As mentioned earlier, the geoid isn't something we can just go out and find. It is mean sea level plus the natural continuing of mean sea level under the landmass, our topography. This extension must be determined mathematically or modelled. The geoid model is actually based on gravity data collected worldwide. Once we determine the geoid, we can compute the difference between the two surfaces, the ellipsoid and the geoid anywhere in the country.

What is the difference between the two surfaces?

In Canada, the lowest point of the geoid is found over Hudson Bay. There, the geoid is about 49 metres below the internationally accepted ellipsoid. The highest geoid values are situated at the farthest points of the country in St. John's, Newfoundland and in the Yukon Territory, where the geoid is only 10 and 13 metres respectively below the ellipsoid.

How accurate is the current geoid model?

It depends on the region. In Canada's Rocky Mountains there are greater variations in the earths' gravity field and fewer gravity measurements available. The geoid is not as accurate there as in the Prairies or Eastern Canada where the earth's gravity field is more consistent and better defined. In general, though, the accuracy of our current geoid with respect to the centre of the earth is +/- 5 cm. It differs from coast to coast by about 40 cm.

The more accurately we can determine the geoid, the more accurate heights we can get using GPS resulting in easier, faster and cheaper construction of highways, sewers and other projects.