Geographic Coordinate System

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Map projections allow us to convert from sphere to planar (flat) representation. Geographic coordinate systems allow for the measurements of distances and to determine direction. Except for the Geographic Grid all coordinate systems are based on some form of map projection.

Coordinates

In a coordinate system, the X value given first and referred to as an easting, the Y value given second and referred to as a northing. There are no west or southern values.  Coordinates always measure right and then up.

Universal transverse Mercator (UTM)

Universal transverse Mercator (UTM) is a geographic coordinate system and the most prevalent plane grid system used.  UTM divides the earth into 84˚ north latitude to 80˚ south and is numbered into 60 vertical zones (each 6˚ latitude wide). All measurements are positive. There are two starting points (all measurements in meters north and east); one at the equator and another at 80˚ south (1000,000,000 meters south of the equator).

UTM is used for remote sensing (satellite and aerial imagery analysis) because it allows precise measurement using the metric system. UTM uses transverse Mercator projection (conformal). This coordinate system is used by the US federal government for their data. The equator is designated as the origin for Northing (y) and 500,000 meters is the false easting (x).

State Plane Coordinate System (SPCS)

The state plane coordinate system uses a unique set of coordinates for each of the 50 states, using either a transverse Mercator or Lambert’s conformal conic project tied to a national geodetic framework. Each SPCS zone has its own projection and coordinate grid. SPCS uses feet as the measurement. This coordinate system is highly accurate (four times as accurate as UTM).  For more: state plane coordinate system.

Datums

Grid systems rely on accurate measurements of the earth. The starting point for such a measurement is called the datum and is based on ellipsoidal representation.

  • First NA datum (1866) – Flatness ratio of 1/294.9787 1-b(polar axis)/a(equatorial axis)
  • NAD 27 used the same ratio
  • NAD 83 based on estimates of distance to center of the earth
  • Now the standard (GRS80)


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