Height Reference System Modernization
Height Reference System Modernization (Height Modernization) is a project at Natural Resources Canada for the development, implementation and promotion of a gravity-based height reference system for Canada. In other words, it is the realization of a new vertical datum for Canada by geoid modelling, rather than by geodetic levelling. It will enable measurements of elevations with respect to a consistent vertical datum everywhere across the country using the Global Positioning System (GPS) and emerging Global Navigation satellite System (GNSS) technologies. 
This new approach will allow reduction dependency on monumented networks for height determination. It will reduce the physical maintenance from some 80,000 federal benchmarks to some 250 stations making the Canadian Active Control System (CACS) and Canadian Base Network (CBN). These networks will be augmented by the provincial High Precision Networks (HPN).
The project includes two main components:
- Strategic planning and implementation; and
- Improvement of the geoid model.
The Strategic planning and implementation component is accomplished in collaboration with provincial and territorial geodetic agencies. Its activities include a consultation with our stakeholders; the development of an implementation plan and a communication plan; preparation of educational materials; and promotion of the new vertical datum through the internet, conferences and workshops.
The Improvement of the geoid model component is the technical aspect of the project. It includes the enhancement of the geoid model and its error model based on the latest theory; the collection of new gravity data (terrestrial and spatial) and digital elevations models; the validation of the geoid model by conducting GPS and levelling surveys; and development of tools to make use of the new vertical datum. This component also includes contribution from academic institutions through research projects funded by GEOIDE Networks of Centers of Excellence and supported by Natural Resources Canada.
The implementation of the new vertical datum for Canada is now expected in 2013.
Why a new datum?
Even though the Canadian Geodetic Vertical Datum 1928 (CGVD28) remains overall very precise locally, it does not represent today's required national accuracy. Furthermore, the maintenance and expansion of the vertical network by spirit levelling is costly, time consuming and laborious. A readjustment of the levelling networks [JPEG, 221.7 kb, 1183 X 764, notice], similar to the NAVD88 project, would only be a temporary solution, albeit more accurate than CGVD28, and would not solve the problem of its limited coverage and cost of maintenance.
On the other hand, geoid modelling is a viable alternative for the realization of a long term vertical datum for Canada. The geoid is a stable surface that can be determined accurately all across the Canadian territory. It is defined in relation to a reference ellipsoid (geoid height), making it compatible with space-based positioning technologies (e.g., GPS, satellite radar altimetry). The ellipsoidal heights (h) determined from GNSS technologies can be converted to orthometric heights (H) through geoid heights (N): H = h - N
With the adoption of a geoid-based vertical datum, benchmarks will not be abandoned immediately. Each benchmark will be assigned a new elevation with respect to the new vertical datum. These elevations will be determined by readjusting first-order levelling networks with constrains at some CACS and CBN stations where the ellipsoidal heights are known accurately to the sub-centimetre level. Furthermore, the adoption of a new vertical datum does not mean that the current datum will suddenly fade away. CGVD28 will continue to co-exist with the new datum as long as it will be required, but it will eventually disappear physically with the destruction or instability of most benchmarks over the years.
What will be the impact on my activities?
An initial impact could be the necessity to convert legacy height data into the new vertical datum. However, this conversion might not be necessary depending on the accuracy of your legacy data and the urgency to convert these data into the new datum (as mentioned above, CGVD28 will continue to co-exist). But, it is important to point out that the height conversion could be as simple as adding a constant if the datasets cover relatively small geographical areas. The difference between CGVD28 and the new vertical datum is a smooth distortion ranging nationally from -65 cm in the region of Halifax to +55 cm in the region of Banff (based on preliminary results [JPEG, 227.7 kb, 1234 X 862, notice]). Thus, in the short term, the current height system and benchmarks will remain in place, but federal, provincial and territorial geodetic agencies will also disseminate a new geoid model and a new set of elevations at benchmarks with respect to the new vertical datum. The geoid model will allow direct transformation of GPS ellipsoidal heights to orthometric heights in a consistent vertical datum at any locations across Canada.
The long-term impact will be the disappearance or reliability of benchmarks across the country. This will require stakeholders to install their own vertical control stations. This can be accomplished by installing new benchmarks from the existing ones before their disappearances using spirit levelling techniques and by maintaining them. On the other hand, it can also be done more efficiently by installing on-the-fly new benchmarks in the area of interest by conducting GPS surveys with respect to CACS, CBN or HPN stations or by conducting GPS Precise Point Positioning (PPP). The local height survey can be resumed either by GPS or spirit levelling technique.
Contact
For more information, please contact our office or your provincial agency.