change las 1.4 to las 1.2

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Knoor, Claudia

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Jul 11, 2018, 8:57:51 AM7/11/18
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Hello all,

 

I would like to “downgrade” my data from v1.4 pdrf 6  to v1.2, pdrf 1.

Changing the LAS-Version I get some warnings because V1.2 does not accept return numbers > 5:

“WARNING: there are 30038 points with return number 6”

“WARNING: there are 4178 points with return number 7”

 

Should I accept these warnings because they are obviously correct, or is it better to adjust the data somehow?

 

Thank you for your help

Claudia

 

 

I used:

las2las -i "dat.laz" -set_version_major 1 -set_version_minor 2 -set_point_type 1 -odix "_v121" -olaz

 

Header Version 1.4:

lasinfo (180620) report for 'D:\Dknoor\DGM\befliegung\2018\beispieldatensatz\dat.laz'

reporting all LAS header entries:

  file signature:             'LASF'

  file source ID:             0

  global_encoding:            1

  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:        'las2las (version 180620)'

  file creation day/year:     255/2017

  header size:                375

  offset to point data:       377

  number var. length records: 0

  point data format:          1

  point data record length:   28

  number of point records:    11839103

  number of points by return: 7873785 2208808 1115925 465060 141309

  scale factor x y z:         0.01 0.01 0.01

  offset x y z:               0 5000000 0

  min x y z:                  385000.00 5648000.00 110.15

  max x y z:                  385999.99 5648999.99 245.12

  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: 11839103

  extended number of points by return: 7873785 2208808 1115925 465060 141309 30038 4178 0 0 0 0 0 0 0 0

the header is followed by 2 user-defined bytes

LASzip compression (version 3.2r4 c2 50000): POINT10 2 GPSTIME11 2

reporting minimum and maximum for all LAS point record entries ...

  X            38500000   38599999

  Y            64800000   64899999

  Z               11015      24512

  intensity           6        397

  return_number       1          7

  number_of_returns   1          7

  edge_of_flight_line 0          1

  scan_direction_flag 0          1

  classification      0          0

  scan_angle_rank   -24         24

  user_data           0        251

  point_source_ID  1274       2015

  gps_time 165075151.591076 174642822.121647

number of first returns:        7873785

number of intermediate returns: 1770949

number of last returns:         7841986

number of single returns:       5647953

overview over extended number of returns of given pulse: 5647953 2178388 1958546 1305717 562352 156935 29212 0 0 0 0 0 0 0 0

histogram of classification of points:

        11839103  never classified (0)

 

 

Header Version 1.2:

lasinfo (180620) report for 'E:\0_las\2018\testdaten\version\dat_v121.laz'

reporting all LAS header entries:

  file signature:             'LASF'

  file source ID:             0

  global_encoding:            1

  project ID GUID data 1-4:   00000000-0000-0000-0000-000000000000

  version major.minor:        1.2

  system identifier:          'LAStools (c) by rapidlasso GmbH'

  generating software:        'las2las (version 180620)'

  file creation day/year:     255/2017

  header size:                227

  offset to point data:       229

  number var. length records: 0

  point data format:          1

  point data record length:   28

  number of point records:    11839103

  number of points by return: 7873785 2208808 1115925 465060 141309

  scale factor x y z:         0.01 0.01 0.01

  offset x y z:               0 5000000 0

  min x y z:                  385000.00 5648000.00 110.15

  max x y z:                  385999.99 5648999.99 245.12

the header is followed by 2 user-defined bytes

LASzip compression (version 3.2r4 c2 50000): POINT10 2 GPSTIME11 2

reporting minimum and maximum for all LAS point record entries ...

  X            38500000   38599999

  Y            64800000   64899999

  Z               11015      24512

  intensity           6        397

  return_number       1          7

  number_of_returns   1          7

  edge_of_flight_line 0          1

  scan_direction_flag 0          1

  classification      0          0

  scan_angle_rank   -24         24

  user_data           0        251

  point_source_ID  1274       2015

  gps_time 165075151.591076 174642822.121647

number of first returns:        7873785

number of intermediate returns: 1770949

number of last returns:         7841986

number of single returns:       5647953

WARNING: there are 30038 points with return number 6

WARNING: there are 4178 points with return number 7

overview over number of returns of given pulse: 5647953 2178388 1958546 1305717 562352 156935 29212

histogram of classification of points:

        11839103  never classified (0)

 

 

 

 

 

Evon Silvia

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Jul 11, 2018, 5:51:31 PM7/11/18
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LAS 1.2 does support 6th and 7th returns, but for reasons beyond my understanding the LAS 1.2 header only provides return counts up to the 5th return. lasinfo is warning you that the tallies by return in the header don't include all of the points. As long as you're aware of that, you're fine. No actual data loss has occurred.

