attribute 0 -step 1 -average -otif

This worked just as expected with data from one supplier, but another dataset I tried to process was created by Terrascan, which includes a few other user attributes (in my case: Distance, Group, Normal) before (?) the height-above-ground attribute.

Below is the lasinfo result:

reporting all LAS header entries:
  file signature:             'LASF'
  file source ID:             0
  global_encoding:            0
  project ID GUID data 1-4:   00000000-0000-0000-0000-000000000000
  version major.minor:        1.4
  system identifier:          'LAStools (c) by rapidlasso GmbH'
  generating software:        'lasclassify (230330) commercial'
  file creation day/year:     286/2023
  header size:                375
  offset to point data:       2167
  number var. length records: 4
  point data format:          7
  point data record length:   59
  number of point records:    0
  number of points by return: 0 0 0 0 0
  scale factor x y z:         0.001 0.001 0.001
  offset x y z:               1000000 5000000 0
  min x y z:                  1982560.000 5675280.000 9.977
  max x y z:                  1982891.369 5675999.999 101.372
  start of waveform data packet record: 0
  start of first extended variable length record: 0
  number of extended_variable length records: 0
  extended number of point records: 6266815
  extended number of points by return: 6137996 114666 13359 771 23 0 0 0 0 0 0 0 0 0 0
variable length header record 1 of 4:
  reserved             0
  user ID              'LASF_Projection'
  record ID            34735
  length after header  40
  description          'by LAStools of rapidlasso GmbH'
    GeoKeyDirectoryTag version 1.1.0 number of keys 4
      key 1024 tiff_tag_location 0 count 1 value_offset 1 - GTModelTypeGeoKey: ModelTypeProjected
      key 3072 tiff_tag_location 0 count 1 value_offset 2193 - ProjectedCSTypeGeoKey: NZGD2000 / New Zealand Transverse Mercator 2000
      key 3076 tiff_tag_location 0 count 1 value_offset 9001 - ProjLinearUnitsGeoKey: Linear_Meter
      key 4099 tiff_tag_location 0 count 1 value_offset 9001 - VerticalUnitsGeoKey: Linear_Meter
variable length header record 2 of 4:
  reserved             0
  user ID              'LASF_Spec'
  record ID            4
  length after header  768
  description          'by LAStools of rapidlasso GmbH'
    Extra Byte Descriptions
      data type: 6 (long), name "Distance", description: "Distance", min: 2147483649, max: 2147483647, scale: 1, offset: 0 (not set)
      data type: 5 (unsigned long), name "Group", description: "Group", min: 0, max: 4294967295, scale: 1, offset: 0 (not set)
      data type: 5 (unsigned long), name "Normal", description: "Normal vector 2+15+15 bits", min: 0, max: 0, scale: 1, offset: 0 (not set)
      data type: 4 (short), name "height above ground", description: "vertical point to TIN distance", scale: 0.01, offset: 250
variable length header record 3 of 4:
  reserved             0
  user ID              'LASF_Spec'
  record ID            4
  length after header  192
  description          ''
    Extra Byte Descriptions
      data type: 1 (unsigned char), name "confidence", description: "confidence values", scale: 1 (not set), offset: 0 (not set)
variable length header record 4 of 4:
  reserved             0
  user ID              'LASF_Spec'
  record ID            4
  length after header  576
  description          'TerraScan Extra Bytes'
    Extra Byte Descriptions
      data type: 6 (long), name "Distance", description: "Distance", min: 2147483649, max: 2147483647, scale: 1, offset: 0 (not set)
      data type: 5 (unsigned long), name "Group", description: "Group", min: 0, max: 4294967295, scale: 1, offset: 0 (not set)
      data type: 5 (unsigned long), name "Normal", description: "Normal vector 2+15+15 bits", min: 0, max: 0, scale: 1, offset: 0 (not set)
LASzip compression (version 3.4r3 c3 50000): POINT14 3 RGB14 3 BYTE14 3
reporting minimum and maximum for all LAS point record entries ...
  X           982560000  982891369
  Y           675280000  675999999
  Z                9977     101372
  intensity           6       1625
  return_number       1          5
  number_of_returns   1          5
  edge_of_flight_line 0          1
  scan_direction_flag 0          0
  classification      1         31
  scan_angle_rank   -41         38
  user_data           0          0
  point_source_ID  3501       3601
  gps_time 351317.008705 351518.726303
  Color R 2560 65280
        G 4352 65280
        B 0 65024
  extended_return_number          1      5
  extended_number_of_returns      1      5
  extended_classification         1     31
  extended_scan_angle         -6833   6333
  extended_scanner_channel        0      0
  attribute0          6       1625  ('Distance')
  attribute1          0 4.29497e+009  ('Group')
  attribute2          0          0  ('Normal')
number of first returns:        6137996
number of intermediate returns: 14143
number of last returns:         6138281
number of single returns:       6023605
overview over extended number of returns of given pulse: 6023605 202375 37730 2990 115 0 0 0 0 0 0 0 0 0 0
histogram of classification of points:
           38503  unclassified (1)
           11019  ground (2)
          333127  high vegetation (5)
         5884145  building (6)
              20  noise (7)
               1  Reserved for ASPRS Definition (31)

