Weird Grids showing on NZ Topos

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Weird Grids showing on NZ Topos

Postby wildwanderer » Tue 25 Sep, 2018 11:04 pm

Im downloading some NZ topo maps from the NZ Govt site https://data.linz.govt.nz/data/category/topographic/ however im having trouble understanding the grids that are printed on the map as they dont appear to correspond to the UTM grid which im familiar with.

Screenshot.jpg
Black UTM and blue Printed Grids


You can see in the above screen shot. The light blue grids are the printed grids that come with NZ topo. they are 1 square km. The black grids lines are 1 square km UTM grids overlayed by orux maps.

As you can see they dont line up at all and as indicated in the upper section of the screenshot the printed blue girds reference grid lines of 84 85 86 etc. Which dont correspond to any UTM grids in the area.

Ive checked and the UTM grid references given for locations (in orux maps) and they are correct, so it doesnt appear to be a map calibration issue in Orux maps as I understand things. and the datum is correct.

As an example For siberia hut. The lat long given in google maps corresponds exactly with the UTM grid ref provided in a online service convertor. See below screenshot. (which is exactly the same as the UTM and lat long refs I get in orux maps)

I’m obviously missing something here.. help please! :D

EDIT - doing a bit of reading here https://www.linz.govt.nz/system/files_f ... download=1 which is the guide to NZ Topos. Apparently the blue grids are something called the New Zealand transverse mercator 2000. So NZ has a different Grid reference system to australia and countires who use UTM grids?

grids.JPG
Online service lat/long to UTM convertor
grids.JPG (77.7 KiB) Viewed 5410 times
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Re: Weird Grids showing on NZ Topos

Postby tom_brennan » Mon 01 Oct, 2018 7:31 am

Keep in mind that the grid references are a combination of the projection (various Transverse Mercator projections) and the datum. Just because a map is projected using UTM, doesn't mean that the grids will agree.

In the case of Australia, we currently use UTM with a datum that is sufficiently similar to WGS84 (used by GPS) that the grids are for most casual purposes, the same.

However, prior to the current Australian grid, the previous map grid was a UTM projection of a different datum, which is about 200m different in NSW. So you'd see that those grids do not match either.
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