Map Datum

A map 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.

Maps, Charts And Map Datum

Lines of latitude and longitude on a map or chart are referenced to a specific map datum.  The map datum is usually listed in the title block of the chart.  A map datum is used to match the location of features on Earth's surface to coordinates and locations on the map or chart.

Maps all start with some form of survey.  Early maps and surveys were carried out by teams of surveyors on the ground using transits and distance measuring "chains".  Surveyors start with a precise reference position and measure from it to locate other features.    These methods do not work well over continents or political borders.   The precision reference positions on earth, survey markers, are the information that the map datum is based on.  With the help of space technology, a new standardized datum has been put in place; based on the center of the earth.

Working Between A GPS Device And A Map/Chart

When  Plotting Coordinates On A Chart Or Map To Be Use On A GPS Device, Or When Taking Coordinates From A GPS Device And Trying To Plot Them On a Map, The GPS Device Must Be Set To Use The Datum Of The Map Or Chart Being Used.  The datum used to create a map or chart is generally indicated in the legend of the map or chart.