This readme file explains some of the details of the distortion grid file
and the transformation software GD2000it version 2.01 dated 25 August 2000. 

The grid file has been computed by Land Information New Zealand 
(www.linz.govt.nz) at approximately a 20 km interval for the conversion 
between NZGD49 and NZGD2000.  The grid values are those to convert from a 
NZGD49 Latitude and Longitude to NZGD2000 Latitude and Longitude.  For
further information on NZGD2000 visit 
www.linz.govt.nz/rcs/linz/pub/web/root/core/SurveySystem/GeodeticInfo/GeodeticDatums/geodeticdatum2000/index.jsp

A New Zealand specific version (GD2000it) of the Australian GDAit software is
available for converting coordinates via the distortion grid.  The GD2000it 
software uses bi-linear interpolation to read the grid file and compute the 
corrections.

GD2000it can be downloaded from
http://www.linz.govt.nz/rcs/linz/pub/web/root/core/SurveySystem/GeodeticInfo/Software/gd2000itdownload/index.jsp 
That file contains this readme file, the GD2000it executable and help files and
the binary version of the distortion grid (NZGD2KGRID0005.GSB) in the Canadian 
format.  

To install the software unzip the contents of gd2000it.zip into a directory called
GD2000it.  Then run the GD2000it_v201.exe file and select the new grid shift file 
(NZGD2KGRID9911.GSB). If you have any difficulties loading the distortion grid 
the ASCII version is available as a separate file (NZGD2KGRID9911.ASC) from the
download site.

The grid file is in the Canadian grid format, known as National Transformation 
Version 2 (NTv2). Information on NTv2 can be found on the Geomatics 
Canada homepage (http://www.geod.emr.ca/).

The original release of the GD2000it software contained a binary grid file 
(NZGD2KGRID9911.GSB) that did not strictly conform with the NTv2 format,
although the ASCII file did conform.  This has been corrected with version 2
of the software.  The reformatted grid file has not changed the transformation 
values.  For further information on the changes to the format refer
to the "NTv2 Binary Files" section in the online Help files supplied with
the GD2000it software.

Version 2.01 dated 25 August 2000
This version fixes a minor bug that was recently identified. The bug
affects the output of coordinates in HP format and will only occur if:
- an input file is being used,
- coordinates are being output in HP notation, and
- the output coordinate is 0.0xxxx" (where x is a number from 0 to 9)
The bug causes the seconds output value to be shifted left 1 place. In the
worst case this will produce an error of (0.9 - 0.09)" which is
approximately 24 metres.

Visit the University of Melbourne site (http://www.sli.unimelb.edu.au/gda94/) 
for further information on the GDAit software and the NTv2 file format.  

Prepared by
Office of the Surveyor-General
Land Information New Zealand
