What is Datum
According to Merriam-Webster Datum is defined as “1. Something given or admitted especially as a basis for reasoning or inference 2. something used as a basis for calculating or measuring” 1
Datum is a relationship in regards to mapping, GPS and the earth is very important. Without a common datum we would not have a starting point.
To start the explanation we need to consider the shape of the earth. The earth is not completely round like a circle, but more of an ellipsoid. Furthermore, if you try to draw a circle around the earth, parts of the earth will fall below or above the circumference of the circle since the surface of the earth is irregular. The irregularities are caused by mountain’s valleys and even the ocean. sea level is not always constant, it varies based on tides, barometric pressure as well as storm surges.
Clark Spheroid of 1866 and NAD 1927
Back in 1866 Colonel Alexander Ross Clarke (1828–1914), a British Geodesist who mapped Britain back in 1858 via triangulation, developed the Clarke spheroid of 1866. This spheroid helped geodetic determine the size and shape of the earth and exact points on its surface, The North American Datum (NAD) of 1927 used a point on the Clarke spheroid of 1866. This point was Meades Ranch in Kansas 39°13’26.686″N latitude by 98°32’30.506″W longitude from the Greenwich prime meridian which also is used. This triangulation station was the starting point in developing the NAD 1927.
Back in the late 1800’s surveyors started a monumental project of placing brass or aluminum markers. Each marker was connected using techniques like triangulation. From this network of markers and the
result of the calculations the North American Datum of 1927 (NAD 27) was created. The NAD 27 is the datum was used for Topographic mapping in the United States. Later geodesists developed NAD 83 which is a more accurate datum still used today NAD 83. Between these two datums they provide the latitude and longitude framework for the whole earth.
NAD27 and NAD83
The difference between the NAD 27 and NAD 83 is that NAD 83 uses data from terrestrial and doppler satellites which corrects some distortions from NAD 27. NAD 83 uses about 250,000 markers compared to 26,000 for NAD 27. NAD 83 also uses an Earth-center reference rather than a station on the surface. The error for the center of the earth is about 2 meters.
Once Global Positioning System (GPS) became part of the mainstream World Geodetic System (WGS84) became the reference for this system. We now have the ability to pinpoint virtually any point on earth with great accuracy. WGS84 also has a Earth-center point and is estimated to be within 2 cm to the center of the earth’s mass2.
Because of the differences in accuracies when comparing your location from a GPS to a map created with NAD 27 datum you will have an error in that you location on the map will be incorrect unless corrected. Most modern GPS have the ability to change their datum reference. On Topographical maps there is usually a key located at the lower left corner. Older maps may use NAD 27, newer maps will have either been updated to NAD 83 or have a correction from NAD 27 to NAD 83. For instance the key may tell the difference between the two is that all locations have moved 60 meters east and 6 meters north. If your map is using NAD 27 configure your GPS to use the same. If you are using a NAD 83 then have your GPR use either NAD 83 or WGS84.