With these files, the result of lasgrid produced unexpected results. So, I started playing with the -attribute 0  flag in lasgrid and couldn't get anywhere (I tried every position from 0 to 9 without success). How do I grid the height-above-ground values for this dataset?

A (cumbersome and not batch-able) way is to import them into CloudCompare and then only export the desired attribute, but that's not the most efficient way to do it. But I couldn't figure out how to address the right attribute for gridding. Any ideas that would help?

Cheers,

Jo

[Jochen Rapidlasso]

Hi Jo,

within your extra_bytes_vlr the height attribute seems to be the 4th one - index starting with 0 - so please try

-attribute 3

as argument. Optional check out the new alternate_z mechanism which should simplify the process.

Cheers,

Jochen @rapidlasso

[Jochen Bind]

Hi Jochen,

That was exactly what my thinking was. But somehow, that didn’t work.

 

Below is the output from

las2txt -parse xyz0123 -i BF44_1000_1935_hgt_0_3.laz -stdout | more

WARNING: attribute '3' does not exist. skipping ...

2060320.023 5752320.299 157.859 1014 327157 0 1014 1014

2060320.042 5752320.005 158.009 1035 523786 0 1035 1035

2060320.037 5752320.014 158.016 1011 589298 0 1011 1011

2060320.023 5752320.090 158.002 1051 523802 0 1051 1051

 

When I prepare a ‘cleaned’ file with CloudCompare (cleaned in that I only export the height-above-ground attribute, deselecting the other attributes in the saving procedure) and run this command, I get the expected results:

las2txt -parse xyz0 -i BF44_1000_1935_hgt_0_3_Jo.laz -stdout | more

2060320.023 5752320.299 157.859 -0.00

2060320.042 5752320.005 158.009 0.14

2060320.037 5752320.014 158.016 0.14

2060320.023 5752320.090 158.002 0.13

2060320.123 5752322.626 157.683 -0.00

2060320.042 5752322.766 157.704 -0.00

2060320.067 5752320.888 157.936 0.15

2060320.093 5752321.233 157.737 -0.00

 

The Warning in the first instance suggests to me that the height-above-ground is not saved as the 4^th attribute in this file, but where is it? I used the same file (BF44_1000_1935_hgt_0_3.laz) to open in CloudCompare and export it as BF44_1000_1935_hgt_0_3_Jo.laz. It really confuses me and I have no idea what’s happening.

 

Cheers,

Jo

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Jochen Bind Hydrodynamics and GIS Technician +64-3-343-8021 +64-21-087-54696 National Institute of Water & Atmospheric Research Ltd (NIWA) 10 Kyle Street Riccarton Christchurch New Zealand Connect with NIWA: niwa.co.nz Facebook LinkedIn Twitter Instagram YouTube

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[Jochen Rapidlasso]

Hi Jo,

do you have a small data sample for us? We would like to analyze this.

Thanks,

Jochen @rapidlasso