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Even though they are easy to fold up and carry around,
neither greatly distorted maps nor disassembled globe
gores have much practical use. For this reason,
cartographers have developed a number of map projections,
or methods for translating a sphere into a flat surface. No
projection is perfect - they all stretch, tear or compress the
features of the Earth to some degree. However, different
projections distort different qualities of the map. "All maps
have some degree of inaccuracy," Turner explains. "We're
taking a round Earth and projecting it onto a two-
dimensional surface -- onto a piece of paper or a computer
screen -- so there's going to be some distortion."
Fortunately, the variety of available projections makes it
possible for a cartographer to choose one that preserves
the accuracy of certain features while distorting less
important ones.
Creating a map projection is often a highly mathematical
process in which a computer uses algorithms to translate
points on a sphere to points on a plane. But you can think
of it as copying the features of a globe onto a curved
shape that you can cut open and lay flat -- a cylinder or a
cone. These shapes are tangent to, or touching, the Earth
at one point or along one line, or they are secant to the
Earth, cutting through it along one or more lines. You can
also project portions of the Earth directly onto a tangent or
secant plane.
Projections tend to be the most accurate along the point or
line at which they touch the planet. Each shape can touch
or cut through the Earth at any point and from any angle,
dramatically changing the area that is most accurate and
the shape of the finished map.
A planar projection
IMAGE COURTESY NATIONAL ATLAS
Some projections also use tears, or interruptions, to
minimize specific distortions. Unlike with a globe's gores,
these interruptions are strategically placed to group related
parts of the map together. For example, a Goode
homolosine projection uses four distinct interruptions that
cut through the oceans but leave major land masses
untouched.
A Goode projection of the Earth
IMAGE USED UNDER THE GNU FREE DOCUMENTATION
LICENSE
Different projections have different strengths and
weaknesses. In general, each projection can preserve
some, but not all, of the original qualities of the map,
including:
Area: Maps that show land masses or bodies
of water with the correct area relative to one
another are equal-area maps. Preserving the
correct area can significantly distort the shapes
of the land masses, especially for views of the
entire world.
Shapes: In the pseudoconical Robinson
projection, the continents are shaped correctly
and appear to be the correct size -- they look
"right." However, distances and directions are
incorrect on a Robinson projection. It's a good
tool for studying what the world looks like but
not for navigating or measuring distances.
Distances: Maps that maintain correct
distances between specific points or along
specific lines are equidistant maps.
Directions: Many navigational maps have
straight rhumb lines, or lines that intersect all of
the parallels or meridians from the same angle.
This means that, at any point on the map,
compass bearings are correct.
You can learn more about the specific map projections and
their strengths and weaknesses from NASA , the National
Atlas of the United States and the U.S. Geological Survey .
Choosing the right projection is just one part of creating a
successful map. Another is finding the right data. We'll
look at where map information comes from in the next
section.
A map of the world's time zones.
It's easy to think of maps as sets of visual directions.
Whether you're trying to get to the top of Mount Everest or
to a friend's new home, a map can help you find your way.
But maps can do more than help you figure out where you
are and where you're going. They are representations of
information that can describe nearly anything about the
world.
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If you wanted to get an idea of which dog breeds are most
popular in different regions, you might spend days looking
at lists and charts. Or, you could look at a map and get an
instant grasp of the same information. Learning about the
physical features, imports, exports and population densities
of different countries would take ages if you relied on
written descriptions in a book. But with a map, all of the
numbers, patterns and correlations are right in front of you.
As Ian Turner, senior cartographer at GeoNova, puts it, "a
map is a type of language. It's a graphic language. It
presents information in hopefully a way that is very easy to
understand."
It's the job of a mapmaker, or cartographer, to put all of
this information into a format that people can understand
and learn from. Exactly what a person can learn depends
on the type of map. Most maps start with an outline of a
location, like a piece of land or a body of water. Then, they
provide information about the location's attributes.
Different maps incorporate different attributes. For
example:
Physical maps illustrate landforms like
mountains, deserts and lakes. With a physical
map, you can get a basic sense of what all or
part of the planet looks like and what its
physical features are. Physical maps usually
show differences in elevation through
hypsometric tints, or variations in color.
Topographic maps, on the other hand, illustrate
the land's shape and elevation using contour
lines.
