Some Concepts That Underpin GIS - Introducing Geographic Information Systems with ArcGIS - page 18

northern and a southern part. In any event, a coordinate system is imposed on the resulting projection
such that the numbers in any given zone:

Are always positive

❏

Always increase from left to right (west to east)

❏

Always increase from bottom to top (south to north)

❏

The representation of our previously discussed object (at 38 o N and 84.5 o W) in the UTM coordinate
system, when that system is based on WGS84, is a “northing” of 4,208764.4636 meters and an “easting”
of 719,510.3358 meters. The northing is the distance to the point, in meters, 15 from the equator measured
along the surface of an “Earth” that has no bumps. The easting is somewhat more complicated to explain
because it depends on the zone and a coordinate system that excludes negative numbers. Consult a
textbook on geodesy or cartography, or review the thousands of Web pages that come up when you type
the words

UTM “coordinate system” “Transverse Mercator”

into an Internet search engine (e.g., www.google.com ).

One version of the UTM coordinate system is based on NAD27. In this case, our object would have
different coordinates: northing 4,208,550.0688 and easting 719,510.6393. This produces a difference of
about 214 meters. If you combined WGS84 UTM data with NAD27 UTM data, the locations they depict
might be in error by just enough not to be obvious at some scales, but great enough to cause trouble. 16

Each state of the United States has one or more State Plane Coordinate Systems (SPCSs). They were
developed, originally in the 1930s, by the U.S. Coast and Geodetic Survey. These systems are based on
different projections (usually Transverse Mercator or Lambert Conformal Conic), depending on whether
the state is mostly north-south (like California) or mostly east-west (Tennessee). The units of an SPCS
may be international feet, survey feet, or meters, depending on decisions made by the state itself. 17 Zone
boundaries frequently follow county boundaries. The coordinate system(s) used in one state are not
applicable in neighboring states. Nor can you apply the SPCS of one zone to areas in the state only a
short distance away in another zone. Furthermore, the difference between NAD27 and NAD83 (WGS84)
can be startlingly large. In Kentucky, for example, 38.0000000 o (North) and 84.5000000 o (West) would
translate into a northing of 1,568,376.1900 feet and an easting of 182,178.3166 feet when based on WGS84.
However, when the basis in NAD27, the coordinates are 1,927.939.8692 and 182,145.9821, which makes a
difference of some 68 miles!

Why the large differences in projected coordinate systems based on NAD27 and those based on WGS84?
Because those responsible for the accuracy of the other coordinate systems took advantage of the

15 In fact, the meter was originally defined as one tenth-millionth of the distance from the equator to the North Pole,
along a meridian that passed through Paris.
16 Comparing these coordinates with the WGS84 UTM coordinates, you see that virtually all of the difference is in the
north-south direction. While true for this particular position, it is not true in general.
17 The meter is the standard unit of length in most places in the world. Two different lengths of “foot” are defined in
terms of the meter. A “U.S. Survey Foot” is one in which a meter is considered to be 39.37 inches, exactly; the other sort
of foot is the international foot, where an inch is 0.0254 meters, exactly. The survey foot and the international foot are
almost, but not exactly, the same length.