[Liblas-devel] Liblas-devel Digest, Vol 79, Issue 1

[Alexandre Guyot]

Hi Martin,
I've actually add a look at this user_data field but have a single value of '2' for all my points when reading with libLAS while the HEIGHT_ABOVE_GROUND attribute showing in GlobalMapper is ranging from -0.002 to 33.909.
Cheers,
Alex
  

      De : Martin Isenburg <martin.isenburg at gmail.com>
 À : Alexandre Guyot <alexguyot at yahoo.fr> 
Cc : Howard Butler <howard at hobu.co>; "Van Dyke, Daryl" <daryl_van_dyke at fws.gov>; "Liblas-devel at lists.osgeo.org" <liblas-devel at lists.osgeo.org>
 Envoyé le : Mercredi 9 novembre 2016 22h30
 Objet : Re: [Liblas-devel] Liblas-devel Digest, Vol 79, Issue 1
   
Hi,
in this case you are indeed just be storing HAG as a quantized value in decimeters (or decifeet) in the 8 bit unsigned user_data field that ranges from 0 to 255 (e.g. 0 to 25.5 meters or feet). See the README file of lasheight for details.
http://lastools.org/download/ lasheight_README.txt

Furthermore you seem then to merely visualize the user_data field in Global Mapper and not a dedicated HAG field.
Regards,
Martin
On Wed, Nov 9, 2016 at 9:55 PM, Alexandre Guyot <alexguyot at yahoo.fr> wrote:

Hi,
Thanks for your help.Actually, I havent' specified any switch in lasheight.

My file is in LAS 1.2 and effectively has the HAG value as it can be read by some commercial software such as GlobalMapper (as "HEIGHT_ABOVE_GROUND" attribute).
Is it therefore stored somewhere else than in extra-bytes or was LAS 1.2 already using extra-bytes ?
Thanks,Alex
  

      De : Howard Butler <howard at hobu.co>
 À : Martin Isenburg <martin.isenburg at gmail.com> 
Cc : "Van Dyke, Daryl" <daryl_van_dyke at fws.gov>; "Liblas-devel at lists.osgeo.org" <liblas-devel at lists.osgeo.org>
 Envoyé le : Mercredi 9 novembre 2016 21h29
 Objet : Re: [Liblas-devel] Liblas-devel Digest, Vol 79, Issue 1
  

> On Nov 9, 2016, at 2:20 PM, Martin Isenburg <martin.isenburg at gmail.com> wrote:
> 
> Hi,
> 
> I believe that Alex talks about the HAG stored as an additional point atttribute to an "Extra Bytes" field via the '-store_as_extra_bytes' switch.
> 
> http://lastools.org/download/ lasheight_README.txt
> 
> There was a discussion on this in the LAStools user forum whether cm resolution with a scale factor of 0.01 and a 16 bits signed integer storage are enough for most trees.
> 
> http://groups.google.com/ forum/#!searchin/lastools/ lasheight$20store_as_extra_ bytes
> 
> "Extra Bytes" fields are described in the LAS 1.4 specification and are therefore also more and more readable in other software.

All of this is accurate information.

Additionally, libLAS does not support LAS 1.4 or support writing LAS 1.4 extra bytes descriptor VLRs. You'll need to use LAStools or PDAL or LASpy to achieve this task if open source is a priority for you.

Howard
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