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New Zealand Geodetic Datum 1949 (NZGD1949)
The New Zealand Geodetic Datum 1949 (NZGD1949) was the official geodetic datum for New Zealand before NZGD2000 was introduced in 1998. Like most datums of its era it was defined using astronomical observations to establish the coordinates of a relatively small number of fiducial stations. Triangulation was then used to connect these stations together. Successive densification of these networks enabled the provision of the datum (normally in the form of trig-beaconed marks) to users.
NZGD1949 was a static datum. This means that the coordinates of its fiducial stations were fixed (at the 1949 positions) and the newer observations adjusted (or more often distorted) to fit them. This was particularly problematic in the later years of the datum when the effect of 70 years of horizontal deformation needed to be absorbed into the network.
The NZGD1949 is a two-dimensional datum. Although many of its constituent marks do have heights in relation to mean sea level, these heights are actually in terms of different local vertical datums rather than NZGD1949.
The key reference for NZGD1949 is Lee (1978). The main parameters for the datum are summarised below:
|Name:||New Zealand Geodetic Datum 1949|
|Reference ellipsoid:||International 1924|
|Reference frame:||Astronomic datum|
Physical marks with coordinates in terms of NZGD1949 are classified into different geodetic networks. These networks relate to the type of mark used and the accuracy of its coordinate. The accuracy classifications are referred to as Orders.
While NZGD49 served New Zealand's needs well for over 50 years, a number of deficiencies led to its eventual replacement with NZGD2000. These include:
- regional distortions in the network of up to five metres caused by the lower precision survey observations and techniques available at the time the datum was defined
- the effects of crustal deformation since the datum was surveyed
- the lower-Order densification of the network has been built up and defined in a piecemeal fashion resulting in localised distortions
- being incompatible with global geodetic reference systems (eg GPS)
- its limited spatial extent, covering only the land mass of New Zealand and not the offshore islands
- not defining a vertical datum
- being a static datum, meaning the coordinates of the first-Order stations which define NZGD1949 are held fixed, irrespective of subsequent movements.
Lee LP (1978) First-order geodetic triangulation of New Zealand 1909-49 1973-74, Technical Series No. 1, Department of Lands and Survey, Wellington.
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