# Map Datum

An important change has been made in National Topographic System (NTS) maps. Natural Resources Canada (NRCan) uses the North American Datum 1983 (NAD83) as its geodetic reference system. As a map user, this is something you need to know, especially if you use GPS receivers, because it will modify the coordinates you will use to position yourself on the map. Geographical (latitude and longitude) and UTM (Universal Transverse Mercator) coordinate systems are positioned on the shape of the world by a "datum". The model for the shape of the world that is used is called an "ellipsoid".

## What is an Ellipsoid?

As everybody knows now, the world is not flat. Actually, it is not round either: it is flattened at the poles. It resembles a three dimensional ellipse called an ellipsoid. An ellipsoid can be defined by a number of mathematical characteristics. As our knowledge of the world has increased over the centuries, we have come to know the dimensions and shape of the world more accurately. Mathematical models of the shape of the world have therefore become more and more precise, and it has become necessary to change to a more accurate model. The new, more accurate model is called Geodetic Reference System 1980 (GRS80).

## What is a Datum?

A datum defines how a coordinate system is seated over the ellipsoid. The origin of the new NAD83 system is the centre of mass of the Earth, whereas the old North American Datum 1927 (NAD27) had a different origin, making it useful only in North America. GPS receivers also use the centre of mass of the Earth as their system's origin.

## What does this all mean?

This means that, because of the change in ellipsoid and datum, there will be a shift in every position on the map between the old edition (using NAD27) and the new one (using the new NAD83). Also, adjoining sheets may be in different datums, causing a loss or addition of detail at the map's neatline. When a map is revised, it is automatically converted to NAD83. All of our digital products have been converted as well. So when using topo maps with adjoining sheets or with a GPS receiver, you should always check the datum note first. This note can be found on the south margin of the map and will state either "North American Datum 1927" or "North American Datum 1983". You must set your GPS receiver to the proper datum in order to be able to use a topo map directly.

NRCan has added a note at the bottom of its maps enabling users to convert coordinates to and from the different datums. For users needing more precise coordinate conversions, the Geodetic Survey Division has developed a software package called the National Transformation Version 2 (NTv2) for converting coordinates between the NAD27 and NAD83 reference systems in Canada.