Latitude and longitude is a standard way of specifying a location on the earth. However, not all latitudes and longitudes are equal. These coordinates change depending on the geodetic datum in use.
The problem is that the earth is not a perfect sphere. It is not even a perfect spheroid. It's a little bit lumpy. Historically, datums have been designed for use within a specific area, and are accurate over that area only. An appropriate datum can be used as a reference frame both to describe locations and to describe distance and direction between two points.
Latitude and longitude coordinates are often written as degrees, minutes, and seconds of arc as follows: 43°35′53.3″ S, 170°08′33.0″ E or as decimal degrees: -43.5981468, 170.1425128. One second of arc (1″) equates to 30m, or 0.00028 in decimal degrees.
NZGD49 and NZGD2000
The New Zealand Geodetic Datum 2000 (NZGD2000) is an example of a datum designed for use within a limited area -- the New Zealand landmass. This datum is the basis of most of the topographic maps in circulation today. This datum supercedes NZGD49, which was in use until 1998.
A key point here is that latitudes and longitudes are about 200m different between these datums.
Aside from this shift, the datums are fundamentally different. NZGD2000 differs from NZGD49 in that it is designed to move and deform with the country as a whole. It is called a "plate-fixed" datum.
GPS technology has made the creation of globally-compatible datums both possible and necessary. GPS units use the World Geodetic System 1984 (WGS84), although they may translate to another datum for display. WGS84 is a global datum. It is fixed to the earth rather than to any one spot, meaning coordinates drift over time.
Unfortunately, there are several datums that call themselves "WGS84". However these only vary from each other by up to 2 metres.
For tramping purposes, the NZGD2000 and WGS84 may be considered identical for now — differences are on the scale of 1-2 metres.
A projection is a technique for translating the curved shape of the earth onto a flat map. A good projection minimises distortion and provides accurate distance and angle measurements. Projections are only successful over a small part of the globe, as larger areas require ever greater distortions.
Coordinate systems may also be devised for a particular map projection. Easting and northing coordinate pairs may be used on the New Zealand topographic maps. These are measures of the distance in metres both east and north of a standard reference point. Aoraki / Mount Cook is referenced as: 5169132, 1369317.
The official projection for topographic mapping in New Zealand is the New Zealand Transverse Mercator 2000 (NZTM2000).
If you look at eastings and northings on a Topo50 map, you are reading numbers from the NZTM2000 projection.
The New Zealand Map Grid (NZMG) is a projection based on the NZGD49 datum. This map grid was used on old NZMS260 maps. Eastings and northings on these maps are no longer current.
Remember that any older NZMS260 maps you may have were using the NZGD49 datum, and will be about 200m out from your GPS.
A grid reference is a simplified reading designed for convenience over accuracy. The first part of the reference is the map sheet code. The remainder combines the easting and northing components. Since the reference is read off the paper of the map itself, it is only supplied with an accuracy of 100m. Here is how to produce a grid reference:
- Write down the sheet number
- Find the first vertical grid line left of the point. Read the value from the margin. Write down the two-digit number.
- Estimate tenths of a square between the vertical grid line and the point. Write down the number of tenths (between 0 and 9).
- Find the first horizontal grid line below the point. Read the value from the margin. Write down the two-digit number.
- Estimate tenths of a square between the horizontal grid line and the point. Write down the number of tenths (between 0 and 9).
The result should look like this: BX24 771737.
Coordinates on this website
Coordinates are stored as NZGD2000 data, which for tramping purposes is currently interchangeable with WGS84 data.
If you are interested in the techniques and mathematics of geodetic conversions in use on this site, take a look at the following resources:
- Understanding datums and projections
LINZ introduction to the topic.
- Three & Seven Parameter Similarity Transformations (NZGD1949 to NZGD2000)
The technique employs an iterative formula and complex numbers.