Evon

Lewis Graham

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Jul 12, 2018, 2:13:27 AM7/12/18
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For some reason in the original LAS format (before ASPRS), I guess five was thought to be more returns than any LIDAR would ever produce.

The array in the header (up to LAS 1.4) was defined as 5 longs.  No one on the LAS committee wanted to “break” the header by adding two additional longs.

 

 

Lewis Graham

GeoCue Group Inc.

9668 Madison Blvd., Suite 202

Madison, AL 35758 USA

256-461-8289

www.geocue.com

Software tools for LIDAR &  Drone Mapping

Bespoke Enterprise Workflow Solutions

Drone Mapping Services

Technical Consulting

 

From: last...@googlegroups.com <last...@googlegroups.com> On Behalf Of Evon Silvia
Sent: Wednesday, July 11, 2018 4:50 PM
To: last...@googlegroups.com
Subject: Re: [LAStools] change las 1.4 to las 1.2

 

LAS 1.2 does support 6th and 7th returns, but for reasons beyond my understanding the LAS 1.2 header only provides return counts up to the 5th return. lasinfo is warning you that the tallies by return in the header don't include all of the points. As long as you're aware of that, you're fine. No actual data loss has occurred.

 

Evon

Martin Isenburg

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Jul 12, 2018, 4:42:04 AM7/12/18
to LAStools - efficient command line tools for LIDAR processing
Hello,

you can ignore those WARNNGs, Claudia. In fact even though you have a LAS 1.4 file it really already has the contents of a LAS 1.2 file - at least according to your lasinfo report - because the stored point type is 1 with a size of 28 bytes. This point type does not need to be stored in a LAS 1.4 file. So when you do the las2las command 

las2las -i dat.laz -set_version 1.2 -set_point_type 1 -odix _v121 -olaz

nothing, absolutely nothing, happens to the points. There is no actual downgrading happening. They are

  point data format:          1
  point data record length:   28

in the 'dat.laz' file before the conversion and they are 

  point data format:          1
  point data record length:   28

in the 'dat _v121.laz' file after the conversion. The only thing that actually happened was that the LAS header went from

  header size:                375
  offset to point data:       377
  point data format:          1
  point data record length:   28
  number of point records:    11839103
  number of points by return: 7873785 2208808 1115925 465060 141309
  extended number of point records: 11839103
  extended number of points by return: 7873785 2208808 1115925 465060 141309 30038 4178 0 0 0 0 0 0 0 0

to

  header size:                227
  offset to point data:       229
  point data format:          1
  point data record length:   28
  number of point records:    11839103
  number of points by return: 7873785 2208808 1115925 465060 141309

so that the two entries 30038 (number of 6th returns in the file) and 4178 ( number of 7th return  in the file) from the LAS return number histogram are "missing" in the LAS header. Instead two WARNINGs inform about these two numbers.

These WARNINGs exist because when I was implementing lasinfo I tried to warn about anything that was not quiet according to the specification and I believe that the wording of the LAS 1.0 to 1.3 specifications is such that the "return number" fields and the "number of returns" fields of point types 0 through 5 are only supposed to store a number between 1 and 5. However, these 3 bit fields can also hold the values 0, 6 and 7. As modern scanning systems became able to extract more and more returns the arbitrary limitation to the range from 1 through 5 seemed silly given that a 1 through 7 range is supported by a 3 bit field. However, the histograms in the LAS header that hold the distribution of return numbers in a LAS file still reflect the original decision that "five returns is probably more than you ever need". However, who ever uses those histograms for anything anyways? I mainly use them to see whether the contents of a file are LiDAR point clouds or other point clouds (that do not have return information).

I mentioned it a few times but never got confirmation. I believe it was Paul Galla of Leica Geosystems who suggested the LAS header to offer space for an incredible number of up to 5 returns because at the time the format was designed the newest airborne LiDAR system from Leica was able to (occasionally) produce a whopping 5 returns. One has to keep in mind that LiDAR scanners started producing only one return, then two, then Optech had up to 4 and Leica then up to 5. It was not until RIEGL pushed those boundaries by making the entire production of returns a fully digital process via waveform digitization and processing that more than 5 returns were possible. But I believe now also Optech has a system that can produce up to 8 returns (without waveform processing)?

Does anyone have a copy of all of the messages from the yahoo forum that documented the LAS format development process from 2000 through to 2004 before it was moved to the (in-)famous CLICK BB of the USGS that was then taken down in the name of US homeland security or something like that ... (-:

I had once copied and pasted a few of the old messages that happened long before I started working with LiDAR:


Regards,

Martin
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