[OSRS-PROJ] Re: [GRASS5] adding a new datum/projection?

Paul Kelly paul-grass at stjohnspoint.co.uk
Wed Jul 23 07:09:14 EDT 2003


On Wed, 23 Jul 2003, H Bowman wrote:

> Hi again --
> I've got some new data in yet another new projection, which uses its own
> new datum.
> The datum is New Zealand Geodetic Datum 2000 (NZGD2000), which is based
> on the International Terrestrial Reference Frame 1996 (ITRF96). This is
> pretty much the same as WGS84 but uses the GRS80 ellipsoid (slightly
> different flattening parameter).
> So as far as I understand: NZGD2k (==ITRF96) is the WGS84 datum combined
> with the GRS80 ellipsoid.

We are pushing at the limits of the accuracy of the current GRASS/PROJ
system here. I looked at the document and to me it seems that NZGD2000 is
based on the ITRS datum. It is 'realised' (which I think means a
description of the current location of the earth's plates based on
measurements from a number of stations around the world) by several ITRF
defintions, and each can be realised at any epoch (point in time).

For high accuracy you would define a shift that related each epoch to WGS84
(would change because of plate tectonics) and each would have a separate
datum entry in GRASS. It would get out of hand very quickly if these were
all included but it may be possible to use GRASS in this way if you could
get figures for the shift (for New Zealand you need ITRF96 at 2000.0
epoch). http://www.iers.org/iers/products/itrf/ looks relevant but I don't
really know.

> Except g.setproj won't let me do that, either with a new location or
> retroactively. The old 5.0.2 version asks for ellipsoid first, then
> datum, but complains when you try to mix wgs84 and grs80 and quits
> without writing anything. The latest g.setproj never gives you a chance
> to set the ellipsoid, it is chosen from your datum.

No it doesn't seem logical to do that. GRASS uses a very limited definition
of a datum which really just comes down to the ellipsoid used, so by
changing the ellipsoid you are really taking away everything the datum
stands for.

> I'm not sure if I get the correct result if I just change the ellps:
> line to grs80 in the PROJ_INFO file?

ellps: grs80
towgs84: 0,0,0

and make sure there is no 'datum' line.

> Is anything else based on ITRF96? Would it be worth adding that as a
> common denominator datum? I think the Map Grid of Australia might be in
> a similar situation (??).

It might be worth adding ITRS (International Terrestrial Reference
*System*), but to be compatible with other GIS we should probably
explicitly add New_Zealand_Geodetic_Datum_2000 and the others. But as
they are the same as far as the level of accuracy GRASS/PROJ currently
provides goes, it seems like we could end up with a lot of clutter.
However I have already done that for several countries in Europe which use
the international ellipsoid but have different datum names, so there is no
point in discriminating....

> see:  (102k)
> http://www.linz.govt.nz/rcs/linz/17880/difference_wgs84_nzgd2000.pdf
> As for the new projection, it is New Zealand Transverse Mercator (NZTM),
> which is in terms of that new NZGD2k datum.
> Details:
> Datum: NZGD2k
> Origin Latitude: 0° South
> Origin Longitude: 173° East
> False Northing: 10 000 000m N
> False Easting: 1 600 000m E
> Scale Factor: 0.9996
> Setting projection as tmerc and entering those terms goes smoothly.
> Should NZTM get its own projection entry or is entering by hand with the
> tmerc projection every time the way to go?

Yes that is the way to do it until GRASS has co-ordinate system support,
which I don't see happening any time soon.. you have it very easy with
New Zealand Map Grid which puts in all the parameters for you.

> for more details see:  (44k)
> http://www.linz.govt.nz/rcs/linz/5684/nztransverse_mercator.pdf
> That PDF also has a page of useful ellipsoid to grid formulae which
> might be useful to someone.
> It doesn't help much that half the country is moving 5cm/year in the
> opposite direction to the other half..

It would be useful to keep an eye out for any Free Software that handles
that level of accuracy
http://www-gpsg.mit.edu/~simon/gtgk/ has some interesting links

> thanks for any insight,

Well hopefully someone will correct the inevitable errors and
misunderstandings in what I've written above

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