Progress in Bathymetric Surveys: Combining High Precision Positioning in Real Time with a Continuous Vertical Datum in Remote Areas

For most of its bathymetric survey activities, the Canadian Hydrographic Service (CHS) requires high precision, three dimensional positioning. As part of a pilot project, one of its launches was equipped with a GNSS receiver processing a high precision correction service in real time (HP-GPS*C) via the internet using satellite telecommunication. This service was provided by Natural Resources Canada/Canadian Geodetic Survey (NRCan/CGS). The bathymetric data from a survey in eastern Hudson Bay performed by CHS in Fall 2013 was post –processed using different standard methods. This resulted in high precision positions that were compared with positions corrected with the real-time precise point positioning (PPP) service (HP-GPS*C) from NRCan/CGS.

 CHS bathymetric surveys must be referred to chart datum, the hydrographical vertical datum defined for use on nautical charts. In the Canadian north, another limitation to high precision bathymetric work is the availability of tide observations and/or predictions. The territory is vast and tide data is limited in space and in time while predicted tides are not always accurate. This makes reductions of bathymetric soundings to Chart datum difficult. To address this problem, CHS and NRCan/CGS have collaborated to produce a Continuous Vertical Datum for Canadian Waters (CVDCW), which incorporates data from NRCan's geoid model, tide gauge and GPS data, satellite altimetry, and ocean models. Thus high precision positioning provides ellipsoidal heights for the bathymetric depths, and the CVDCW allows to correct these ellipsoidal heights to chart datum. Comparisons of the bathymetry from the pilot survey corrected for tide data versus the bathymetry referred to its ellipsoidal height corrected to chart datum with the CVDCW are given to demonstrate the relative changes to the depths. This also illustrates the advantage of a continuous vertical datum with its potential to be combined with real-time high precision positioning.