As part of a project to incorporate good support for vertical datum transformation support in libLAS, I, Frank Warmerdam, have been contracted to incorporate support for vertical datum transformations in PROJ.4 in the sprint of 2010. This page is intended to collect information on the changes made to PROJ.4, grid shift files, and other notes of interest.
To support transformation between a variety of vertical datums. At the minimum between vertical datums such as WGS84, ITRFxx and NAVD88. Also desirable would be support for tidal datums.
There seems to be a few classes of vertical datums, as described at http://vdatum.noaa.gov/docs/datumtutorial.html
This is a fairly US oriented treatment, and it is not immediately clear how well it applies worldwide.
These are things like ITRFxx, WGS84 and NAD83 and they are meters above the ellipsoid for a given traditional horizontal datum. These are already supported by PROJ.4. It may be desirable to add better support in the default dictionaries for some datum shifts.
Orthometric (Geoidal) Datums
The NAVD88 seems to be related to the geoid rather than to the ellipsoid. Programs like VDatum provide transformations between NAD83/WGS84(transit) and NAVD88 using the geoid shift files.
Other datums like NGVD29 (the NAD27 equivelent) and IGLD85 (great lakes) are normally computed from WGS84(transit) by first converting to NAVD88 using the geoid file, and then applying an additional grid shift file to the particular datums.
There are a variety of tidal datums possible (low water, high water, etc) but in VDatum at least LMSL (Local Mean Sea Level) is used as a tidal pivot datum. Shifts from NAVD88 to LMSL are made with a grid shift file, and then another grid shift file is applied to convert to the other tidal datums.
Implementation in PROJ.4
I have added a new PROJ.4 option call +geoidgrids= similar to +nadgrids= to specify a list of vertical datum grid shift files that can be used to transform from the coordinate systems vertical grid to our pivot vertical datum (WGS84 Ellipsoidal). The vertical datum grids must be in .gtx format.
Also note that from version 4.8.0 on there is support for +vunits / +vto_meter parameters to control the vertical units.
Vertical Datum Shift Grid Collection
- Global EGM 96 15 minute Geoid grid converted to .gtx format.
- Global EGM 2008 2.5 minute Geoid grid converted to .gtx format.
- USA Geoid grids from 1999, 2003 and 2009.
- VERTCON NGVD29 to NAVD88 grids.