1.0 Introduction
A modern society urgently requires elevations of land, roads, buildings, infrastructure and resources relative to mean sea level that are provided easily, accurately and at the lowest possible cost. The current system of elevations in Canada, the Canadian Geodetic Vertical Datum of 1928 (CGVD28), was established by classical spirit leveling technique and has prohibitively high maintenance costs. It is not compatible with modern space-based technologies (e.g., GPS) and inhibits cost-saving utilization of these new technologies. With the situation further exacerbated by the deterioration of the existing infrastructure, a decision regarding the future of the system is urgent.
GPS has become the tool of choice for the positioning component of a number of activities: environmental studies, forestry and other resource applications, oil and gas exploration and development, land development, and precision agriculture, among others. GPS users want to obtain 3D positions referenced to the Canadian Spatial Reference System (CSRS) to ensure compatibility with data from other sources and to meet regulatory requirements. The current vertical datum has limited Canadian coverage and is not well integrated within the CSRS, resulting in extra effort from contemporary users to obtain CGVD28 elevations. An opportunity exists to define a new datum - one that is compatible with international standards and enables cost-saving implementation of space-based technologies such as GPS.
2.0 Levelling Technique
The spirit leveling technique is a well-known approach that has been conducted for more than 200 years. Newer technologies have been employed but no cheaper alternative was available. Although an inherently accurate method for determining height differences, spirit levelling is costly, time consuming and laborious. It involves making differential height measurements between two vertical graduated rods, approximately 100 metres apart, using a tripod mounted telescope whose horizontal line of sight is controlled to better than one second of arc by a spirit level vial or a suspended prism. This process is repeated in a leap-frog fashion to produce elevation differences between established bench marks that comprise the vertical control network.
2.1 Network Maintenance
From 1972 to 2000 nearly the entire network was re-surveyed with 124,000 km of levelling carried out. Until 1993, GSD carried out an average of 4000 to 5000 km of levelling annually. Approximately 65% (~3000 Km) of the levelling was for maintenance purposes, the other 35% (~1500km) was related to network expansion. From 1994 to 2000 (following federal government Program Review and associated budget reductions), GSD performed an average of 1200 km of levelling annually, with a steady decline over the years. GSD has performed only minimal leveling since 2001.
Assuming the vertical network continued to be maintained on a 25-year cycle, approximately 5,600 km of levelling would be required annually. At a rate of $250-300 per km, the O&M cost of re-survey alone would range between $1.4M to $1.7M annually. Furthermore, this cost does not include repair or replacement of damaged bench marks ($1000-$2500 per bench mark depending on the type), nor the salary costs related to the surveys coordination, mathematical adjustment and related data management. Even a skeletal network of about 30,000 km proposed as the minimum vertical framework for Canada would cost about $400K (O&M) per year to maintain and potentially pre-empt or delay the work essential to establishing a modernized solution.
2.2 Physical State of the Network
Due to the extent of the network and the related time required to carry out a full inspection, it is difficult to assess the exact state of the network at a specific point in time. We can only extrapolate based on statistics currently available in our databases or derived from the most recent inspections of small sections of the network.
Although only 5% of the 80,000 bench marks on the GSD database show reports of dubious condition (damaged, destroyed, not found, displaced or inaccessible), it is expected that the current state of the vertical network is in much worse shape. Until 1996, as part of the network maintenance program, GSD inspected 3000 to 4000 km annually. These inspections found 11 to 22% of the BMs inspected were unusable or destroyed. Assuming roughly a 20-year inspection cycle was in place, this extrapolates to an estimated rate of degradation of 16% in 20 years. In urban or near-urban settings the rate can be much higher. A systematic inspection of some 400 primary bench marks established 25 years ago in the Greater Vancouver Regional District (GVRD) reported 32% of the bench marks were either not found, inaccessible or destroyed. Somewhat consistent with these statistics, a student summer project recently carried out by the Ontario MNR, yielded a level of destruction of 22 % based on the inspection of 110 bench marks selected randomly in and around six cities. On the other hand, Alberta provincial agency records show that they have inspected 12% of the federal bench marks on their database since 1988, with less than 2.5 % reported as destroyed or having an "anomalous" condition. At the other end of the spectrum, the Newfoundland provincial survey agency estimates the destruction rate at about 2.5 % per year (yielding a rate of about 40% over 20 years) based on the destruction rate of their own control monuments in the province (not based on inspection of vertical bench marks alone).
In summary, we can probably estimate the degradation rate of the network across Canada to be in the range of 15% to 20% per 20 years. In urban or near-urban areas the degradation rate could reach 35% for the same period. In the context of datum modernization, this implies that a significant portion of the existing monumented networks should remain intact and enable a transition period of a few decades. However, additional network maintenance may be necessary in certain areas where damage to the physical network occurs at a rate unacceptable to a successful transition.
