[Liblas-devel] Vertical Datum Transformation

[David C. Finnegan]

Frank,

The work you have done to incorporate vertical datum support is really
great!

Is it possible for users to incorporate their own vertical datum
descriptions beyond what is supported by default.

Dave


On 10/29/10 5:31 PM, "Frank Warmerdam" <warmerdam at pobox.com> wrote:

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