The current height reference system is based on the Canadian Geodetic Vertical Datum of 1928 (CGVD28) which was
adopted by a federal Order in Council in 1935. CGVD28 was determined using classic differential leveling techniques. The datum reference level was defined as mean sea-level determined from data collected at five tide gauges on the east and west coasts. The datum is accessed by users through an extensive network of 80,000 precisely leveled benchmarks provided by federal and provincial agencies. The definition of this height reference system has been separate from the definition of the horizontal reference system historically, but with the implementation of the Canadian Spatial Reference System (CSRS) this is changing.
A number of limitations (described in a technical papers provided) are associated with the continued use of CGVD28, including:
Global Navigation Satellite System (GNSS) users require 3D positions referenced to the CSRS to ensure compatibility with data from other sources and to meet regulatory requirements. Therefore, the modernization of the vertical component of the CSRS is critical to providing Canadians with a truly three-dimensional integrated datum.
The new vertical datum has been designated CGVD2013. It will be realized primarily through the use of an accurate Canada wide geoid model, CGG2013. This will allow users to determine accurate elevations anywhere in Canada including the high Arctic and remote areas presently without nearby benchmarks through the use of GNSS.
The implementation of CGVD2013 is in progress now. Natural Resources Canada, Geodetic Survey Division (GSD) has completed a readjustment of stable and accurate benchmarks across Canada at a spacing of approximately 500km. The results of this adjustment will be provided to GeoBC, which will undertake a new adjustment of all GCMs over the next 2 years. Upon completion of this adjustment, CGVD2013 as well as existing CGVD28 elevations will be available to users through MASCOT.
The new datum will result in changes in benchmark elevations across BC. The new primary vertical benchmark heights will differ from the current published heights from a few cm to 50cm, depending on the area of the province. See table 1for approximate shifts at various locations across the province.
|South Vancouver Island (CRD)||+0.13|
|Central Vancouver Island (Nanaimo)||+0.15|
|North Vancouver Island||+0.21|
|Central Coast (Prince Rupert)||+0.23|
|Northwest BC (Dease Lake)||+0.26|
|Northeast BC (Fort Nelson)||+.040|
|East Central BC (Fort St. John)||+0.04 - +0.07|
|North Okanagan (Kamloops)||+0.18|
The new datum will be accessible directly through space-based positioning tools (e.g. GPS) via CGG2013, as well as through monument networks including the federal and provincial Active Control Points (ACP), the Canadian Base Network (CBN), and the provincial High Precision Network (HPN). Both traditional and space-based techniques will coexist throughout the transition period.
The expected relative accuracy of elevations determined using CGG2013 is approximately 2 cm for points separated by up to 100km, even in the Cordillera (Rocky Mountains) area of the province.
More details on Height Modernization can be found at http://webapp.geod.nrcan.gc.ca/geod/hmod/hmod.php
In order to ease transition to the new system, a number of steps will be taken:
The CGVD28 datum will continue to co-exist with the new datum for the foreseeable future and current CGVD28 elevations will be available through MASCOT. It should be noted, however, that the existing network has its limitations and the revision in benchmark heights will not account for, or correct for, poorly established benchmark heights as originally surveyed due to systematic errors, benchmarks that have moved over the years, nor for changes in the Earth's crust (uplift/subsidence) that affect the accuracy of individual bench marks. The gradual deterioration of the existing network of ground benchmarks is expected to correspond to a reduction in its use as space-based positioning tools are increasingly adopted.
Base Mapping & Cadastre is a member of the Canadian Geodetic Reference System Committee (CGRSC) - a Federal - Provincial working committee of the Canadian Council on Geomatics (CCOG). The CGRSC was tasked to plan and coordinate maintenance and improvement of the Geodetic Reference System in Canada. Although the CGRSC is well aware of the technical issues related to the modernization of the vertical reference system, there are a number of practical issues that need to be taken into consideration in the development of an implementation plan. A key concern is that stakeholders be consulted to ensure the envisioned modernization and related transition are conducted in a manner that minimizes negative impacts to business and users, while maximizes benefits to all concerned.
As a result, the CGRSC has prepared a high level implementation plan for a modernized Canadian Height Reference System. Their study has just been completed and has:
The first part of this work has been completed - namely the stakeholder consultation process. This was done via interviews with approximately fifty stakeholders across Canada ranging from industry representatives, to national agencies, to local municipal governments.
Given the limited reach of that overall cross-country consultation and the need to plan British Columbia's response and work ahead, including the need to prepare clients, stakeholders and users in British Columbia appropriately, GeoBC is canvassing provincial stakeholders/clients. If you would like to provide comments on this initiative and its effects on you or your organization’s business, please contact GeoBC.
Base Mapping and Cadastre
3400 Davidson Ave
Victoria BC V8Z 3P8