Map Projections
The only true representation
of the earth, free
of distortion, is a globe. Maps are flat, and the process by which
geographic
locations (latitude and longitude) are transformed from a
threedimensional
sphere to a twodimensional flat map is called a projection.
PROPERTIES
Every map projection distorts at least three, and
sometimes all four,
of the following properties: Shape, Area, Distance, and
Direction.
Conformal: No map can preserve the shape
of large areas,
but a conformal projection maintains shape in small, localized
areas.
Equal Area: These projections show the
areas of all regions
on the map in the same proportion to their true areas on the
globe.
Equidistant: No map can show distance
correctly between
all points on the map, but only from one, or at most two, points to any
other point.
Azimuthal: This projection correctly shows
directions
(azimuths) from a single point to all other points on the map.

To better
understand what happens
during the projection process, imagine the earth as a large inflated
balloon.

Cut it apart, and
flatten it to make
a map. It will be stretched in some places, shrunk in others. Think of
where and how the original balloon has been distorted.

VISUAL COMPARISONS
Visualize the properties of a map projection by comparing the
arrangement
of its meridians and parallels with the characteristics of the
graticule
on the globe: Parallels are spaced equal distances apart.

The equator is the longest parallel. Length of parallels decreases
moving
toward the poles (which are points).

All meridians are of equal length.

Meridians converge at the poles.

At a given latitude, meridians are equally spaced.

Latitude and longitude meet at right angles.
This diagram illustrates how a Mercator (cylindrical) projection
distorts
the relative sizes of areas on the earth. Note how areas are
increasingly
enlarged towards higher lattitudes. 
CLASSIFICATION OF PROJECTIONS
One method of classifying map projections is to group
them by the type
of surface onto which the graticule is theoretically being projected:
Cylindrical,
Conic, and Azimuthal. Map projections that do not fit within these
three
classes are described as Pseudo or Miscellaneous projections. Though
few
maps are truly the result of such projection (most are derived from
mathematical
formulas), it is a useful way to visualize and understand the
transformation
process. Where the projection surface touches (is tangent to) the
globe,
scale is true. This can be at a point, or along one or two lines
(called
standard lines, or, if along a line of latitude, standard parallels).
Distortion
increases with increasing distance from the standard point or
lines.

CYLINDRICAL
Imagine a light bulb in the center of a globe, with a
sheet of paper
wrapped around it in the form of a cylinder. Meridians and parallels
would
be "projected'' onto the cylinder as straight, parallel lines. Because
meridians on these projections do not meet at the poles, as they do on
the globe, these maps are increasingly stretched and distorted toward
the
poles.
Pseudocylindrical projections normally have straight
parallels and curved
meridians (usually equally spaced). The Robinson Projection is a
popular
example. It was created to make the world "look" right by keeping
angular
and areal distortions to a minimum.


AZIMUTHAL
A perfectly flat piece of paper (a plane) would touch
the globe at a
point. This projection is a good choice for maps with circular or
square
shapes. When the point of tangency is one of the poles, meridians are
shown
as straight lines radiating from the pole. If parallels are then drawn
as equally spaced concentric circles, this projection would be
equidistant
(scale is true along any line radiating from the center point, in this
case the pole).
Gnomonic Projection: Great circle routes
(the shortest
distance between two points on the globe) appear as straight lines on
this
Azimuthal


CONIC
A cone of paper placed over a globe would touch its
surface along one
standard line (usually a parallel). A cone that sliced through the
globe
would intersect it twice, creating two standard parallels. Such a
projection
is wellsuited for showing areas in the middlelatitudes with a mostly
eastwest extent (like the United States).


