New Zealand Geodetic Datum 1949 (NZGD1949)

Read about the New Zealand Geodetic Datum 2000 (NZGD2000)

New Zealand Geodetic Datum 1949 (NZGD1949) was the official geodetic datum used in New Zealand until March 1998. It was a described as a horizontal or ‘2.5-dimensional datum’, providing a consistent 2-dimensional position for a coordinate in the format latitude (Φ) and longitude (λ), while the height of NZGD1949 marks were normal-orthometric (h), referenced to a local vertical datum.

Local Mean Sea Level Datums

A geodetic datum is a reference system that defines a how positions are represented. This includes a coordinate system, a reference ellipsoid and sometimes additional models such as a deformation model.

Coordinate systems used in New Zealand
Reference ellipsoids
NZGD2000 Deformation Model

New Zealand Geodetic Datum 1949 (NZGD1949) was in use until March 1998, when it was officially replaced by New Zealand Geodetic Datum 2000.

New Zealand Geodetic Datum 2000 (NZGD2000)

NZGD1949 is described in Lee (1978) First-order geodetic triangulation of New Zealand 1909-49 1973-74 (Referenced below). The key parameters for NZGD1949 are summarised in the following table:

More information about New Zealand Geodetic Datum 1949 (NZGD1949)

Astronomic datum

Unlike modern datums (such as New Zealand Geodetic Datum 2000, International Terrestrial Reference Frame (ITRF), and World Geodetic System 1984 (WGS84), which rely on continuously operating reference stations to monitor and maintain the connection to the datum, NZGD1949 was developed using astronomical observations. These were used to calculate the coordinates of a network of fiducial stations which were interconnected using triangulation. The triangulated coordinates were then extended to a network of marks such as trig stations, systematically establishing connections to the datum.

International Terrestrial Reference Frame (ITRF)
World Geodetic System 1984 (WGS84)
PositioNZ
About trig stations and geodetic marks

Static datum

After determining the coordinates of the NZGD1949 fiducial stations through astronomical observations, these positions were established as fixed. NZGD1949 was a static datum, so these positions have remained unchanged since 1949 as the re-coordination of the stations would have resulted in the creation of a new datum. This means that any discrepancies in the triangulation networks (due to factors such as tectonic deformation or error) had to be adjusted back through the networks rather than altering the coordinates of the fiducial stations.  

Two-dimensional datum

NZGD1949 is a horizontal (2-dimensional) datum, so while many NZGD1949 marks include height information these are not ellipsoidal heights. Instead, heights were published as local vertical datum heights, leading some to describe NZGD1949 as a 2+1, or a 2.5-dimensional datum. When converting coordinates to and from NZGD2000, separate transformations are required for the horizontal and vertical components.

Local Mean Sea Level Datums
New Zealand Geodetic Datum 2000 Ellipsoidal Heights

Height orders and accuracies

NZGD2000 and New Zealand Vertical Datum 2016 (NZVD2016) mark accuracy is determined using statistical analysis, this is used to assign their tier, class, and order. In contrast, NZGD1949 orders were assigned based on factors such as which triangulation network it was part of, the type of equipment used, and the physical properties of the mark itself. For instance, if a NZGD1949 station was observed using a first-order measuring procedure, then it was assumed to meet the first order standards. This means that the accuracy of NZGD1949 marks could only be evaluated when direct observations were made between marks within the same network.

New Zealand Vertical Datum 2016 (NZVD2016)
Coordinate Orders
NZGD1949 Orders

Datum limitations

While NZGD1949 met New Zealand's geodetic datum requirements for more than 50 years, a number of deficiencies led to its eventual replacement by NZGD2000. These include:

• its static nature which meant that the coordinates defining NZGD1949 remained fixed regardless of subsequent movements or changes
• regional distortions of up to 5 meters within the network, due to the survey methods and techniques available when NZGD1949 was defined
• effects of crustal deformation since the original survey of the fiducial network
• the fragmented nature of its network densification, leading to localized distortions
• its incompatibility with global geodetic reference systems such as those used for GPS
• limited spatial coverage, encompassing solely the mainland of New Zealand
• its lack of a defined vertical datum.

Reference

Lee (1978) First-order geodetic triangulation of New Zealand 1909-49 1973-74