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## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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New Zealand grid coordinates
From: Paul Hirose
Date: 2019 Oct 7, 12:44 -0700

```On 2019-10-04 9:02, David C wrote:
> Until  2000 NZ used the NZMG (NZ mapping grid) which was described as
exhibiting minimal scale distortion over New Zealand. However it was a
projection unique to New Zealand and so could be difficult to use or
program into computer software or positioning devices (eg, GPS receivers).
> In 2000  a change was made to the the New Zealand Transverse Mercator
2000 (NZTM2000) projection.  I understand that NZTM2000 is less accurate
than the NZMG but the change was made for the reasons given above.
> The Chatham Islands uses the Chatham Islands Transverse Mercator 2000
(CITM2000) projection.
> So NZ switched to an old mapping standard??????

Old doesn't mean obsolete. Transverse Mercator coordinates are widely
used in the US for large scale engineering projects (such as highway
construction) under the name "state plane coordinates." The state plane
system was created in the 1930s by the Coast & Geodetic Survey as a
scheme to express the positions of their survey marks in plane
coordinates (feet x and y). That allowed surveyors to tie their work to
the national triangulation frame with simple Cartesian math.

Conversion of latitude and longitude to plane coordinates is
accomplished by a map projection. The C&GS chose two. The LCC (Lambert
conformal conic) projection was for areas wide in longitude and narrow
in latitude, TM (transverse Mercator) for the opposite. The original
goal was distortions no greater than one part in 10 000. At that
accuracy each state would require its own projection (thus the "state"
in "state plane"). In fact most states had to be divided into several
zones, sometimes with a mix of projections. For example, New York has
three TM zones and one LCC zone, the latter for Long Island.

You might think the arrival of cheap computing power would make state
plane coordinates obsolete. However, computers also gave us CADD
(computer aided drafting and design) and GIS (geographic information
systems). These represent data in rectangular coordinates, and so the
state plane system got new life. I suspect it's used more than ever.

The unavoidable distortions in any projection are not so important in
CADD and GIS. Computers easily convert with utmost accuracy between
plane coordinates and the true angles and distances on the ground. On
paper or a monitor the distortions are insignificant. Thus there's a
trend nowadays to accept more distortion in exchange for larger zones.
For instance, the original state plane system split Montana into three
LCC zones. The current version has one for the whole state.

And that brings us to the grids David mentioned. The old New Zealand
grid has less distortion, but it's a nonstandard projection which has to
be specially coded in software. In exchange for more distortion, the new
one can be computed with a generic transverse Mercator routine. You need
only supply the projection parameters such as central meridian, scale
factor, etc. Apparently the New Zealand authorities believe that's a
better compromise for modern applications.
```
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