Some Concepts That Underpin GIS - Introducing Geographic Information Systems with ArcGIS - page 15

The datum described as the World Geodetic System of 1984 (WGS84) offers the most recent, widely
accepted view of where the center of the Earth is, its shape, and the location of its poles. The ellipsoid
of WGS84 is virtually identical to the GRS80 ellipsoid. 11 In the coterminous states of the United States,
this datum is virtually identical (within millimeters) to the North American Datum of 1983 (NAD83),
although they result from different approaches and calculations.
According to the WGS84 latitude-longitude graticule, the object previously described would be at
latitude 38.00007792 o and longitude 84.49993831 o . The difference might seem insignificant, but it amounts
to about 10 meters on the Earth's surface. Or consider it this way: According to NAD83, a second object
placed in the ground at 38 o N and 84.5 o W would be 10 meters away from the first one. Does that sound
like a lot? People have exchanged gunfire in land disputes over smaller distances. Given a latitude and
longitude, a GIS must know the datum that is the basis for the numbers. Hundreds of datums exist, and
many countries have their own.
Geodesy, Coordinate Systems, Geographic
Projections, and Scale
First, a disclaimer: This text does not pretend to cover in detail such issues as geodetic datums,
projections, coordinate systems, and other terms from the fields of geodesy and surveying. Nor will the
text rigorously define most of these terms. Simply knowing the definitions would mean little without a
lot of study. Many textbooks and Web pages are available for your perusal. These fields, concepts, and
principles may or may not be important in your use of GIS, depending on your projects. However, the
datum, projection, coordinate system designations, and measurement units must be identical when you
combine GIS or map information. If not, your GIS project may well produce inaccurate results.
How we apply the mathematically perfect latitude-longitude graticule to points on the ground depends
partly on human's understanding of the shape of the Earth. This understanding changes the more
we learn. Geodesy is the study of the shape of the Earth and the validity of the measurements human
beings make on it. It deals with such issues as spheroid and datum. You don't have to know much about
geodesy to use a GIS effectively, provided your data are all based on the same spheroid and datum (and
projection and units, as you will see later). It is the application of geodesic knowledge that caused the
differences in the coordinates of that hypothetical object I discussed earlier that was put into the ground
six or seven decades ago. The object hasn't moved. We simply have a better idea of the location of the
object relative to the latitude-longitude graticule.
Projected Coordinate Systems
For several reasons, it's often not convenient to use latitude and longitude to describe a set of points
(perhaps connected by straight lines to make up a coastline or country's boundaries) on the Earth's
surface. One is that doing calculations using latitude and longitude—for example, determining the
distance between two points—can involve complex operations such as products involving sines and
cosines. For a similar distance calculation, if the points are represented on the Cartesian x-y plane, the
worst arithmetic hurdle is a square root.
11 GRS80 is a global geocentric system based on the ellipsoid adopted by the International Union of Geodesy and
Geophysics (IUGG) in 1979. GRS80 is the acronym for the Geodetic Reference System of 1980.