Why do we need a geoid-based datum?

Even though the current vertical datum (CGVD28) is very precise over short distances (e.g., 30 km), it includes significant distortions at the national scale. Its access is only available at benchmarks, which are mostly located in southern Canada. Furthermore, the maintenance of a levelling network is very expensive. Thus, the best alternative to levelling is geoid modelling. A geoid-based datum would be accessible by Global Navigation Satellite Systems (e.g., GPS) at any location across the Canadian territory.

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Who really need a new vertical datum?

Already, a large number of stakeholders rely on GPS as their tool of choice for accurate positioning and require a geoid model to convert their ellipsoidal heights to orthometric heights. In addition, an increasing number of stakeholders are conducting surveys in remote regions where the vertical datum is not accessible through traditional benchmarks.

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Why not delay the adoption of a new vertical datum?

NRCan has stopped maintaining the levelling network since 1996. It is important that NRCan implements a new datum before the current datum deteriorate to such a level that it might be difficult to have a smooth transition period between the old and new datums. Furthermore, it exists already a large community of GPS users, who require an accurate geoid-based vertical datum across the country.

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Do the elevations of benchmarks have to change?

Unfortunately, CGVD28 includes significant distortions across the country. These distortions will be corrected in the new datum meaning that absolute heights may change by almost one metre in certain regions. For several regions, the change in heights will be less than a few cm. The local height differences will remain the same in the two datums.

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How can the confusion of having two vertical datums be minimized?

Having two vertical datums during the transition period may bring confusion to some stakeholders. NRCan and the provincial agencies will indicate clearly the datum of the data when disseminating heights. Also, it is important for stakeholders to identify properly the datum used in their documents. Proper identification will remove a lot of confusions.

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Does the geoid-based datum represent mean sea level better than the levelling-based datum CGVD28?

First, lets mention that the mean sea level (MSL) is not a level surface. Just like land, oceans have permanent topography, albeit ranging from -2.0 to 2.0 m worldwide. CGVD28, which is constrained to a series of tide gauges across the country, represents well the MSL at these specific locations. However, these same constrains are responsible in part for systematic errors in CGVD28. On the other hand, the geoid is a level surface; it does not coincide with MSL. However, the geoid-datum will be near the MSL on the east coast because the geoid will be selected such as it represents MSL at the tide gauge in Rimouski. This location is one of the constraints for CGVD28 and the only constraint for NAVD88 (US vertical datum). NRCan will make available a model representing the separation between the geoid and MSL along the Canadian coast.

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How do heights estimated by GPS and corrected using a geoid model agree with CGVD28 heights?

CGVD28 contains systematic errors, which can reach close to one metre in an absolute sense. However, local GPS-corrected height differences will agree well with CGVD28.

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How will I validate the precision/accuracy of my heights when I use a GPS/geoid approach?

The stations from the federal Canadian Base Network (CBN) and provincial High Precision Network (HPN) can be used by stakeholders to validate GPS/geoid procedure for the determination of accurate 3D positions. In addition, several benchmarks have 3D positions; however, the quality of these 3D positions may vary depending on the GPS epoch of observations. Also, one has to be careful about the stability of the benchmarks.

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Can I still do surveys with spirit levelling and integrate them into the new vertical datum?

Yes, most benchmarks will have an elevation in the new datum. If there are no benchmarks within a reasonable distance from your project, you can install your own control stations by GPS in the project area and resume the work locally by levelling technique.

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Will I have to update elevations in my database or my topographic maps?

It will depend on the accuracy of your datasets. It might not be necessary if the changes are smaller than the error associated to your datasets. A conversion tool will be made available in order to convert heights between the two datums. If your datasets are local, the conversion can be as simple as adding a constant.

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How will I maintain compatibility between historical and new height surveys?

The compatibility between historical and new height survey can be maintained by conducting the first new survey in the two datums. This will allow you to determinate the relation between historical and new data. If the project is local, the conversion should be as simple as adding a bias to the old or new datasets.

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What will be the name of the new vertical datum?

It has not been decided yet. However, the new datum might be identified by the following acronym CGVD2010(CGG2008), where the name between parenthesis is the geoid model realizing the vertical datum. If we update the geoid model in 2015 in the same reference system, the datum would be identified as CGVD2010 (CGG2015).

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Why a new name for the vertical datum?

