- MAPS:
A map is a stylized (simplified) drawing of a portion of the Earth's surface.
Maps of small areas (like a city, or a park) can show shapes and distances accurately, without distortion.
- Map projections:
Because the surface of the Earth is curved, no flat map can show any significant portion of the Earth's surface without distortion.
A map projection is a mathematically defined pattern that shows some or all of the Earth's surface on a flat map.
All map projections show distortion. (Only a spherical globe can show the true shape of the Earth's surface.)
Some map projections are designed to depict certain features accurately:
- An equal-area projection maintains the proportions of areas but distorts their shapes.
If one area is twice the size of another, its image on the map is also twice as big.
- An equidistant projection shows certain distances in correct proportion,
but no flat map can show all distances in true proportions.
- An equiangular projection shows certain angles correctly. If the angles that are shown correctly are those
radiating out from a central point, the projection is called an azimuthal projection.
- Some map projections are compromises that are neither equidistant nor equal-area, but are
designed to minimize the distortions. Distortions are generally minimized in the center of the map,
but become greater towards the margins.
- Some oft-used map projections:
Mercator projection: This is the oldest and most famous map projection, first used in 1569 by Gerardus Mercator.
It is neither equidistant nor equal-area, but it accurately shows the angles used by navigators on ships to show their
true 'heading' or 'bearing', the angle that the ship's direction needs to make with true North. Mercator's projection
is most accurate near the Equator (where most ships sailed from 1500-1800), but is increasingly distorted towards the poles.
Greenland, for example, looks bigger than the Continental United States on Mercator's projection, even though it it much smaller;
its shape is also distorted so that it looks much taller and narrower.
Rectangular projection: This is perhaps the simplest projection but is seldom used because it is neither equal-area nor
equidistant (except along the Equator). Latitude lines (parallels) and longitude lines (meridians) are equally spaced.
A rectangular projection is similar to Mercator's, but is less distorted near the poles. It is also a projection that can
show the entire world, while Mercator's can never show the poles.
Conic projection: Often used for maps of single continents or large countries, this projection shows distances accurately
along a standard parallel, usually chosen to lie near the middle of the map. Distortions increase north or south of
the standard parallel.
Polyconic projection: Similar to the conic projection, but using two standard parallels,
with small distortions between them, and larger distortions to the north and south.
Rectangular equal-area projection: This distorts shapes badly, especially towards the poles, but,
because it is equal-area, it can be used to compare sizes of different map areas (countries, agricultural regions, etc.) where
shapes are not important.
Azimuthal equidistant projection: Used mostly to show polar regions, this projection shows both angles and distances
at true scale radiating out from a
central point. Also sometimes used to show airline distances from a central starting point.
Azimuthal equal-area projection: Shows angles correctly from a central point but distorts shapes badly at increasing distances from the center.
Robinson projection: A frequently used compromise projection to show the entire world with minimal distortion
in low and middle latitudes but maximum distortion near the poles.
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