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Coming to Terms

ArcUser April-June 2001
CNT (Canadian National Conversion)
The Canadian standard transformation program for conversion between NAD27 and NAD83. Accuracy is often in the range of 0.10 meter and it predicts to within 0.50 meter for 93 percent of all cases.

CONUS (CONtinental United States)
A small-scale conversion set, intended for use across the continental United States by NADCON. CONUS is often available as a regional option in datum transformation programs. Accuracy is often in the range of 0.15 meter.

A set of parameters and control points used to accurately define the three-dimensional shape of the earth. The datum defines part of a geographic coordinate system that is the basis for a planar coordinate system. For example, the North American Datum for 1983 (NAD83) is the datum for map projections and coordinates within the United States and throughout North America.

datum transformation, geographic transformation
A method that converts data between two geographic coordinate systems (datum). Also known as a datum transformation.

ellipsoid, spheroid
When used to represent the earth, the three-dimensional shape obtained by rotating an ellipse about its minor axis. This is an oblate ellipsoid of revolution.

geographic coordinates
A measurement of a location on the earth's surface expressed in degrees of latitude and longitude. See also projected coordinates.

The true three-dimensional shape of the earth considered as a mean sea level extended continuously through the continents.

HARN (High Accuracy Reference Network), HPGN (High Precision GPS Network)
An ongoing effort at the state level to readjust the NAD83 datum to provide a higher level of accuracy. This is a cooperative project between the United States National Geodetic Survey and individual states. While all states have been resurveyed, not all the data has been released to the public. HARN or HPGN is often available as a high accuracy option in datum transformation programs. Accuracy is often in the range of 0.05 meter.

The North American Datum of 1927 uses the Clarke 1866 spheroid 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 NAD 1927 control points were calculated from observations taken in the 1800s. These calculations were done manually and in sections over many years. Therefore, errors vary 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. To address these problems, the North American Datum of 1983 was developed. It consistently covers North America and surrounding areas and is based upon both earth and satellite observations using the GRS80 spheroid. The origin for this datum is the earth's center of mass rather than the point on the earth (as with NAD27), which causes the locations of previous control points in North America to shift, sometimes as much as 500 feet. A 10-year multinational effort tied together a network of control points for the United States, Canada, Mexico, Greenland, Central America, and the Caribbean. Because NAD83 is an earth-centered coordinate system, it is compatible with global positioning system (GPS) data. The raw GPS data is actually reported in the World Geodetic System 1984 (WGS 1984) coordinate system.

NADCON (North American Datum CONversion)
The standard NAD27-NAD83 datum transformation program, created by the United States National Geodetic Survey. Transformation is derived from a minimum curvature surface from the National Geodetic Reference System. Approximate accuracy of 0.15-0.50 meter. NADCON is the fastest, simplest, and most accurate datum transformation for mapping at scales of 1:200 and smaller and is intended for conversion of NAD27 to NAD 83 in the continental United States, Puerto Rico, and the Virgin Islands.

projected coordinates
A measurement of locations on the earth's surface expressed in a two-dimensional system that locates features based on their distance from an origin (0,0) along two axes, a horizontal x-axis representing east-west and a vertical y-axis representing north-south. A map projection transforms latitude and longitude to x,y coordinates in a projected coordinate system. See also geographic coordinates.

projected coordinate system
1. A reference system used to measure horizontal and vertical distances on a planimetric map. A coordinate system is usually defined by a map projection, a spheroid of reference, a datum, one or more standard parallels, a central meridian, and possible shifts in the x- and y-directions to locate x,y positions of point, line, and area features.

2. In ArcInfo, a system with units and characteristics defined by a map projection. A common coordinate system is used to spatially register geographic data for a given area.
3. A reference system consisting of a set of points, lines, and/or surfaces and a set of rules used to define the positions of points in space either in two or three dimensions.

A mathematical formula that transforms feature locations between the earth's curved surface and a map's flat surface. A projected coordinate system includes the information needed to transform locations expressed as latitude and longitude values to x,y coordinates. Projections cause distortions in one or more of these spatial properties--distance, area, shape, and direction.

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