The following "Letter to the
Editor" appeared in Geological Society of New Zealand
Newsletter 124: pp 18-19 (March 2001), and is reproduced here
with permission.
Under the above heading,
the Geological Society‚s Newsflash 19 of 9 February 2001
has again publicised LINZ‚s stated desire to replace the
existing New Zealand Map Grid Projection (NZMG) with a new Transverse
Mercator projection (TM), and thus force the obsolescence of the
current NZMS260 1:50 000 topographic map series, and hence of
all other map series and data-bases that are based on NZMS260 and its
projection.
I have already commented
at some length on this proposal, and its defects (Reilly,
2000).
The argument for change
follows on the adoption by LINZ of a new national geodetic datum
˜ New Zealand Geodetic Datum 2000 (NZGD2000) ˜ which has
associated with it a reference ellipsoid that differs from the
existing one, on which NZMG is based. It is claimed that, to capture
the benefits of moving to NZGD2000, a new projection is needed, which
is true; and that the best would be a Transverse Mercator projection,
which is highly dubious, to say the least.
A change to a Transverse
Mercator projection will force all users of topographic maps, and of data-bases that depend
on NZMG, to replace their existing systems, at a cost which will be
borne by all the individuals, companies, and agencies involved, not by LINZ.
This wholesale
disruption is quite unnecessary. I have calculated the parameters of
a new map projection ˜
NZGeoMap© ˜ the
details of which will be published in the March 2001 issue of the
Survey Quarterly, a journal of the New Zealand Institute of
Surveyors (Reilly,
2001). It has been designed on the
same principle as NZMG, and adjusted to fit NZMG as closely as
possible. Its principal features include
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with NZGD2000;
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<![endif]>A
root-mean-square scale error of 1.22×10-4, exactly the
same as for NZMG, as compared with 9.32×10-4 for TM;
<![if
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<![endif]>A
root-mean-square discrepancy of 2.5 m between the apparent position
of the same point in NZGeoMap© and in
NZMG coordinates (447 m for TM vs. NZMG);
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<![endif]>A
maximum discrepancy of 5.6 m (1447 m for TM vs.
NZMG).
(The comparisons between
projections were made over the 228 points at half-degree intervals
used in designing NZMG (and NZGeoMap©
)).
The adoption of
NZGeoMap© rather than TM would
have the following advantages for users who wish to change to
NZGD2000:
To conclude: the costly
disruption to existing mapping systems that would follow from the
adoption of LINZ‚s proposal for a new Transverse Mercator
projection can be avoided by the use of a little common sense, and
some simple but effective mathematics.
Ian Reilly
12 March, 2001
Reference: