Latitude and longitude coordinates are not unique

Latitude and longitude coordinates are not unique

Latitude and longitude are measurements on a model of the earth, normally an ellipsoid. Several hundred models have been defined and about forty different models remain in daily use. The selection of model together with its position and orientation relative to the earth is defined through a geodetic datum. If the model or its position or orientation is changed, that is if the geodetic datum is changed, the values of latitude and longitude at a point will usually change. The same values of latitude and longitude referenced to different datums will refer to different locations (see Figure B.1). Conversely, for coordinate values to be unambiguous, the datum to which they are referenced must be identified.

Roger Lotts picture

NOTE "WGS 84", "ED50" and "OSGB 1936" are the identifiers of Coordinate Reference Systems.

Figure 1 - Locations with identical latitude and longitude values on three different Datums

Projected coordinates are derived from geographic (latitude and longitude) coordinates. For the projected coordinates to be unambiguous the datum for their source geographic coordinates must therefore also be identified.

 

The differences in coordinate values of a point caused by change of geodetic datum are typically about 50 to 500 metres but can be considerably more in extreme cases. When dealing with coordinates with an accuracy of approximately 1 kilometre or worse, these differences are not significant. For applications requiring an accuracy of better than approximately 1 kilometre, if coordinates are to be unambiguous, the identification of their datum is essential.

 Roget Lott, OGP and formerly Head of Survey BP Exploration.