2.3 Other Limitations of the Network
The heights currently published are a construct of annual survey observations that date back to 1904. Despite great care to minimize potential errors, the network was established piece-meal, each year with data adjusted locally. This resulted in significant regional distortions in the current published heights that are, over time, further influenced by crustal motion. Comparisons of the heights currently published against more recent scientific re-adjustments of the network and with the most recent geoid model, indicate regional distortions of up to one metre. While the consistency of heights at a local level (relative heights) probably still have sub-centimetre precision, the application of new technology such as GPS is impeded by the inability to obtain accurate point heights consistent with the current datum.
As an extension of the latter difficulty, the current published heights are also based on a datum that assumed the Pacific and Atlantic oceans were at the same height. In fact, the water level at Vancouver could be higher than the water level at Halifax by 40 to 70 cm. This discrepancy causes a national-scale tilt in the published heights that has significant impacts on different scientific applications. It also has implications for potential discrepancies in heights along the Canada/US border, where the US Government adopted NAVD88.
Subsidence or uplift of individual bench marks due to frost or other local instability is another weakness of the network, significantly affecting its accuracy (or equivalently, confidence in that accuracy) at a local level. Occasional reports of such inconsistencies in the levelling network are expected to increase as the time period since the last maintenance increases.
3.0 Geoid Modelling
The alternative approach to spirit leveling for the realization of a vertical datum is geoid modeling. If the two approaches would be errorless, they would define the same vertical datum. For the levelling technique, the datum is realized in relation to the topography, which is an unstable surface due to geodynamic effects and local uplift and subsidence. For geoid modeling, it is defined in relation to an ellipsoid (e.g., GRS80). The separation between the ellipsoid and the geoid (vertical datum) is the geoid height and is determined from the Earth's gravity field. A geoid-defined datum is easily accessible all across Canada (land and water) through space-based technologies such as GPS and satellite radar altimetry.
3.1 Adoption of a Geoid Model for Datum
With the adoption of a new height datum compatible with space-based positioning techniques, a drastic reduction in reliance on the dense monumented ground network is expected. This reduction would go hand-in-hand with the increasing adoption by the geomatics community of new technologies with their related improvements in accuracy and efficiency.
In this new context, NRCan will continue to be responsible for realizing a relevant and viable datum as a standard by carrying out the required investigation, monitoring, R&D, etc. and making recommendations for required improvements. The principal means of obtaining heights consistent with this datum will be available to GPS users through a file of geoid heights that will enable direct determination of orthometric heights (e.g., heights above "Mean Sea Level"). In fact, any 3D positions referenced to NAD83 (CSRS) could be converted directly into heights referenced to this new datum.
3.2 Monumented Framework for Heights in the New Datum
The highest-level monumented framework for heights will then consist of the federal Active Control Points (ACP) and Canadian Base Network (CBN) points, with NRCan continuing their physical and mathematical maintenance. Provincial High Precision Network (HPN) points will serve as densification to this. NRCan will also continue the mathematical maintenance of the Primary Vertical Network with respect to the new datum through the existing links to the CBN and ACPs. This requires an overall readjustment of the network, not only to generate heights with respect to the new datum, but also to remove much of the distortion resulting from the piece-meal construction of the network. It should be noted however that the current network has its limitations and a new adjustment will not account for or correct for monuments 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 availability of heights referenced to the new datum for the existing network should greatly facilitate the transition to this new datum. To help ease the potential burden associated with moving information to a new datum and as further incentive, NRCan will also compute and maintain a set of transformation parameters (grid shift file) and corresponding software tools to support the conversion of existing data sets referenced to CGVD28.
GSD also recognizes that during the transition period and beyond, there may be some additional requirements for monumented bench marks or levelling lines on a local basis. These situations should be evaluated on a case-by-case basis and where justified, could be addressed in a cost-shared/collaborative fashion with the appropriate provincial geodetic agency. In this regard, NRCan will strive to maintain its precise levelling expertise.
4.0 Summary
The Canadian Geodetic Vertical Datum of 1928 (CGVD28) does not provide today's required national accuracy. Furthermore, the maintenance and expansion of the vertical network by levelling is too costly, time consuming and laborious. A readjustment of the levelling network, 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. The only viable alternative for the realization of a long-term vertical datum for Canada is a geoid model. It would define the datum in relation to an ellipsoid, making it compatible with space-based technologies for positioning (e.g., GPS and satellite radar altimetry). It would allow easy access to heights above mean sea level throughout the Canadian territory. The current first-order levelling would be readjusted by constraining it to ellipsoidal heights and geoid heights at selected CBN stations across Canada. The new datum would bring changes in heights ranging from 0 to 1 m across Canada. However, the height differences locally would remain with the same precision of a few cm or better. CGVD28 will continue to co-exist with the new datum as long as required, and would eventually disappear mainly due to the destruction over time of most bench marks.