the projection itself does. This is connected to the general problem ofscaleon map
projections. Due to the inaccuracies and distortions of casting a three-dimensional
surface onto a two-dimensional map, true scale cannot be maintained from a given
point in all directions. Some projections are constructed so as to present scale
accurately between two pointsalong a given line, but scale will still be inaccurate
in other directions from the point of reference. Another aspect of a projection
affecting its appearance, accuracy and utility, is how the projection is centered;
that is, what point or latitude representsthe main line of reference in the center
of the projection? Frequently the equator is chosen for centering a projection; this
is called an equatorial projection, but for certain reasons, one of the poles may be
chosen as the centering point, resulting in a polar projection. Any other point or
line may be used as well, and changing the centering point will dramatically alter
the appearance of the map.
In mathematical terms, to create a projection of the globe, a “developmental
surface” must be achieved, meaning a surface that may be flattened out into two
dimensions. This may be accomplished in a large number of ways, but most pro-
jections use certain geometric shapes that are aligned with the axis of theEarth,
which are then “opened” into a two-dimension representation. The most common
of these shapes are a cylinder, a cone, and a plane—the latter is also sometimes
referred to as an azimuthal projection. Each of these has certain advantages,
depending on the nature and purpose of the map being constructed. Generally,
the graticule of each of these follows certain properties. On a cylindrical projec-
tion, the lines of the graticule will remain parallel, with each pair of lines meeting
at a right angle. On such a projection, the poles are impossible to represent. A
conic projection will often display the graticule lines as converging toward one
pole and diverging in the opposite direction. The azimuthal projection shows a
graticule that radiates outward from apoint or set of points, representing where
the plane intersects that portion of the globe the projection represents. Azimuthal
projections are not particularly well suited to showing large areas but are quite
useful in calculating direction from a knownlocation; therefore, they have wide
application in navigation and scientific studies.
One of the most famous map projections is also one of the oldest. The Mercator
projection was introduced in 1569 by Gerardus Mercator, a Belgian cartographer
who was interested in designing a chart that would be accurate for determining
true compass bearings for oceanic navigation. Mercator’s projection is no doubt
the most widely used projection in history, as it was adopted not only by mariners
but over the centuries became popular with other map users as well. The projection
is a cylindrical projection centered on theequator, and thus the graticule lines,
both for latitude and longitude, appear as parallels running at right angles. Maps
constructed using this projectioncontainverylittledistortioninregardto220 Map Projections