The new vertical datum must be identified by a different acronym because CGVD28 and the new datum are not in the same reference system, i.e., they are not defined the same way. In this case, we are not talking about levelling versus geoid modelling because these are only techniques to realize the vertical datum. Rather that CGVD28 is defined by MSL at five tide gauges across Canada while the new datum is defined by the equipotential surface that coincides with mean water level at the tide gauge in Rimouski. The mean water level in Rimouski is defined such as the geoid heights (N) at benchmarks in the Rimouski/Pointe-au-Père area are equal to the difference between NAVD88 orthometric heights and ellipsoidal heights: N_NAD83(CSRS) = h_NAD83(CSRS) - H_NAVD88 in the Rimouski/Pointe-au-Père sector. Thus, the new vertical datum for Canada will have basically the same reference system as NAVD88.

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Will NAVD88 and the new datum in Canada coincide along the border?

Even though NAVD88 and the new datum have the same reference system, these two datums will not coincide along the border. Current information shows that NAVD88 is tilted in relation to the latest gravimetric geoid model (CGG2005) developed at NRCan. NAVD88 indicates that MSL next to Vancouver is higher than MSL next to Halifax by 1.5 m. On the other hand, CGG2005 indicates a difference of 0.5 m. Thus, the separation between the two realizations of the reference system increases gradually from 0.0 m in Rimouski to 1.0 m in Vancouver.

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How Precise is the geoid-based vertical datum?

The geoid-based datum will have a better absolute accuracy than CGVD28 across the country. Still, the geoid model will not be errorless, but no systematic errors should be larger than the decimeter (95% confidence) across the country. CGVD28 have systematic errors reaching close to the metre level. Overall, the accuracy of the geoid model will be approximately 3 to 5 cm. On the other hand, the relative precision of the geoid model will be comparable to spirit levelling. Naturally, the relative precision of the GPS-derived orthometric heights will also depend on the precision of your ellipsoidal heights.

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How can the geoid model be further improved?

Between the late 1980s and today, geoid models have improved quite significantly. We saw changes at a level of a few metres. Today, the theory takes care of many terms that were formally omitted or neglected; more gravity data are available from terrestrial and spatial techniques; and better Digital Elevation Models (DEM) are available. This new information is currently stabilizing the realization of the new geoid models. Furthermore, geoid modeling is gaining global acceptance as the future technique to define national or continental vertical datum. Thus, national and international academic institutions and governmental agencies are developing new techniques to achieve higher accuracy in geoid modelling. NRCan is keeping abreast with all new developments.

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How much will an orthometric height measured with GPS and corrected with a geoid model vary in time (h-dot and N-dot)?

The Earth is a dynamic planet; it is always changing. Some changes can be quite drastic (e.g., landslide, earthquake) while others are can be more subtle (e.g., post-glacial rebound). When you have drastic events, the benchmarks can move significantly or be destroyed completely. In this case, you loose access to the vertical datum. On the other hand, the impact of a drastic event on the geoid is very small; thus, it is possible to take new GPS measurements and install new control stations immediately. Subtle changes are difficult to detect because they expend over a very large regions. These changes are usually not detected when conducting local relative surveys (e.g., levelling or differential GPS). However, GPS technique such as Precise Point Positioning (PPP) will show that the terrain can move by as much as 1 cm per year. This dynamic change of the topography will also bring a 10% change to the geoid. Thus, the long wavelength components of the geoid can change by approximately 1 cm every ten years.

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How is the geoid model validated?

A geoid model can be validated in two ways: error propagation and independent datasets. For the former, the challenge is to associate a realistic error model to the input data required in the determination of a geoid model. This internal accuracy can be too optimistic because it will not consider systematic errors and omissions. For the latter, GPS on BMs is the most common approach. It consists of comparing the geoid models (N) to geoid heights determine from GPS ellipsoidal heights (h) and spirit leveled orthometric height (H): h - H - N = ε. The discrepancies ε should be zero (or a constant) if each height would be errorless. The problem with this technique is the difficulty to disassociate errors from the geoid model, levelling data, GPS measurements and stability of the BMs. Other independent techniques for validation could be satellite radar altimetry and astro-geodetic deflections of the vertical.

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Without new levelling lines, how will it be validated in the future?

Existing benchmarks (BMs) are here to stay for many more years and most of these BMs are fairly stable. Even if we do not conduct new levelling surveys, we still have a lot of levelling lines to validate geoid models for many more years. In addition, we can also validate the geoid model at tide gauges. By the time that most BMs will disappear, validation of geoid models by "GPS on BMs" will not be a priority.

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How often will a new geoid model be published?

