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NZGD2000 Frequently Asked Questions
The New Zealand Geodetic Datum 2000 (NZGD2000) was implemented in 1998. This section describes some of the common questions about the development of the datum and the transition from the New Zealand Geodetic Datum 1949 (NZGD1949).
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- What is NZGD2000?
The NZGD2000 is the coordinate system for surveying, mapping, and positioning in New Zealand. It was introduced in 1998 to replace the New Zealand Geodetic Datum 1949 (NZGD1949). NZGD2000 is a geocentric datum that is based on a global framework of reference stations, the International Terrestrial Reference Frame (ITRF).
- Why did New Zealand change to NZGD2000?
Before NZGD2000 was adopted surveying and mapping in New Zealand was in terms of NZGD1949. This datum was not accurate enough for modern survey work. Its initial accuracy was limited by the capabilities of the survey equipment, and ongoing earth deformation that has moved points up to 2.5m since its definition. It is also inefficient to work with NZGD1949 coordinates in an international environment where positioning, navigation and information systems relate to global Earth models.
The NZGD2000 datum overcomes these issues because:
- It is a three dimensional datum compatible with international geodetic systems such as the International Terrestrial Reference Frame (ITRF);
- The accuracy of the datum supports modern survey techniques and provides a network of modern, accessible survey marks; and
- The effects of crustal deformation are managed to maintain the accuracy of NZGD2000.
- How is the effect of crustal deformation managed in NZGD2000?
Plate tectonics cause New Zealand to be affected by slow crustal deformation amounting to about 5cm/year. These effects are modelled in NZGD2000 by using a deformation model to convert observations and coordinates to their predicted positions at the reference epoch of 1 January 2000. The deformation model also enables LINZ to generate coordinates at times other than the datum reference epoch. The NZGD2000 coordinates of points change when new observations provide more accurate values, or when marks are affected by unpredictable deformation such as earthquakes.
- What is the difference between NZGD1949 and NZGD2000 coordinates?
The NZGD2000 coordinates (latitude, longitude & ellipsoidal height) of a point will be different to its NZGD1949 coordinates. This difference is mainly because NZGD2000 is a geocentric datum whereas NZGD1949 is not. The effect of moving to a geocentric datum is as if New Zealand had shifted approximately 190m north and 10m east. The shift in ellipsoidal height varies across the country. Additionally, NZGD1949 had distortions in it due to the effects of crustal deformation and errors in the early surveys used to define the datum.
- What is the technical difference between NZGD1949 and NZGD2000?
NZGD1949 is a local horizontal datum. It is a static datum because the coordinates of the trig stations defining the datum (first Order stations) have been held fixed since they were defined in 1949. It uses the International (Hayford) Spheroid. The origin station for NZGD1949 is the assumed position of Papatahi, in the Rimutaka Range near Wellington.
NZGD2000 is a geocentric three dimensional datum. It is a semi-dynamic datum incorporating a deformation model with coordinates aligned to the International Terrestrial Reference Frame 1996 (ITRF96) at a reference date of 1 January 2000 (epoch 2000.0). It uses the Geodetic Reference System 1980 (GRS80) ellipsoid which has a geocentric origin (located at the centre of mass of the earth).
- What is the difference between NZGD2000 and WGS84?
Given that the NZGD2000 and WGS84 reference frames are very closely aligned, for most practical purposes NZGD2000 and WGS84 coordinates can be considered the same and no transformations are required.
- Do I have to change to NZGD2000?
Unless you are carrying out cadastral surveys you don't have to change to NZGD2000. However, NZGD2000 coordinates are more accurate than NZGD1949 coordinates and LINZ will no longer support NZGD1949. Also cadastral and topographic data will be presented in terms of NZGD2000. Nonetheless you will be able to convert data to NZGD1949 if you wish to do so.
- Does NZGD2000 affect heights?
The adoption of NZGD2000 has no effect upon the normal-orthometric height of marks, which is the height of the mark above sea level as determined by precise levelling surveys.
However NZGD2000 is a three dimensional coordinate system - positions of marks are defined by their latitude, longitude, and ellipsoidal height. The ellipsoidal height is the height above a mathematically defined ellipsoid and is commonly determined by GPS observations. Marks have both ellipsoidal and normal-orthometric heights, though one or both of these may not be known for specific marks (because the appropriate surveys have not been done).
The difference between the normal-orthometric height and the NZGD2000 ellipsoidal height varies from 0 to 35 metres across New Zealand. The two heights can be related through a geoid model such as NZGeoid2009.
- How do I transform coordinates from NZGD1949 to NZGD2000?
There are three options to transform coordinates between NZGD1949 and NZGD2000. These are the grid transformation, and the three- and seven-parameter similarity transformations.
- Why do some high order NZGD1949 geodetic marks have Order 6, 9 or 10 NZGD2000 coordinates?
Where NZGD1949 geodetic marks have been resurveyed in terms of NZGD2000 they have assigned to them an Order of 0-5 depending on the accuracy of the survey and standard of the ground mark. Where NZGD1949 marks have not been resurveyed their NZGD1949 coordinates have been transformed to NZGD2000. In these cases the NZGD2000 coordinate has been assigned Order 9. Where these marks have subsequently been used in a cadastral survey they may be upgraded to Order 6, 7 or 8.
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