The name of the model used is displayed in the Model box. The active model may be changed by selecting a new name in the list or by clicking on the Browse button. The Browse button activates the file selection window to select a .byn file by default. If the data are valid, the new model will be added to the existing list. In addition, the Browse button allows the selection of the .slv file, which is the GSD former format for geoid models, and the .bin file, which is the U.S. National Geodetic Survey (NGS) format for their geoid models. Details of the US NGS format are available at http://www.ngs.noaa.gov/GEOID/.
A brief description of the model is available by activating the Model button located in the Formats section.
Orthometric heights derived from GPS ellipsoidal heights and pure gravimetric geoid heights are not equivalent to CGVD28 heights due to errors in the realization of both CGVD28 and the pure gravimetric geoid model, unless use with ellipsoidal heights at bench marks. Here, a Height Transformation is developed based upon survey stations positioned across Canada using GPS and levelling observations. The GPS-derived NAD83 (CSRS98) ellipsoidal heights and levelling-derived CGVD28 orthometric heights, in addition to a pure gravimetric geoid heights, provided the basis for establishing the Height Transformation.
The Height Transformation (HT) allows users to obtain GPS-derived orthometric heights compatible with the surrounding CGVD28 vertical control, without the need to occupy benchmarks during field operations. Overall accuracy of the height Transformation is estimated as ± 5 centimetres (with 95% confidence) in the southern regions of Canada, but may amount to a few decimetres in remote or northern regions where there are few accurate CGVD28 heights to derive a reliable transformation (Véronneau, 2001a).
List of geoid models for Canada
CGG2000: Scientific model of the geoid for North America solely from gravity data collected up to 2000. The approach is a Helmert-Stokes spherical approximation. The calculation method is spherical 1D-FFT. The solution is optimum for Canada (Véronneau, 2001b).
GSD95: Scientific model of the geoid for Canada determined solely from gravity data collected up to 1995. The calculation method is spherical 2D-FFT (Véronneau, 1997).
GSD91: Scientific model of the geoid for Canada determined solely from gravity data collected up to 1991. The calculation method is Cartesian FFT (Véronneau and Mainville, 1992).
WW15MGH: World geoid model developed jointly by NASA, NIMA and OSU. This model is recommended for all regions of the world for which no national or regional model is available (Lemoine et al., 1997).
List of Height Transformations for Canada
The Height Transformations are not valid for USA and over seas. The US national datum is NAVD88. This datum can be different by more than one metre along the US-Canada border in British Columbia. The discrepancies diminish going eastward.
HT2_0 = CGG2000 + HRG01 + Transformation ITRF97-NAD83 (CSRS98): The CGG2000 geoid model adjusted to the Canadian primary vertical control (CGVD28) by means of 1285 NAD83 (CSRS98) ellipsoidal heights throughout Canada. HT2_0 makes possible direct transformation of NAD83 (CSRS98) ellipsoidal heights to CGVD28 orthometric heights (Véronneau, 2001a).
HT1_01 = GSD95 + HRG97 + Transformation ITRF96-NAD83 (CSRS98): The GSD95 geoid model adjusted to the Canadian primary vertical control (CGVD28) by means of 1323 NAD83 (CSRS98) ellipsoidal heights throughout Canada. Ht1_01 makes possible direct transformation of NAD83 (CSRS98) ellipsoidal heights to CGVD28 orthometric heights (Mainville, Craymer and Blackie, 1997).
HT97 = GSD95 + HRG97 + Transformation ITRF92-NAD83 (CSRS96): The GSD95 geoid model adjusted to the Canadian primary vertical control (CGVD28) by means of 1323 NAD83 (CSRS96) ellipsoidal heights throughout Canada. Ht97 makes possible direct transformation of NAD83 (CSRS96) ellipsoidal heights to CGVD28 orthometric heights (Mainville, Craymer and Blackie, 1997).
(See also Reference System )
The new identification for the Height Transformations is a version number instead of an epoch. At GSD, only datums will be identified with an epoch, e.g., GSD95, NAD83 (CSRS98) and CGVD28. A version number will identify software, packages and transformation grids.
Binary file format:
At the Geodetic Survey Division, the current format of the binary files for software GPS·H is as follows:
The file is identified by the extension .byn. The first 80 bytes of the file correspond to the header and include information related to the grid (e.g. boundaries, spacing, reference systems, ...). The data (short or long integer) are stored by rows starting from the north; and each row is stored from the west to the east. Thus, the first and last data in the file are the northwest and southeast corner, respectively. The size of the .byn file is 80 bytes for the header plus the number of rows multiplied by the number of columns time 2 or 4 (Size = 80 bytes + (2 or 4 bytes)*n*m).