Vertical Control Networks (CGVD28/CGVD2013)
In 2015, Canada adopted a new height reference system: Canadian Geodetic Vertical Datum of 2013 (CGVD2013). It replaced the Canadian Geodetic Vertical Datum of 1928 (CGVD28). The new system corrects for systematic errors in the old datum, and is realized by a geoid model instead of a network of benchmarks whose heights are measured by levelling. To facilitate a smooth transition, benchmarks of the vertical control network are currently assigned mean sea-level elevations in both CGVD28 and CGVD2013.
CGVD28 elevations result from an adjustment of all vertical levelling lines observed in Canada over a period of almost a century. As a result, CGVD28 elevations can be subject to systematic biases reaching several decimetres, as well as decimetre level distortions due to terrain uplift/subsidence.
CGVD2013 elevations are with respect to an equipotential surface that conventionally represents coastal mean sea level around North America, and is realized by the Canadian Gravimetric Geoid model of 2013 (CGG2013). The CGG2013 geoid heights provide the separation between GNSS ellipsoidal heights (GRS80) in NAD83(CSRS) and the CGVD2013 mean sea level.
Since November 2013 the CGVD2013 datum has been published, providing dual-value CGVD28/CGVD2013 benchmarks across Canada. Access to CGVD28/2013 benchmark heights is available through the NRCan Passive Control web application.
Primary Horizontal Control Network (NAD83)
The primary Horizontal Control Network consists of over 25,000 points with 2D coordinates measured with conventional surveying methods. Monuments spaced 20 to 100 km extend over the entire Canadian landmass. With meter-level accuracy and relative precision of 5–10 ppm, these control points are not suitable to perform quality control on differential GPS surveys. In addition, these points may be inconvenient to use as physical access may be difficult.