Datum F&Q

A datum is a mathematical model of the Earth which approximates the shape of the Earth, and enables calculations such as position and area to be carried out in a consistent and accurate manner. The datum is physically represented by a framework of ground monuments (i.e., trig stations) whose positions have been accurately measured and calculated on this reference surface. Lines of latitude and longitude on a map or chart are referenced to a specific map datum.

Coordinate values resulting from interpreting latitude, longitude, and height values based on one datum as though they were based in another datum can cause position errors of up to one kilometer. Reporting coordinates without datum is analogous to reporting distance as a number without units, i.e. the distance between Frankfort and Lexington is 50 (miles, KM, feet, etc.??).

A reference datum is a known and constant surface which is used to describe the location of unknown points on the earth. Since reference datums can have different radii and can have different centre points, a specific point on the earth can have substantially different coordinates depending on the datum used to make the measurement. There are hundreds of locally-developed reference datums around the world, usually referenced to some convenient local reference point. Contemporary datums, based on increasingly accurate measurements of the shape of the earth, are intended to cover larger areas. The most common reference Datums in use in the North America are NAD 83, and WGS 84. (NAD 27 is no longer accepted)

The WGS84 datum, which is almost identical to the NAD83 datum used in North America, is the only world referencing system in place today. WGS84 is the default standard datum for coordinates stored in recreational and commercial GPS units.

Users of GPS are cautioned that they must always check the datum of the maps they are using. To correctly enter, display, and to store map related map coordinates, the datum of the map must be entered into the GPS map datum field.

If you are using the Kentucky Hunting and Fishing web site or Google Maps to determine coordinates, the datum used there is WGS 84.

If your data was gathered in NAD 27 datum use can use kgs.uky.edu to convert to WGS 84.