For this example, the vertical datum is 'Mean Sea Level' and the horizontal datum is 'North American Datum 1927'.įigure 2 - Example of Co-ordinate Conversion Table from National Topographic System map sheet 41J/2 Algoma, Ed. This note informs users of the horizontal and vertical datum’s that coordinates and elevations are based upon. Maps not scheduled for revision for a long period of time will not be converted, but instead, will be overprinted with notes, usually in the bottom margin, enabling the conversion of the map grid of the geographic co-ordinates to NAD 83.įigure 1 below is an example of a Datum Note. The National Topographic System is revising all existing maps to NAD 83 and since April, 1989, all new mapping has been computed using NAD 83 values. NAD 83 is therefore more compatible with the co-ordinate system used to compute the orbit of satellites used for surveying and for the use of GPS receivers. NAD 83 is based on the Earth's centre of gravity and also is a more accurate mathematical representation (ellipsoid) of the shape of the earth than was NAD 27. Other departments of the Government of Canada and provincial governments have adopted NAD 83 as their geodetic reference system for new maps. The United States, Mexico, Denmark (Greenland) and Canada had reached agreement on the definition of the datum in 1983, hence the name NAD 83. The system as revised in 1983 was officially adopted by the Department of Energy, Mines and Resources (now Natural Resources Canada) in May of 1990. The introduction of navigation satellites has allowed cartographers to define the shape of the earth and the positions of features on the earth more precisely than before.

In 1927 a complete survey of North America was completed to define positions in latitude and longitude and all survey networks which also included the 1000 metre grid pattern that we all have been using on Canadian topo maps. Refer to Figure 1.) but Edition 4 of the same map was based on 'North American Datum 1983' or 'NAD 83'. Edition 3 of our map was based on 'North American Datum 1927' or 'NAD 27' (shown as the Datum Note in the bottom margin of topo maps. We were using two editions of the same map. Was there a misprint on the maps?Ĭanoe trippers should be aware of what has happened. The maps appeared the same but the grid lines had shifted by over 200 metres.

When looking at two maps we realized the grids on our Canadian 1:50 000 topographic sheets were not in the same positions. On a trip in the Algoma District our group was trying to identify exactly where we were on a long shoreline using triangulation and comparing our grid references. This article originally appeared in the May 1999 edition of 'Canews', (the Magazine of the Ontario Recreational Canoeing Association).

Earth energy grid map