Latitude and Longitude, Coordinate Systems and  Projections - NAD27 and NAD83

 

Are you confused about terms like NAD27 and NAD83?    Read on....


Locating Points on the Earth

Locations on the earth are routinely defined using latitude and longitude.  Longitude is a measure east-and-west relative to the prime meridian which runs through Greenwich, near London, in the United Kingdom.  Latitude is a measure north-south relative to the equator. This sounds straight forward, but as discussed below, may contribute to errors in locating towers by several hundred feet. 

Map Projection

A map projection is the method of taking the curved surface and displaying this on a flat surface such as map or computer screen.  There are many projections offering different ability to maintain equal areas or equal distances. To learn more about projections, click here.

Coordinate System

A coordinate system is a means for identifying a point on the earth on a planimetric map.  The coordinates systems is typically defined using an x- and y-ordinate or northing and easting.  Commonly used are State Plane Coordinates and UTM.  Latitude and longitude are actually a special kind of coordinate system, using spherical coordinates. 

Latitude and Longitude

The earth is not a perfect sphere and bulges somewhat around the equator.  What many people are not aware is that different ellipsoids are used in projecting. 

North American Datum of 1927 (NAD27) is a datum based on the Clarke ellipsoid of 1866.  The reference or base station is located at Meades Ranch in Kansas. There are over 50,000 surveying monuments throughout the US and these have served as starting points for more local surveying and mapping efforts.  Use of this datum is gradually being replaced by the North American Datum of 1983.

North American Datum of 1983 (NAD83) is an earth-centered datum based on the Geodetic Reference System of 1980. The size and shape of the earth was determined through measurements made by satellites and other sophisticated electronic equipment; the measurements accurately represent the earth to within two meters. 

There are differences in the two ellipsoids ranging from 200-300 feet in the western US to several tens of feet in the central and eastern US as show below:

                Click here for a larger image.

Converting NAD27 to NAD83

There are several ways to convert between these two systems.  Many desktop GIS software have conversion routines built in.  Additionally you can use online calculators, download the underlying software program, or purchase a geographic calculator.

For more detail, check the additional definitions in this glossaries by ESRI or Mentor Software

 

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