For conversions between ellipsoidal and orthometric height a file of geoid heights is used. This is a binary file which defines the geoid heights at a grid of points. The heights at points of interest are calculated by interpolating from the grid points. Geoid heights represent the height of sea level above the ellipsoid defined by a reference frame, and so depend upon the reference frame being used. The geoid binary file defines the coordinate system for which the heights have been calculated. This system must be defined in the coordsys.def file in order for concord to convert the heights into the input and output reference frames.
The default name for the geoid file is geoid.grd. If a different file name is used the file can be specified either by setting an environment variable GEOIDBIN to the name of the file, or by specifying the name of the file on the command line with the -G switch. The format of the geoid file is defined in the following section.
Snap and concord come with three gridded geoid models
nzgeoid2016.grd |
The New Zealand Geoid 2016 geoid model. This is generally accurate to within about 4cm. It is applicable in the vicinity of New Zealand only. The model has been gridded at a 1" spacing. |
nzgeoid09.grd |
The New Zealand Geoid 2009 geoid model. This is generally accurate to within about 10cm. It is applicable in the vicinity of New Zealand only. The model has been gridded at a 1" spacing. |
egm96.grd |
The EGM96 model. This is a global gravity model which is generally accurate to within 1m. The model has been gridded has been calculated at a 15" spacing. |