A datum is a mathematical model of the Earth which approximates the shape of the Earth and is used as a reference on maps. It enables calculations such as position and area to be carried out in a consistent and accurate manner even though the earth is not perfectly round, but is spheroid in shape. Every map has a map datum reference that is usually listed in the title block or legend of the map.
When using a map and GPS together for navigation, your GPS must be set to the same datum as the map you are using. Failure to do so could result in errors of 200 meters or more in your displayed position. This is because the same position on earth (a hut or trailhead for example) will have different coordinates depending on the datum used to create the map. Most United States Geological Survey (USGS) topographic maps and ALL 10th Mountain Division Hut Association winter maps are referenced using NAD 27. Some newer USGS maps use NAD 83. If you are using your GPS without a map (shame on you for not having a map!), using WGS84 is generally the best datum to use as it is considered the most accurate datum to use with a GPS alone. Consult your GPS owner’s manual to learn how to set the correct datum to match your map's datum