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Datums and Projections

A basic understanding of datums and projections and how they are used is essential for anyone conducting research with a mapping/spatial component to it.

A reasonably good summary of geodetic datums and projections is written here (read this summary first) : http://www.linz.govt.nz/geodetic/find-out/understanding-datums/index.aspx

Some Key points about datums and projections

In general

  • The most important thing to know is what datum + projection the data are in
  • When someone gives you data ask about what datum + projection the data are in.
  • Check what the GPS is set to!
  • People may tell you “The data are in Lat long” this is not a datum, it is a way of expressing the positions in a datum! Latititude and longitude could be given for any datum! (it is most likely that data will be wgs84 datum).

NZ data specifically

  • If you are entering location data into a spreadsheet put the datum or projection in the header (e.g. |Easting_NZTM| Northing_NZTM| )
  • When reporting eastings and northings report eastings first (like other x,y coordinates). For latitude and longitude, latitude is often reported first (not sure why this is).
  • There is ~200m difference between nzgd1949 and nzgd2000 datums
  • For NZMG and NZTM the easting is always the smaller of the two (unless you are using it outside of onshore nz- which you shouldn’t).
  • NZTM and NZMG Eastings/ northings are of the order of a million metres. So if you see one that that is “123456” or “12345678”, they are wrong (probably mistyped).
  • NZMG eastings begin with 2 e.g. 2345346 and NZTM eastings usually begin with 1 (e.g.  1345657). There is no overlap in the coordinates for onshore New Zealand (see map below). This is by design.
Comparison between projections used in New Zealand. One grid shows NZMG, while the other shows NZTM

Comparison between projections used in New Zealand

For more info go to: http://www.linz.govt.nz/geodetic/datums-projections-heights

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