[OSRS-PROJ] Re: [GRASS5] adding a new datum/projection?
Cliff Mugnier
cjmce at lsu.edu
Wed Jul 23 07:19:58 PDT 2003
In regard to New Zealand, I have not researched their implementation in
detail, but from the little that I know; the ITRF is probably NOT the
way to go for "datum shifts" in New Zealand. The original datum was not
recognized or utilized throughout both North Island and South Island,
and local property was referenced to local "meridional circuits" that
were perimeter surveys forming local coordinate systems and cast on
individual aphylactic Cassini-Soldner Grids. Each county was an
"island" unto itself. Topographic mapping at inch to the mile was on
the NZ Transverse Mercator before WWII, and the system remained that way
for decades.
They had a datum readjustment (I think) in the 70s and Lee came up with
the "NZ Map Grid" that covered both islands.
I do know that they have supposedly dropped Lee's projection for both
islands and may have gone back to Transverse Mercator. However, NZ2000
Datum is actually changing monthly if not daily because of the enormous
speed the islands are moving towards Hawaii. (I think maybe a
centimeter a week, not sure!)
Anyway, GIS coordinates need to be "islands" unto themselves (like the
old "meridional circuits"), and I don't think that geodetic "solutions"
are appropriate in this case. I believe that the subject is so complex
for New Zealand, that the transformation from the ITRF_XX to a local
reference frame should be done by geodesists and maybe Land Surveyors,
but certainly not implemented into GIS software.
I may be incorrect in my assessment, but at present I believe that
"PROJ" should certainly NOT implement the current geodetic system (in an
international sense of the ITRF_XX) for New Zealand. Leave it to the
geodesists for now.
Clifford J. Mugnier (cjmce at LSU.edu)
Chief of Geodesy and
Associate Director
Center for GeoInformatics
Department of Civil Engineering
LOUISIANA STATE UNIVERSITY
Baton Rouge, LA 70803
Voice: (225) 578-8536
Facsimile: (225) 578-8652
Pager: 1-(888) 365-5180
=======================================
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http://www.cee.lsu.edu/facultyStaff/mugnier/index.html
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-----Original Message-----
From: owner-osrs-proj at remotesensing.org
[mailto:owner-osrs-proj at remotesensing.org] On Behalf Of Paul Kelly
Sent: Wednesday, July 23, 2003 6:09 AM
To: H Bowman
Cc: grass5 at grass.itc.it; osrs-proj at remotesensing.org
Subject: [OSRS-PROJ] Re: [GRASS5] adding a new datum/projection?
From:Clifford J Mugnier/cjmce/LSU at stjohnspoint.co.uk on 07/23/2003
12:09
PM CET
Hello
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
Paul
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