Political maps display cultural information
about countries, their borders and their major
cities. Most political maps also include some
physical features, like oceans, rivers and large
lakes. You can check out political maps of the
world at our interactive atlas.
Thematic maps add information on a specific
theme, or subject. Examples of common themes
are population density, land use, natural
resources, gross domestic product (GDP) and
climate. Thematic maps can also show
extremely specialized information, such as the
availability of Internet access in different parts
of the world.
This combination of locations and attributes makes it
possible to put lots of information into a very small space.
A single map can show you all of the countries on a
continent, their borders, their approximate populations and
their primary imports and exports. People can also use
specialized thematic maps to analyze trends and patterns
in all kinds of data. A map showing communication costs in
different parts of the world, for example, could help a
nonprofit organization decide where to build a low-cost
wireless network. As Turner explains, "Maps geography
more than about capitals and countries - it's really about
how economics and climate and natural features, how all
the different variables that make up a society relate to one
another."
Common conventions help cartographers present all this
information in a way that makes sense. We'll look at them
in more detail in the next section.
Contour lines are the greatest distinguishing feature of a
topographic map . Contour lines are lines drawn on a map
connecting points of equal elevation, meaning if you
physically followed a contour line, elevation would remain
constant. Contour lines show elevation and the shape of
the terrain. They're useful because they illustrate the
shape of the land surface -- its topography-- on the map.
Here's a cool way to understand how to interpret contour
lines: Take an object like a ball or a pile of laundry , and
shine a red laser pointer along the object's side. The line
you see will look like a contour line on a topographic map.
In order to keep things simple, topographic maps show
lines for certain elevations only. These lines are evenly
spaced apart. We call this spacing the contour interval. For
example, if your map uses a 10-foot contour interval, you
will see contour lines for every 10 feet (3 meters) of
elevation -- lines at 0, 10, 20, 30, 40, and so on. Different
maps use different intervals, depending on the topography.
If, for example, the general terrain is quite elevated, the
map might run at 80- to even 100-foot (24.4- to 30.5-
meter) intervals. This makes it easier to read the map --
too many contour lines would be difficult to work with.
Look in the margin of your map to find out its contour
interval.
To make topographic maps easier to read, every fifth
contour line is an index contour. Because it's impractical to
mark the elevation of every contour line on the map, the
index contour lines are the only ones labeled. The index
contours are a darker or wider brown line in comparison to
the regular contour lines. You'll see the elevations marked
on the index contour lines only. To determine elevations,
pay attention to the amount of space in between lines. If
the contours are close together, you're looking at a steep
slope. If the contours have wide spaces in between -- or
aren't there at all -- the terrain is relatively flat.
Another thing you need to understand about a topographic
map is scale. Obviously, maps aren't life-sized. Otherwise
we'd never be able to fit them in our backpacks. Instead,
cartographers plot maps on a ratio scale, where one
measurement on the map equals another larger amount in
the real world.
The first number of the scale is always one. It's your unit
of measurement, usually an inch. The second number is
the ground distance. For example, if your U.S. Geological
Survey (USGS) map has a scale of 1:24,000, it means that
one inch on the map is equal to 24,000 inches (2000 feet
or 609.6 meters) in the real world. Your map's scale legend
will always be at the bottom.
For USGS topographic maps, 1:24,000 is the scale most
often used. Maps based on metric units use a scale of
1:25,000, where one centimeter equals 0.25 kilometers.
You'll find most of the United States mapped at the
1:24,000 scale, with only a few exceptions. Puerto Rico, for
example, maps at 1:20,000 or 1:30,000 because the
country originally mapped at a metric scale. A couple of
states map at 1:25,000, and most of Alaska (due to its
size) maps at 1:63,360. The more populated areas of
Alaska, though, map at the typical 1:24,000 or 1:25,000.
A 1:24,000 map is large and provides a lot of detail about
the area -- it will include buildings, campgrounds, ski lifts,
among other things. You may also see footbridges and
private roads on a map of this scale.
It requires about 57,000 maps at this scale to cover the 48
contiguous United States, Hawaii and territories [source:
USGS]. However, you can find maps of various areas at all
different scales. The scale corresponds to its intended use.
For example, township engineers generally need extremely
detailed maps that show sewers, power and water lines,
and streets. The common scale for these maps is 1:600. If
you want to see one large area on a single sheet but with
less detail, a smaller scale map like 1:250,000 is better.