The Universal Transverse Mercator (UTM) system divides the curved surface of the earth into a series of flat rectangles with local coordinate systems. Yellowstone National Park is located in grid (rectangle) 12T.

The methods used to create flat mercator maps may differ either in the formulae that they use to describe the shape of the earth (the spheroid) or in the survey data that were used to locate the origin point of the grid (the map datum).

The UTM coordinates in this report are based on the World Geodetic System of 1984 (WGS84). This system is the default option in most GPS units and is compatible with Arcview GIS layers supplied by the NPS Spatial Analysis Center. In contrast, some of the paper topographical maps still in circulation (e.g. the USGS 7.5 minute quadrangles) are based on the older North American Datum of 1927 (NAD27). Based on the few test calculations that I have run (with Arcview), it seems that using WGS84 coordinates on NAD27 maps can create as much as 0.2 mile error in horizontal location.

The Park Service sometimes refers to NAD83 coordinates in their reports. I can find absolutely no difference between coordinates generated under the WGS84 and NAD 83 systems.


Here is more information copied from the Arcview help files:

To make mathematical calculations easier, the Earth is often treated as a sphere, having a radius valued at 6,370,997 meters. This assumption that the Earth is a sphere can be used for small-scale maps, those less than 1:5,000,000. At this scale, the difference between a sphere and a spheroid cannot be detected on a map; however, to maintain accuracy for larger-scale maps (scales of 1:1,000,000 or larger), the Earth must be treated as a spheroid. Spheroids are also referred to as ellipsoids.

A datum is a set of parameters defining a coordinate system, and a set of control points whose geometric relationships are known, either through measurement or calculation (Dewhurst, 1990). A datum is defined by a spheroid, which approximates the shape of the Earth, and the spheroid's position relative to the center of the Earth. There are many spheroids representing the shape of the Earth, and many more datums based upon them.

A horizontal datum provides a frame of reference for measuring locations on the surface of the Earth. It defines the origin and orientation of latitude and longitude lines. A local datum aligns its spheroid to closely fit the Earth's surface in a particular area and its 'origin point' is located on the surface of the Earth. The coordinates of the 'origin point' are fixed and all other points are calculated from this control point. The coordinate system origin of a local datum is not at the center of the Earth. NAD27 and the European Datum of 1950 are local datums.

In the last fifteen years, satellite data has provided geodesists with new measurements to define the best Earth-fitting ellipsoid, which relates coordinates to the Earth's center of mass. An Earth-centered, or geocentric, datum does not have an initial point of origin like a local datum. The Earth's center of mass is, in a sense, the origin. The most recently developed and widely used datum is the World Geodetic System of 1984 (WGS84). It serves as the framework for supporting locational measurement worldwide. GPS measurements are based upon the WGS84 datum.

There are two horizontal datums used almost exclusively in North America. These are the North American Datum of 1927 (NAD27) and the North American Datum of 1983 (NAD83).


The North American Datum of 1927 uses the Clarke spheroid of 1866 to represent the shape of the Earth. The origin of this datum is a point on the Earth referred to as Meades Ranch in Kansas. Many NAD27 control points were calculated from observations taken in the 1880s. These calculations were done manually and in sections over many years. Therefore, errors varied from station to station.


Many technological advances in surveying and geodesy since the establishment of NAD27 - electronic theodolites, GPS satellites, Very Long Baseline Interferometry, and Doppler systems - revealed weaknesses in the existing network of control points. Differences became particularly noticeable when linking existing control with newly established surveys. The establishment of a new datum would allow for a single datum to cover consistently North America and surrounding areas.

The North American Datum of 1983 is based upon both Earth and satellite observations, using the GRS80 spheroid. The origin for this datum is the Earth's center of mass. This affects the surface location of all latitude-longitude values enough to cause locations of previous control points in North America to shift, sometimes as much as 500 feet. A ten-year multinational effort tied together a network of control points for the United States, Canada, Mexico, Greenland, Central America, and the Caribbean.


For the United States there is an ongoing effort at the state level to readjust the NAD83 datum to a higher level of accuracy using state-of-the-art surveying techniques that were not widely available when the NAD83 datum was being developed. It is a GPS readjustment of NAD83. This project, known as the High Accuracy Reference Network (HARN), or High Precision GPS Network (HPGN) is a cooperative project between the National Geodetic Survey and the individual states.
HARN /HPGN is also known as NAD91 or NAD93. 35 states have been readjusted so far.