A
map is a visual representation of an area—a
symbolic depiction highlighting relationships between elements of
that space such as
objects,
regions, and
themes.
Many maps are
static two-dimensional, geometrically accurate
representations of
three-dimensional space, while
others are dynamic or interactive, even three-dimensional. Although
most commonly used to depict geography, maps may represent any
space, real or imagined, without regard to
context or
scale; e.g.
Brain
mapping,
DNA mapping, and extraterrestrial
mapping.
Geographic maps
200 px
Cartography, or
map-making is
the study, and often practice of crafting representations of the
Earth upon a flat surface (see
History of cartography), and one who
makes maps is called a
cartographer.
Road maps are perhaps the most widely used maps today, and form a
subset of navigational maps, which also include aeronautical and
nautical charts, railroad network
maps, and hiking and bicycling maps. In terms of quantity, the
largest number of drawn map sheets is probably made up by local
surveys, carried out by
municipalities,
utilities, tax assessors, emergency services providers, and other
local agencies.
Many national surveying projects have been
carried out by the military, such as the British
Ordnance Survey
(now a civilian government agency internationally
renowned for its comprehensively detailed work).
In addition to location information maps may also be used to
portray contour lines (isolines) indicating constant values of
elevation,
topography,
temperature,
rainfall
etc.
Orientation of maps
The orientation of a map is the relationship between the directions
on the map and the corresponding
compass directions in reality. The word
"
orient" is derived from
Latin oriens, meaning East. In the Middle
Ages many maps, including the T and O maps, were drawn with East at
the top (meaning that the direction "up" on the map corresponds to
East on the compass). Today, the most common – but far from
universal – cartographic convention is that North is at the top of
a map. Several kinds of maps are often traditionally not oriented
with North at the top:
- Maps from non-Western traditions are oriented a variety of
ways. Old
maps of Edo show the Japanese
imperial palace
as the "top", but also at the centre, of the
map. Labels on the map are oriented in such a way that you
cannot read them properly unless you put the imperial palace above
your head.
- Medieval European
T and O maps such as the Hereford Mappa Mundi were centred on
Jerusalem
with East at the top. Indeed, prior to the
reintroduction of Ptolemy's Geography to Europe around 1400, there was
no single convention in the West. Portolan charts, for example, are oriented to
the shores they describe.
- Maps of cities bordering a sea are often conventionally
oriented with the sea at the top.
- Route and channel maps have traditionally been oriented to the
road or waterway they describe.
- Polar map of the Arctic or Antarctic
regions are conventionally centred on the pole, and
the concept of orientation does not apply; the direction North
would be towards or away from the centre of the map,
respectively.
- Reversed maps, also known as
Upside-Down maps or South-Up maps, reverse the "North is up"
convention and have South at the top.
- Buckminster Fuller's Dymaxion maps are based on a projection of the
Earth's sphere onto an icosahedron. The
resulting triangular pieces may be arranged in any order or
orientation.
Scale and accuracy
Many, but not all, maps are drawn to a
scale, expressed as a
ratio
such as 1:10,000, meaning that 1 of any unit of
measurement on the map corresponds exactly, or
approximately, to 10,000 of that same unit on the ground. The scale
statement may be taken as exact when the region mapped is small
enough for the curvature of the Earth to be neglected, for example
in a town planner's city map. Over larger regions where the
curvature cannot be ignored we must use map projections from the
curved surface of the Earth (sphere or ellipsoid) to the plane. The
impossibility of flattening the sphere to the plane implies that no
map projection can have constant scale: on most projections the
best we can achieve is accurate scale on one or two lines (not
necessarily straight) on the projection. Thus for map projections
we must introduce the concept of
point
scale, which is a function of position, and strive to keep its
variation within narrow bounds. Although the scale statement is
nominal it is usually accurate enough for all but the most precise
of measurements.
Large scale maps, say 1:10,000, cover relatively small regions in
great detail and small scale maps, say 1:10,000,000, cover large
regions such as nations, continents and the whole globe. The
large/small terminology arose from the practice of writing scales
as numerical fractions: 1/10000 is larger than 1/10000000. There is
no exact dividing line between large and small but 1/100000 might
well be considered as a medium scale. Examples of large scale maps
are the 1:25000 maps produced for hikers; on the other hand maps
intended for motorists at 1:250,000 or 1:1,000,000 are small
scale.
It is important to recognize that even the most accurate maps
sacrifice a certain amount of accuracy in scale to deliver a
greater visual usefulness to its user. For example the width of
roads and small streams are exaggerated when they are too narrow to
be shown on the map at true scale; that is, on a printed map they
would be narrower than could be perceived by the naked eye. The
same applies to computer maps where the smallest unit is the pixel.
A narrow stream say must be shown to have the width of a pixel even
if at the map scale it would be a small fraction of the pixel
width.
Sometimes the scale of a map is distorted deliberately. For example
the map of Europe shown here has been distorted to show population
distributions. Clearly the basic scale is approximately uniform for
the rough shape of the continent is still visible. This is an
example of a
cartogram.
Another example of distorted scale is the famous
London Underground map. The basic geographical
structure is respected but the tube lines (and the River Thames)
are smoothed to clarify the relationships between stations. Near
the centre of the map stations are spaced out more than near the
edges of map.
Further inaccuracies may be deliberate. For example cartographers
may simply omit military installations or remove features solely in
order to enhance the clarity of the map. For example, a road map
may or may not show railroads, smaller waterways or other prominent
non-road objects, and even if it does, it may show them less
clearly (e.g. dashed or dotted lines/outlines) than the highways.
