Geography Reference
In-Depth Information
is the most accurate representation of the earth's surface. It accurately
describes the location of objects to a common reference at a certain point.
The difference between the sphere, ellipsoid, and geoid at any place can be
as much as several hundred meters (yards). The ellipsoid and geoid models
of the earth are defined and updated at irregular intervals. Should you
become involved with very detailed and accurate measurements of location,
you should also be aware that the geoid of the earth is constantly changing
and locations recorded with an older geoid may not match a newer geoid.
(See Plate 1 for geoid undulations.)
2. A projection makes compromises . Every projection either preserves one
projection property or makes some compromises between projection prop-
erties. In either case, some projection properties are compromised by every
projection. Because there are theoretically an unlimited number of projec-
tions, it is important to organize projections by projection properties. Which
projection is used in making geographic information or a map has much to
do with how geographic characteristics and relationships are preserved. The
four projection properties, along with the cartographic terms in parentheses
for each, are:
Angles
Preservation of the angles (including shapes) of small
areas (conformal)
Areas
Preservation of the relative size of regions (equivalent
or equal area)
Distance
Partial preservation of distance relationships (equidis-
tant)
Direction
Certain lines of direction are preserved (azimuthal)
Most projections preserve area, although a large number are compromise
projections , which means that they sometimes preserve area, but sometimes
preserve shape. Usually compromise projections are used for showing the
globe, but they can be used for smaller areas. All things considered, the pro-
jections that preserve area are more common because people usually need
maps of smaller areas where geographic relationships and area comparisons
are very important. However, the projections showing the globe are signifi-
cant because they are the only way for almost all people to see and under-
stand the world. Global projections make very significant trade-offs between
projection properties. One of the most common projections used for show-
ing the entire world, the Mercator projection, is a classic case of how a pro-
jection always trades off among projection properties. In the case of the
Mercator projection, it preserves the shape and distance relationships of
small areas, but only locally; it preserves lines of constant bearing; it fails to
preserve area (the sizes of Greenland and Africa are greatly distorted); it par-
tially preserves continuity, breaking Eurasia into two halves. These trade-offs
mean that the Mercator projection is a good choice for representing small
areas and large areas, but only for navigation.
3. Distortions will occur . Every projection, in making trade-offs between
Search WWH ::

Custom Search