This zip file contains the following files:

nzgeoid05.grd.zip :

Lists the 2 arc-minute grid of geoid values for each grid cell 
within the area of computation (160E-170W, 25S-60S).


nzoffset05.txt :

Lists the offset from NZGeoid05 to each of the thirteen major
New Zealand levelling datums.


readme.txt :

The file you are currently reading.



The grid is in the following format :

nrow ncol latitude longitude lat_interval lon_interval
record record record record record record record record record record
record record record record record record record record record record
...

where:

nrow is the number of rows in the grid
ncol is the number of colums in the grid
latitude is the latitude of the first record (top left corner of grid)
longitude is the longitude of the first record (top left corner of grid)
lat_interval is the latitude interval of the grid in arc-minutes
lon_interval is the longitude interval of the grid in arc-minutes

The records are listed in rows of longitude with a <return> after every 
10 records and at the end of a row (i.e. 1051 records).


The first 3 rows of nzgeoid05.grd are shown below

1051  901 -25.0000 160.0000  2.00  2.00
48.159    47.984    47.812    47.647    47.497    47.364    47.249    47.143    47.043    46.951
46.868    46.797    46.745    46.710    46.692    46.692    46.712    46.750    46.800    46.855



To convert from ellipsoid heights to levelling heights subtract the
geoid and add the offset to the ellipsoid height.

For example: CE 21 (Geodetic Code: ADND), Auckland 1946

Latitude:           36 58' 25.78152" S
Longitude:          174 59' 31.29868" E
NZGD2000 height:    120.660 m

To convert to Auckland 1946 levelling datum:

Interpolated geoid: 34.5516 m
Auckland offset:    0.496 m

Auckland height = NZGD2000 - geoid + offset
                = 120.660 - 34.5516 + 0.496
                = 86.6044 m


