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Organization:
Alexa Crawls
Starting in 1996,
Alexa Internet has been donating their crawl data to the Internet Archive. Flowing in every day, these data are added to the
Wayback Machine after an embargo period.
Starting in 1996,
Alexa Internet has been donating their crawl data to the Internet Archive. Flowing in every day, these data are added to the
Wayback Machine after an embargo period.
The Wayback Machine - https://web.archive.org/web/20151003074029/http://www.progonos.com:80/furuti/MapProj/Normal/CartProp/cartProp.html
Useful Map Properties
Matching Projection to Job
No map projection is perfect for every task. One must
carefully weigh pros and cons and how they affect the intended
map's purpose before choosing its projection. The next sections
outline desirable properties of a map, mentioning how
projections can be used or misused.
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Like the real Earth, this globe's north-south axis is
tilted at 23°27'30" from the orbital plane.
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For any map, the most important parameters of accuracy can be expressed as:
Globe Properties
Unfortunately, only a globe offers such
properties for any points and regions. Since crafting a
globe is only a matter of reducing dimensions (no projection is
involved), every surface feature can be reproduced with
precision limited only by practical size, with no loss of
shape or distance ratios. As a bonus, a globe is a truly
three-dimensional body whose surface can be embossed
in order to present major terrain features. But globes
suffer from many disadvantages,
being:
- bulky and fragile, clumsy to transport and store;
- expensive to produce, especially at larger sizes, thus
impractical for showing fine details;
- difficult to look straight at every point, therefore
- cumbersome for taking measurements or setting
directions;
- able to show a single hemisphere at a time;
- completely unfeasible for widespread reproduction by
printed or electronic media
Changing minor map properties
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Several maps using
the Mercator
projection show how fundamental projection properties are
not affected by changes in minor features like scale,
aspect, and choice of mapped area.
What is constant
in all Mercator maps is how scale quickly
changes the farther one gets from a reference line (which
may or may not be horizontal), but remains constant along a direction
parallel to that line; this ensures all shapes are locally preserved,
in detriment of area ratios.
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1 The Mercator
projection is frequently (and usually inappropriately)
employed in world maps. Any Mercator map must be
arbitrarily clipped, in this normal equatorial aspect
at its top and
bottom; a complete map would be infinitely tall.
The cropped, transverse and oblique maps on the
right are much more legitimate uses for this
projection.
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2
Translating the central meridian is a common and
trivial modification strongly dependent on the
map's theme. Here a translation is used to avoid
interrupting the Pacific Ocean. The distortion
pattern remains the same as above, with areas
drastically enlarged at the top and bottom.
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3
Departing from tradition, some maps
half-jokingly show Australia and other
meridional nations "on top of the
world". Here, a bit more of Antarctica was
arbitrarily cropped off. Of course, neither
projection nor aspect differ from the above
maps. Mercator's projection is often naïvely
accused of privileging the northern
hemisphere—even "lowering" the Equator
to aggrandize
First World nations—but an uncropped
equatorial Mercator map is perfectly
symmetrical around the Equator; it's the map's
editor who decides whether a large portion of
Antarctic ice is relevant enough to show
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4
Scaling—in this case up by 100%—and cropping
the map are common editorial decisions which
change no projection properties.
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5 Still a
Mercator map, but in a transverse
aspect scaled
up 25% and rotated 90°. The graticule seems different
and the poles can be visible, however the distortion
pattern remains: scale is constant vertically instead of horizontally,
growing left and right, not up and down. The central meridian, instead
of the Equator, is perfectly represented, while
Africa, bent strongly enough to seem split in two,
is barely recognizable.
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6 This
oblique
Mercator map (scaled up 25% and rotated) lays more of
the Americas in the central band of reduced
scale distortion. The Equator is no longer straight.
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Exploiting Map Properties
So, flat map projections are usually more important and useful
than globes, despite their shortcomings. In particular, no flat
map can be simultaneously conformal (shape-preserving) and equal-area (area-preserving) in every
point.
However, a reasonably small spherical patch can be
approximated by a flat sheet with acceptable distortion.
In most projections, at least one specific region—usually
the center of the map—suffers from little or no distortion. If the
represented region is small enough (and if necessary suitably
translated in an oblique
map), the projection choice may be of little importance.
On the other hand, the fact that no projection can faithfully
portray the whole Earth should not lead to a pessimistic view,
since distorting the planet on purpose makes possible—unlike with a
globe—uncovering important facts and presenting at a glance
relationships normally obscure. Skillfully used, distortion is a
powerful visual tool; this becomes explicit in a kind of
pseudoprojection called cartogram, where the
place a point is drawn depends not only on its location on Earth,
but also on attributes of the mapped region, like a county's
population or a country's economic yield. In 1934 Erwin Raisz presented
cartograms using simple rectangles with areas proportional to the
attribute of interest but no
relation to the original shapes. Waldo Tobler developed the
modern concept of cartogram, which instead uses distorted
but actually recognizable shapes; the amount of repeated calculations
involved makes electronic computers indispensable tools.
No projection is intrinsically good or bad, and a projection
suitable for a particular problem might well be useless or misleading
if applied elsewhere.
Major and Minor Properties
For any projection, its "major" properties—concerning whether
and how well distances, areas and angles are preserved—are
largely independent of changes in scale,
aspect
and the choice of mapped area (this last detail is strongly
associated with the aspect, selection of central meridian, and
any eventual post-projection rotation and cropping), even though the
graticule and other shapes may appear radically
different. Therefore, the same projection may be the source
of many maps, often only superficially unrelated.
Sometimes, for historical or convenience reasons, particular
uses of a single projection are known by distinctive names. E.g.,
the Gauss-Krüger projection is a transverse case of
the ellipsoidal Mercator projection;
the Briesemeister
and Nordic projections are oblique (with or without rescaling)
aspects of the Hammer
projection; and many rescaled
versions of Lambert's equal-area cylindrical projection have been
proposed. Other examples abound for interrupted, averaged and
composite designs.
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![](https://cdn.statically.io/img/web.archive.org/web/20151003074029im_/http://www.progonos.com/furuti/MapProj/Normal/CartProp/Img/vdgG.jpg) |
The projection known as Van der Grinten's third violates all five
graticule properties:
- purple lines are very stretched near the poles; green lines
are longer farther from the vertical axis (the constant vertical
spacing between parallels is deceptive)
- red lines are also slightly stretched closer to the map edges
- the blue cells are also enlarged near the edges; this is more
obvious at high latitudes
- of all meridians, only the central remains straight
- parallels and meridians cross at right angles only at the Equator
and central meridian; elsewhere, there's shearing,
symmetrical around the vertical axis: angles are compressed in
opposite directions east and west of the central meridian
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Especially for a map in the normal aspect, a
quick visual inspection of its graticule provides obvious
clues of whether its projection preserves features. For
instance, if the coordinate grid is uniformly laid (say, one line every
ten degrees),
- along any meridian, the distance on the map between parallels
should be constant
- along any single parallel, the distance on the map between
meridians should be constant; for different parallels, should decrease
to zero towards the poles
- therefore, any two grid "cells" bounded by the same
two parallels should enclose the same area
Also,
- the Equator and all meridians should be straight unbroken
lines, since they don't change direction on the Earth's
surface
- any meridian should cross all parallels at right angles
Again, for any particular projection, violation of any or all
these properties doesn't necessarily make it poorly designed or
useless; rather, it suggests (and constrains) both the range of
applications for which it is suitable and, for each application,
regions on the map where distortion is significant.
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Copyright © 1996, 1997 Carlos A. Furuti