[GRASS5] New Datums

[Paul Kelly]

Hello Radim

On Wed, 28 May 2003, Radim Blazek wrote:

> On Wednesday 28 May 2003 16:16, Paul Kelly wrote:
> > Hermannskogel seems to be in Austria so this name seemed appropriate but
> > there is also S_JTSK_Ferro which applies to the Czech Republic and
> > Slovakia, but are these the same datum?

I mean the name Militar_Geographische_Institut seemed appropriate. [1] Within
GRASS anyway the name is not important as the user can select from a list of
pre-defined transformation parameters or his/her own parameters (see below).
The actual name is not very important at all. The long name is for future
support (well I am working on it now but might put it into GRASS5.1) for
exporting the projection information to other programs. There are some
standards for the datum names based on the European Petroleum Survey Group
(EPSG) but some of them are a bit ambiguous and unclear and there may be
several names for each datum; see
http://gdal.velocet.ca/~warmerda/wktproblems.html

> I am not very sure what the 'datum' is, just from user point of view:

No, neither am I :) But saying it is an ellipsoid and a set of
transformation parameters will do us for now, I think. Technically it has
a point of origin (or datum anchor point as you describe below) but I
don't see how that is needed for GRASS.

> Acording to http://crs.ifag.de/, 'S-JTSK' is 'Datum identifier' (so it could be datum)
> for Czech and Slovak Republic. 'Hermannskogel' seems to be 'Datum anchor point'
> used by more 'datums'. If datum record in datum table is something what should help
> user to define transformation parameters, then we need two records for
> S-JTSK, one for Czech and second for Slovak Republic (different parameters),
> S-JTSK_CZ and S-JTSK_SK?
> On crs.ifag.de is distinguished only 'CRS identifier': 'CZ_S-JTSK / KROVAK'
> and 'SK_S-JTSK / KROVAK'
>
> If you use only 1 record for S-JTSK (but there are no reliable parameters
> for the whole territory available, I think), then user have to either define its own
> parameters or use wrong (not precise enough) parameters.

Just a clarification: The new datum set-up in GRASS makes it possible to
define many different sets of transformation paramters for each datum.
This is done in the datumtransform.table file. E.g. hermannskogel
currently has four sets of 7-parameter transformations, each for Austria,
Czech Republic, Slovakia and Slovenia. Also, these are really just
'suggestions' for the user to pick when setting up his/her location using
g.setproj, and custom parameters can be entered or changed later by editing
the PROJ_INFO file.

It is only if there are no transformation parameters specified (only a
datum name) in the PROJ_INFO file that the default 3-parameter transformation
is looked up from the datum.table and used. This is very unlikely to
happen. In general there should always be specific datum transformation
parameters in the PROJ_INFO file (either 'towgs84:' or 'nadgrids:' line)
and these will be used.

Hope this clarifies things a bit

Paul

[1]. This is how the name is given in the EPSG database. I suspect there
should be an umlaut on the a in Militar which would become Militaer with
the conventional German letters --> ASCII transliteration. And Markus also
suggested it should be Geographisches instead of Geographische which may
be a mistake in the EPSG database. (See Frank's webpage above for the
standard rules for forming these names). It is hard to know if we should
try to produce our own GRASS transformations of the EPSG names (which may
be more correct) or propagate the existing (if unconventional) spellings
in the interests of compatibility.