[GRASS5] Vertical datum support for 5.7

[Helena]

I would like to make a suggestion for adding support for vertical
datum into GRASS or at least add the info to the PROJ_INFO file,
so that the conversion can be added later without the need
to change the PROJ_INFO file, e.g.:

name: State Plane
datum: nad83
vdatum: navd88
....

It would be great if this could go into 5.7 before we have a stable
release with the new vector format.
I noticed that PROJ supports only horizontal datum too,
or am I missing something? Are there any plans to add it to PROJ?
It is really important for working with elevation data -
when the vertical datums are mixed up, place that is sinking may appear growing
and the difference can be from few cm to several meters.

I looked for some public domain software - for USA, NOAA/USGS has some
source code and grids available but the licence has some restrictions,
I am not sure how to interpret it. Also, the software is FORTRAN.
The conversion is relatively simple as it involves interpolation on a grid,
but the grid files are rather large.

Below is a link and info about the NOAA programs, in case that somebody
can look at it. I am not sure what would be available for Europe and other parts
of the world,

Helena

http://chartmaker.ncd.noaa.gov/bathytopo/vdatum.htm

VDatum
VDatum is under development as part of the joint NOAA/USGS Bathymetric-Topographic
Demonstration Project, and is a tool for the conversion of elevation data among
28 different vertical datums. Vertical datums can be based on Mean Sea Level
(such as NAVD88), tidally derived surfaces (nautical charts) or three dimensional
space systems (GPS, for example).

VERTCON (Version 2.1)
Computes the modeled difference in orthometric height between the North American
Vertical Datum of 1988 (NAVD 88) and the National Geodetic Vertical Datum of 1929
(NGVD 29) for a given location specified by latitude and longitude.
This conversion is sufficient for many mapping purposes.
An on-line interactive version is also available from the Geodetic Tool Kit.

# GEOID99

Modeled geoid height values with a 1-minute by 1-minute spacing in latitude
and longitude suitable for the conversion of NAD 83 ellipsoid heights into
NAVD 88 orthometric heights. GEOID99 includes software programs XNTG and INTG
for extracting and interpolating values from the model to discrete points.
INTD is also available on line for interactive computations over the web.
GEOID99 replaces the existing GEOID96 product.

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