[Liblas-devel] Vertical Datum Developments

[Smith, Michael ERDC-CRREL-NH]

Not exciting? This is extremely exciting! Thanks Frank. 

Michael Smith
US Army Corps of Engineers
Hanover, NH
Remote Sensing/GIS Center

On Sep 20, 2010, at 7:47 PM, "Frank Warmerdam" <warmerdam at pobox.com> wrote:

> Folks,
> 
> I owe you all (and Howard and Mike in particular) a status report on my
> vertical datum support efforts for liblas and other client libraries and
> applications.
> 
> First, I will refer you to the RFC I wrote in the spring related to vertical
> coordinate system support at:
> 
>  http://liblas.org/development/rfc/rfc_1_verticalcs.html
> 
> That was implemented, but recently I have added some additional work, aiming
> to actually support applying vertical datum shifts as part of a reprojection
> operation with liblas (ie. las2las).
> 
> The key elements in this are:
> 
> 1) PROJ.4 has been upgraded to support a +geoidgrids= parameter which can
> reference one or more geoid files relating geoid heights to WGS84 ellipsoidal
> heights.  These are used to transform back and forth to/from geoidal vertical
> datums like NAVD88.
> 
> 2) Some geoid files in the .gtx format have been assembled at the following
> url, notably US files from NOAA suitable for use for NAVD88 and global
> models from NGA suitable for approximating other geoidal coordinate systems
> world wide.
> 
>  http://download.osgeo.org/proj/vdatum
> 
> 3) Libgeotiff's EPSG to csv translation script was upgraded to capture some
> information about EPSG vertical coordinate systems, and compound coordinate
> systems (in vertcs.csv and compdcs.csv respectively).  The vertcs.csv includes
> a column indicating what .gtx transformation files can be used to get from
> a given vertical datum to WGS84 ellipsoidal. This has been forcably
> populated for NAVD88 and "EGM96 Geoid Height".
> 
> 4) GDAL/OGR's OGRSpatialReference object has been extended to support
> importing vertical and compound coordinate systems with importFromEPSG().
> A new EXTENSION node on the VERT_DATUM object can be used to list
> geoid grid shift files.
> 
> 5) GDAL/OGR's OGRSpatialReference::SetFromUserInput() method has been
> extended to allow defining a compound coordinate system from any
> EPSG vertical coordinate system and any (GCS or PCS) horizontal coordinate
> system using the form EPSG:4326+5703 (WGS84 + NAVD88 in this case).
> 
> 6) OGRSpatialReference's from and to PROJ.4 functions have been updated
> to support the +geoidgrids= and corresponding WKT EXTENSION node.
> 
> 7) The GeoTIFF to WKT translation code now uses definitions from vertcs.csv
> when available which also picks up the geoid grid files.
> 
> 8) liblas has been updated to use the latest geotiff translation code
> and to ensure that "eCompoundOK" forms of WKT are used for translation
> too and from proj.4 format so we get the geoidgrids.
> 
> As a result of the above, if you were to build liblas, GDAL, PROJ.4,
> and perhaps libgeotiff from SVN "trunk" of each project, and ensure you
> populated your PROJ.4 grid shift file directory (ie. /usr/local/share/proj)
> with the appropriate grid shift files you could do:
> 
>  las2las -t_srs WGS84 srs_vertcs.las wgs84_ellipsoidal.las
> 
> and transform the sample srs_vertcs.las file from NAVD88 geoidal
> heights to WGS84 with WGS84 ellipsoidal heights!  That may not seem
> that exciting to you but for me it's a milestone.
> 
> I have a variety of cleanup, documentation, and validation to do,
> but I feel the essentials of my project are now in place.   Testing
> and feedback are welcome, though you might want to wait till I have
> better documentation in place.
> 
> 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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