We know that it is impossible to realize an errorless geoid model. The model will always be as good as the theory and input data. Certainly, these two elements will improve with time. It is hard to say at this time how often a new model will be published; however, we do not expect that the changes in geoid models will not be larger than what we saw with CGVD28 over the last 75 years. Furthermore, models will not be published at a higher rate than currently, which is approximately every five years.

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To what 3D reference frame realization is the geoid based vertical datum referred to?

A geoid model is determined from gravity measurements. Knowing that gravity points towards the center of mass, we assume that the reference frame is ITRF. Usually, we associate it to the latest ITRF realization (e.g., ITRF00). The geoid model is converted to NAD83 (CSRS) using a seven-parameter transformation (rotations, translations and scale). The epoch of the geoid model is determined from the observations period of the satellite data.

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What method can I use to convert my old CGVD28 elevations to the new datum (or the opposite)?

There are three approaches. First, you can conduct your own GPS surveys on a series on BMs in your area and determine the separation between the two datums (ε = (h-N)_new H_old). The advantage is that it will convert any local datums to the new datum. The disadvantage is that you must be able to determine accurate ellipsoidal heights in the proper reference frame. A second approach is to download benchmarks information in your area. Most BMs will have published heights in the two datums (CGVD28 and new datum). Finally, a third approach is to use the National Height Conversion (NHC) tool, which is a grid containing the CGVD28-New datum separation across Canada. Even though benchmarks are quite dense along a levelling line, levelling lines can be quite sparse in between. Thus, NHC might not be representative of the actual separation in sparse area.

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I am confused with all these acronyms: NAD83, NAD83(CSRS), ITRF, CGVD28, NAVD88, NGVD29, IGLD85, GSD95, CGG2000, HTv2.0, GEOID03, etc. What is the difference?

Traditionally, geodesy consists of two types of reference frames: a 2D horizontal network (latitude and longitude) and a 1D vertical network (height above mean sea level). Today, with the immersion of satellite positioning, we are now taking about 3D networks (latitude, longitude and ellipsoidal height).

1. NAD27 and NAD83 are traditional horizontal networks while NAD83(CSRS) and ITRF are modern 3D networks. The horizontal components of NAD83(CSRS) are more precise than those of NAD83. NAD83, who was though to be at the Earth center of mass at the time, is actually off by approximately two metres. ITRF is a global reference frame with its origin is at the center of mass (±2 cm). There are several realizations of ITRF (e.g., ITRF97, ITRF00). These are new versions which are more precise than the previous ones. Coordinates in ITRF can be converted to NAD83(CSRS), or the opposite, using a seven-parameter transformation. WGS84 is an ITRF reference frame.
2. CGVD28, NGVD29, NAVD88 and IGLD85 are traditional vertical networks. These networks are realized by spirit levelling. CGVD28 is the current vertical datum for Canada. NGVD29 is the former vertical datum for the USA. That datum was replaced in 1995 by NAVD88, which is a minimum constrain adjustment of the levelling data in North America. IGLD85 is a special vertical datum for the St-Lawrence Seaway and Great Lakes.
3. GSD91, GSD95, CGG2000, CGG2005, HTV2.0 and GEOID03 are geoid models. A geoid model is an integral part to a 3D network to convert ellipsoidal heights to orthometric heights (heights above a vertical datum). GSD91, GSD95, CGG2000 and CGG2005 are pure gravimetric geoid models. Each new model is a better representation of the geoid. CGGxxxx is now the standard to identify gravimetric geoid models in Canada. HTv2.0 and GEOID03 are hybrid geoid models, i.e., they are distorted geoid models to represent a spirit-levelled datum. HTv2.0 is a representation of CGVD28. HT stands for Height Transformation. GEOID03 is a representation of NAVD88 in the USA. In Canada, we opted not to name our hybrid models geoid because these are distorted to represent a vertical datum, which includes systematic errors.

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Why the height of benchmarks changes?

There are basically three reasons: 1) new reference system; 2) new realization of the reference system; and 3) the Earth is a dynamic planet.

1. Changes due to a new reference system are rare because we rarely adopt a new reference system. For example, 3D coordinates are different between NAD83(CSRS) and ITRF because these two reference frames do not have the same origin (about 2 m apart).
2. Changes due to a new realization of the reference system are more comment because all new observations will modify the reference frame. For example, control stations will have new updated coordinates based on more precise observations.
3. Finally, the Earth is not static. There are earthquakes, landslide and post-glacial rebound to name a few natural changes to the planet. There are also some changes due to human intervention. For example, there is subsidence due to mining and oil exploration or construction of major hydro projects.

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