More on the Mapmakers
If you want to learn more about geodesy and cartography, read the "Geodesy and Datums" section in The American Practical Navigator by Bowditch or visit the NIMA web site at www.nima.mil. Another excellent book on cartography, written in layman's terms, is The Mapmakers, by John Noble Wilford.
The difference between the datum of the chart and the datum selected in your GPS unit can have a significant impact on navigational accuracy. The problem is that not all datum is the same and unless you're aware of the different datum, you could get a wrong reading of where your are.
First, let's start with background on the problem. To make a paper chart, the mapmaker creates a projection that converts the spherical surface of the earth to a flat chart. Most navigation charts are drawn to a Mercator projection on which latitude and longitude lines are parallel. The cartographer's job is complicated by the fact that the earth's surface is an irregular ellipsoid which is larger in diameter at the equator than at the poles. For that reason, cartographers have developed models to approximate this ellipsoid shape. These models must reflect the local variations caused by the irregular surface of the earth. Geographic datum is a frame of reference used by cartographers to define a chart's grid of coordinates.
Through the centuries, cartographers have used a variety of datums to create nautical charts and maps. There are scores of such local and regional datums. The world's geographic community has attempted to develop standardized worldwide datums to provide a universal system of coordinates. These datums have been refined with the advent of satellites. As a result, cartographers have dramatically improved the accuracy of the charts.
But navigators need to be aware of the various chart datums being used. In electronic charts, depending on the area, certain chart datums have been chosen as default settings.The current U.S. standard is NAD 83. NOAA is in the process of changing from the North American Datum 1927 (NAD 27) to the newer World Geodetic System 1984 (WGS 84) standard. NOAA charts using the WGS 84 standard are listed as NAD 83. Be aware that the differences between NAD 27 and NAD 83 paper charts may range from 100 feet in Maine to 1,500 feet in Hawaii.
In other parts of the world, charts are based on different formats such as WGS 72 or their local and regional datums. For example, in the Bahamas and the Caribbean, some of the paper charts are extremely old and contain errors of 500 feet or more. In other areas of the world, the errors may be several miles.
GPS uses WGS 84 as its global reference. When using your GPS unit, find out the datum for the chart on which you are navigating. The GPS default setting is WGS 84. If the electronic or paper chart is based on another datum, such as NAD 27, you may well notice errors that can be significant.
Select that chart's datum for your GPS. Refer to your GPS operating manual for specific information on how to make this change in the proper datum default setting. All GPS units have the capability of displaying latitude and longitude coordinates in many different datums.
The majority of electronic charts in vector format are based on WGS 84 for compatibility with GPS. If not, the datum used will be displayed on the electronic chart. Raster-formatted charts are based on the datum of the scanned chart. Check the listed datum of the chart and select the appropriate datum in your GPS unit. Failure to do so can place you at risk, especially if you are relying solely on your GPS and electronic charting system for navigation.