Does the geoid-based datum represent mean sea level better than the levelling-based datum CGVD28?
First, lets mention that the mean sea level (MSL) is not a level surface. Just like land, oceans have permanent topography, albeit ranging from -2.0 to 2.0 m worldwide. CGVD28, which is constrained to a series of tide gauges across the country, represents well the MSL at these specific locations. However, these same constrains are responsible in part for systematic errors in CGVD28. On the other hand, the geoid is a level surface; it does not coincide with MSL. However, the geoid-datum will be near the MSL on the east coast because the geoid will be selected such as it represents MSL at the tide gauge in Rimouski. This location is one of the constraints for CGVD28 and the only constraint for NAVD88 (US vertical datum). NRCan will make available a model representing the separation between the geoid and MSL along the Canadian coast.
