News & Events
Canadian HT2 Geoid Model in different Epochs: 1997, 2002, and 2010
True North | Winter 2020
Even though the Canadian Geodetic Vertical Datum of 2013 (CGVD2013) has been the official vertical datum of Canada since 2013, there are still many users in survey, engineering, construction and mining sectors using the HT2 hybrid geoid model to convert the heights (ellipsoidal heights) to elevation (orthometric heights) during a GNSS survey. The current version of the HT2 hybrid geoid model is at epoch 1997.0. This means that it must be used in a GNSS survey when the geometrical coordinates (Latitude, Longitude, Height) are at Epoch 1997.0 of NAD83 (CSRS).
Most of the provincial and federal geodetic agencies updated their reference epochs to either 2002.0 or 2010.0; therefore, in order to use the HT2 hybrid geoid model when conducting surveys using GNSS, new versions of the model in epoch 2002 and 2010 must be used. Table 1 shows the adopted NAD83 (CSRS) epochs for Canadian provinces and territories.
Table 1: NAD83(CSRS) adopted epochs for Canada’ s provincial geodetic agencies
| Province | Epoch |
| British Columbia (Mainland) | 2002 |
| British Columbia (Vancouver Island) | 1997 |
| Alberta | 2002 |
| Saskatchewan | 1997 |
| Manitoba | 2010 |
| Ontario | 2010 |
| Quebec | 1997 |
| New Brunswick | 1997 |
| Prince Edward Island | 2010 |
| Nova Scotia | 2010 |
| Newfoundland and Labrador | 2010 |
| Nunavut | 2010 |
| Northwest Territories | 2010 |
| Yukon | 2002 |
Canadian Geodetic Survey (CGS) recently released two new versions of the HT2 model, HTv2.0 (2002) and HTv2.0 (2010). The name of the previous model was changed to HTv2.0 (1997). The new Epochs of the HT2 model are built directly from HTv2.0 (1997) by simply adding a 5 and 13-year vertical velocity correction to the hybrid geoid heights. Figure 1 shows the correction applied to HTv2.0 (1997) to generate HTv2.0 (2010). The corrections range from -0.087 m to 0.379 m.
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Figure 1: The correction applied to HTv2.0 (1997) to generate HTv2.0 (2010)
These new HT2 models are implemented in the GPS-H applications (desktop and online)
Cansel has converted and tested these new geoid models to the Trimble’ s proprietary format of geoid models (*.GGF) to be used in Trimble Access (field software) and Trimble Business Center (office software). These models can be downloaded.
Depending on the Epoch of the NAD83 (CSRS), the relevant geoid model must be used. For example, the users of the Can-Net system in Ontario must use the HTv2.0 (2010) model because the reference epoch for Can-Net in Ontario is 2010. Another example is when the PPP service is used to obtain the precise coordinates of the GNSS base station in an RTK survey. PPP service provides users with the option to pick a certain Epoch for processing. Depending on the Epoch, the relevant HT2 model must be used. The coordinates that are not at epochs 1997.0, 2002.0 or 2010.0 should be transformed to one of these three reference epochs using, for example, TRX software available from Natural Resources Canada
Please note that the Canadian Geodetic Survey highly recommends the use of CGVD2013 when conducting surveys using GNSS. The development of the new epochs for the HT2 geoid model does not mean that CGS will resume the maintenance of the first-order benchmarks in CGVD28 by levelling or GNSS technique. The new epochs of the HT2 geoid model are temporary products until CGVD2013 is well established within the geomatics community.
Babak Amjadiparvar, P.Eng., Ph.D.
Geospatial Team Lead, Cansel
Acknowledgment:
I would like to thank Marc Vé ronneau from Canadian Geodetic Survey for providing me with some internal reports and information about the new HT2 models.