The most accurate method for transforming coordinates uses a grid of "distortions" which models the difference between NZGD1949 and NZGD2000 horizontal coordinates.
The distortions are provided on a grid with approximately 20 kilometre spacing over the land area of New Zealand. The estimated accuracy of this transformation varies across New Zealand from 0.1 metre to 1.0 metre but typically averages approximately ±0.2 metres.
To convert coordinates from NZGD1949 to NZGD2000:
- use the distortion grid to interpolate a correction at the NZGD1949 coordinate
- add this correction to the NZGD1949 coordinate to determine the NZGD2000 values.
Reverse transformations (ie NZGD2000 to NZGD1949) are more complex. You will need to:
- use the NZGD2000 coordinates in the grid file to determine an estimate of the NZGD1949 to NZGD2000 correction
- subtract this correction from the NZGD2000 coordinates to determine initial estimates of NZGD1949 coordinates
- use the estimated NZGD1949 coordinates to determine a more accurate NZGD1949 to NZGD2000 correction
- subtract this correction from the original NZGD2000 coordinates to determine the final NZGD1949 coordinates.
The grid file and interpolation software can be downloaded. Alternatively, online conversions of coordinates also use this transformation. Example coordinates transformed using the distortion grid are also available.
This distortion grid is provided in the NTv2 ASCII format. NTv2 is a format developed by the Geodetic Survey of Canada specifically for the purpose of distributing grid transformation data.
This shows the differences between NZGD1949 and NZGD2000 after applying the 7 parameter similarity transformation