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<TITLE>Re: [Liblas-devel] Vertical Datum Transformation</TITLE>
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<FONT FACE="Verdana, Helvetica, Arial"><SPAN STYLE='font-size:12.0px'>Frank,<BR>
<BR>
The work you have done to incorporate vertical datum support is really great!<BR>
<BR>
Is it possible for users to incorporate their own vertical datum descriptions beyond what is supported by default.<BR>
<BR>
Dave<BR>
<BR>
<BR>
On 10/29/10 5:31 PM, "Frank Warmerdam" <warmerdam@pobox.com> wrote:<BR>
<BR>
</SPAN></FONT><BLOCKQUOTE><FONT FACE="Verdana, Helvetica, Arial"><SPAN STYLE='font-size:12.0px'>Ibraheem Ali wrote:<BR>
> Dear All,<BR>
> <BR>
> I'm a bit fresh with all LiDAR data processing and analysis.<BR>
> <BR>
> I've got recently a bunch of LiDAR "las" format files, and their<BR>
> Vertical datum is ellipsoidal "WGS 84". The vertical datum is not<BR>
> defined within the las files, it was givin to me as a fact from the vendor.<BR>
> <BR>
> I need to convert these files to NAVD88 Vertical Datum, and I basically<BR>
> need to perform the following steps:<BR>
> <BR>
> 1- Define the vertical datum (WGS 84) ellipsoidal within the files<BR>
> 2- Transform to NAVD 88<BR>
> <BR>
> I've got the "liblas" with "OSGeo w4", but I can not find a clear<BR>
> descriptions to the datums available or sample code regarding the above<BR>
> mentioned issue<BR>
> <BR>
> I would ask you kindly to help me with this, and would you please direct<BR>
> me to the right documentations?<BR>
<BR>
Ali,<BR>
<BR>
Jason's approach with VDatum can certainly be used; however, over this<BR>
year we have put some significant effort into adding vertical datum<BR>
handling within liblas. With OSGeo4W this should be possible, but<BR>
in addition to the liblas commandline tools package you will also need<BR>
to ensure the "proj-vdatum" package is installed from under "Libraries"<BR>
in the advanced install.<BR>
<BR>
If the file already has horizontal coordinate system information based<BR>
on a geographic coordinate system using the WGS84 ellipsoid then the<BR>
source data will already be considered as using WGS84 ellipsoidal heights.<BR>
<BR>
I have a source file (liblas/test/data/srs.las) which is marked as<BR>
UTM 17 / WGS84. I can transform this to UTM 17 / WGS84 / NAVD88 with<BR>
the command:<BR>
<BR>
las2las -i srs.las -o out.las --t_srs EPSG:32617+5703<BR>
<BR>
The coordinate system EPSG:32617+5703 is a shorthand way of telling<BR>
las2las (or any OGRSpatialReference based application) to create a compound<BR>
coordinate system consisting of EPSG:32617 as the horizontal coordinate<BR>
system (UTM 11 / WGS84) and EPSG:5703 as the vertical coordinate system<BR>
(NAVD88).<BR>
<BR>
The las2las command will (via PROJ.4) pick up the G2003conus.gtx file<BR>
from C:\OSGeo4w\share\proj to convert from WGS84 ellipsoidal heights<BR>
to NAVD88 based heights. The data file is also used by VDatum and<BR>
in particular for WGS84/GRS80 <--> NAVD88 transformation I have compared<BR>
the results between VDatum and this process with agreement to several<BR>
decimal places. For the sample file the offsets are roughly 33m.<BR>
<BR>
Currently the only grid based vertical datum shift files provided with<BR>
OSGeo4W are the 2003 geoid data for the USA (for NAVD88 / EPSG:5703),<BR>
and the EGM 15 global data from 1996 (for EGM96 / EPSG:5773).<BR>
<BR>
Please let me know if you have any problems with this process.<BR>
<BR>
Best regards,<BR>
--<BR>
---------------------------------------+--------------------------------------<BR>
I set the clouds in motion - turn up | Frank Warmerdam, warmerdam@pobox.com<BR>
light and sound - activate the windows | <a href="http://pobox.com/~warmerdam">http://pobox.com/~warmerdam</a><BR>
and watch the world go round - Rush | Geospatial Programmer for Rent<BR>
<BR>
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