Co-ordinate systems & projections

Geologists have in the past generally managed

to avoid dealing with different coordinate

systems in any detail, as the areas they were

dealing with were small. The advent of GPS

and computerized data management has

changed this. The plotting of real world data on

a flat surface is known as projection and is the

result of the need to visualize data as a flat

surface when the shape of the earth is best

approximated by a spheroid, a flattened sphere.

For small areas the distortion is not important

but for larger areas there will be a compromise

between preserving area and distance relationships.

For example, the well-known Mercator

projection emphasizes Europe at the expense of

Africa. The scale of the data also governs the

choice of projection. For maps of scales larger

than 1:250,000, either a national grid or a

Universal Transverse Mercator (UTM) grid is

generally used. In the latter projection, the

earth is divided into segments of 6 degrees

longitude with a value of 500,000 m E given to

the central meridian of longitude and a northing

origin of 0 m at the equator, if north of the

equator, or large number, often 10,000,000 m,

if south of the equator.

There are a variety of different values in use

for the ellipsoid that approximates the shape

of the earth, known as the datum. The most

commonly used datum for GPS work is World

Geodetic System (WGS 1984) but the datum

used on the map must be carefully checked,

as the use of different datums can change coordinates

by up to 1500 m. The reader is advised

to read about the problems in more detail in

texts such as Longley et al. (2001) and Snyder

(1987).