Known as decluttering, the practice makes the subject matter that
the user is interested in easier to read, usually without
sacrificing overall accuracy. Software-based maps often allow the
user to toggle decluttering between ON, OFF and AUTO as needed. In
AUTO the degree of decluttering is adjusted as the user changes the
scale being displayed.
World maps and projections
Maps of the world or large areas are often either 'political' or
'physical'. The most important purpose of the political map is to
show territorial borders; the purpose of the physical is to show
features of
geography such as mountains,
soil type or land use. Geological maps show not only the physical
surface, but characteristics of the underlying rock,
fault lines, and subsurface
structures.
Maps that depict the surface of the Earth also use a
projection, a way of translating the
three-dimensional real surface of the
geoid to
a two-dimensional picture. Perhaps the best-known world-map
projection is the
Mercator
projection, originally designed as a form of
nautical chart.
Airplane pilots use aeronautical charts based on a
Lambert conformal conic
projection, in which a cone is laid over the section of the
earth to be mapped. The cone intersects the sphere (the earth) at
one or two parallels which are chosen as standard lines. This
allows the pilots to plot a great-circle route approximation on a
flat, two-dimensional chart.
Electronic maps
From the last quarter of the 20th century, the indispensable tool
of the cartographer has been the computer. Much of cartography,
especially at the data-gathering survey level, has been subsumed by
Geographic Information
Systems (GIS). The functionality of maps has been greatly
advanced by technology simplifying the superimposition of spatially
located variables onto existing geographical maps. Having local
information such as rainfall level, distribution of wildlife, or
demographic data integrated within the map allows more efficient
analysis and better decision making. In the pre-electronic age such
superimposition of data led
Dr.
John Snow to discover the cause of
cholera. Today, it is used by agencies of the human
kind, as diverse as wildlife conservationists and militaries around
the world.
Even when GIS is not involved, most cartographers now use a variety
of computer graphics programs to generate new maps.
Interactive, computerised maps are commercially available, allowing
users to
zoom in or
zoom out (respectively
meaning to increase or decrease the scale), sometimes by replacing
one map with another of different scale, centered where possible on
the same point. In-car
global navigation satellite
systems are computerised maps with route-planning and advice
facilities which monitor the user's position with the help of
satellites. From the computer scientist's point of view, zooming in
entails one or a combination of:
- replacing the map by a more detailed one
- enlarging the same map without enlarging the pixels, hence showing more detail by removing less
information compared to the less detailed version
- enlarging the same map with the pixels enlarged (replaced by
rectangles of pixels); no additional detail is shown, but,
depending on the quality of one's vision, possibly more detail can
be seen; if a computer display does not show adjacent pixels really
separate, but overlapping instead (this does not apply for an
LCD, but may apply for a
cathode ray tube), then replacing a
pixel by a rectangle of pixels does show more detail. A variation
of this method is interpolation.
For example:
- Typically (2) applies to a Portable Document Format (PDF) file
or other format based on vector graphics. The increase in detail
is, of course, limited to the information contained in the file:
enlargement of a curve may eventually result in a series of
standard geometric figures such as straight lines, arcs of circles
or splines.
- (2) may apply to text and (3) to the outline of a map feature
such as a forest or building.
- (1) may apply to the text (displaying labels for more
features), while (2) applies to the rest of the image. Text is not
necessarily enlarged when zooming in. Similarly, a road represented
by a double line may or may not become wider when one zooms
in.
- The map may also have layers which are partly raster graphics and partly vector graphics. For a single raster
graphics image (2) applies until the pixels in the image file
correspond to the pixels of the display, thereafter (3)
applies.
See also: Webpage , PDF , MapQuest, Google Maps,
Google Earth, OpenStreetMap or Yahoo! Maps.
Conventional signs
The various features shown on a map are represented by conventional
signs or symbols. For example, colors can be
used to indicate a classification of roads. These signs are usually
explained in the margin of the map, or on a separately published
characteristic sheet.
Labeling
To communicate spatial information effectively, features such as
rivers, lakes, and cities need to be
labeled. Over centuries cartographers
have developed the art of placing names on even the densest of
maps. Text placement or name placement can get mathematically very
complex as the number of labels and map density increases.
Therefore, text placement is time-consuming and labor-intensive, so
cartographers and GIS users have developed
automatic label placement to ease
this process.
Non geographical spatial maps
Maps exist of the
solar system, and
other cosmological features such as
star
maps. In addition maps of other bodies such as the
Moon and other planets are technically not
geological maps.
Non spatial maps
Many diagrams such as
Gantt charts
display logical relationships between items, and do not display
spatial relationships at all.
Many maps are
topological in nature, and
the distances are completely unimportant, and only the connectivity
is significant.
See also
- General
- Map design and types
- Modern maps
- Map history
- Related Topics
Footnotes
References
- David Buisseret, ed., Monarchs, Ministers and Maps: The
Emergence of Cartography as a Tool of Government in Early Modern
Europe. Chicago: University of Chicago Press, 1992, ISBN
0-226-07987-2
- Denis E. Cosgrove (ed.) Mappings. Reaktion Books, 1999
ISBN 1-86189-021-4
- Freeman, Herbert, Automated Cartographic Text Placement. White
paper.
- Ahn, J. and Freeman, H., “A program for automatic name
placement,” Proc. AUTO-CARTO 6, Ottawa, 1983. 444-455.
- Freeman, H., “Computer Name Placement,” ch. 29, in Geographical
Information Systems, 1, D.J. Maguire, M.F. Goodchild, and D.W.
Rhind, John Wiley, New York, 1991, 449-460.
- Mark Monmonier, How to Lie with Maps, ISBN
0-226-53421-9
- O'Connor, J.J. and E.F. Robertson, The History of Cartography. Scotland : St.
Andrews University, 2002.
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