CGS published several scientific geoid models since 1991 representing their own vertical datums. The geoid model gives the geoid height ( N), which is the separation between the ellipsoid and the MSL. This is problematic as a large number of users are interested in elevations ( H) with respect to mean sea level (MSL). While precise heights can be readily obtained from GNSS surveys, these heights ( h) are given with respect to an ellipsoid in a 3D geometric reference frame (e.g., NAD83(CSRS)). CGVD2013 is a modern vertical datum realized by a geoid model, allowing compatibility with GNSS positioning technique. In 2013, CGS introduced a new vertical datum. Each height transformation is addressed on a case-by-case basis by contacting the Geodetic Information Services at CGS. CGS does not produce vertical grid shift files for the transformation between vertical datums because the levelling lines are too sparse across Canada. Depending of the size of the area, the transformation can go from an offset to a complex surface. When a height transformation is required between two levelling-construct vertical datums, it is necessary to find in the working area benchmarks having heights in the two systems. As CGVD28 was the only vertical datum for years in Canada, there was no real need for height transformations, unless working across the US border where the Americans work in NAVD 88 or tying to a historical vertical datum. Until recently, the Canadian Geodetic Survey (CGS) realized and maintained the vertical datum (CGVD28) using spirit levelling.
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