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A new topographic map series? 

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 Societys Newsflash 19 of 9 February 2001 has again publicised LINZs 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 Compatibility with NZGD2000, exactly the same as for NZMG, as compared with 9.3210. 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). 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:

  • At mapping scales of 1:25 000 or less (e.g. 1:50 000, 1:250 000, etc.), the differences in grid coordinates between NZGeoMap and NZMG are negligible. No conversion of coordinates is necessary: NZMG coordinates can simply be re-labelled as NZGeoMap .
  • At these scales, new or revised map sheets on NZGeoMap can be merged seamlessly with existing sheets on NZMG: the only difference will be that parallels and meridians will be displaced by up to 200 m (and GNS does not even, regrettably, print latitudes and longitudes on its QMAP series!).
  • Those who are working at accuracies of a few metres or less will need to convert coordinates, whatever projection is chosen, if only to remove the distortions inherent in the existing New Zealand Geodetic Datum 1949 (NZGD49). Despite insinuations to the contrary, a projection such as NZGeoMap is as suitable as any other well-defined mapping projection for handling high-accuracy positional data. The inference that modern computing methods are somehow incapable of handling a projection of the type of NZGeoMap is both ludicrous and misleading.

To conclude: the costly disruption to existing mapping systems that would follow from the adoption of LINZs 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 ireilly_at_southern.co.nz

12 March, 2001 Reference:
Reilly, Ian, 2000: Mapping and coordinate systems an update. Geological Society of New Zealand Newsletter 122: 13-16
Reilly, Ian, 2001: NZGeoMap : A New Mapping Projection for New Zealand in the Twenty-first Century. Survey Quarterly 25: 25-29.