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Map Projections
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What is a map projection? |
The process of systematically transforming positions on the
Earth's spherical surface to a flat map while maintaining
spatial relationships, is called map projection. This projection
process is accomplished by the use of geometry and, more commonly,
by mathematical formulas. In geometric terms, the Earth as
a spheroid (that is, a slightly flattened sphere), is considered
an undevelopable shape, because, no matter how the
Earth is divided up, it cannot be unrolled or unfolded to
lie flat. Some of the simplest projections are made onto geometric
shapes that can be flattened without stretching their surfaces.
These shapes or forms are considered to be developable.
Examples of shapes that reflect these properties are cones,
cylinders, and planes.
The CONE, CYLINDER and PLANE are developable geometric shapes. The curved
surface of the Earth can be projected on to these forms which can be unrolled
to make a flat map. (The PLANE is already a flat surface!)
[D] Click for larger version, 11 KB Figure 1. Diagram of Developable Surfaces
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How Map Projections are Derived |
These geometric shapes can either be tangent or secant to the spheroid.
In the tangent case the cone, cylinder or plane just touches the Earth
along a single line or at a point. In the secant case, the cone, or cylinder
intersects or cuts through the Earth as two circles. (The secant case
for the plane intersects as one circle.) Whether tangent or secant, the
location of this contact is important because it defines the line or point
of least distortion on the map projection. This line of true scale is
called the standard parallel or standard line.
With conical and cylindrical projections, the axis of these shapes usually
corresponds to the axis of the spheroid (Earth); the exception is the
oblique case. When a cone or cylinder is cut along any meridian to produce
the final projection, the meridian opposite the cut line is called the
central meridian. Planar projections may be oriented in different
ways: polar, equatorial and oblique.
Projections may be oriented in different ways with respect
to the Earth's axis. For cylindrical projections this is achieved
by changing the position of the lines used for tangency or
secancy. For planar projections, the point of contact with
the Earth can be altered. This point determines the aspect
used and functions as the focus of the projections.
[D] Click for larger version, 13 KB Figure 2. Drawing of Projection Orientation
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Classification of Map Projections |
Most projections are derived from mathematical formulas,
but some are easier to visualize as projected on to a developable
surface. Therefore, projections are commonly classified according
to the geometric surface from which they are derived: conical,
cylindrical, and planar (azimuthal or zenithal).
The many projections that cannot be easily related to these
three surfaces are described as pseudo, modified or individual
(or unique).
Conical Projection
In the conical case, we can visualize the Earth projected
onto a tangent or secant cone, which is then cut lengthwise
and laid flat. The parallels (lines of latitude),
are represented by concentric circular arcs, and the meridians
(lines of longitude), by straight, equally spaced,
radiating lines.
This type of projection is used for mapping mid-latitude regions, such
as Canada and the United States. The result is less overall shape distortion
of land and water areas. The Lambert Conformal Conic projection is a commonly
used version of the conic type.
The polyconic projection (from the Greek, "poly" meaning many),
envelopes the globe with an infinite number of cones, each
with its own standard parallel. The parallels are non-concentric,
while the central meridian is straight. Other meridians are
complex curves. Scale is true along each parallel and along
the central meridian.
Cylindrical Projection
In the cylindrical case, the Earth is projected on to a tangent
or secant cylinder which is also cut lengthwise and laid flat. The result
is an evenly spaced network of straight, horizontal parallels and straight,
vertical meridians. A straight line between any two points on this projection
follows a single direction or bearing, called a rhumb line. This
feature makes the cylindrical projection useful in the construction of
navigation charts.
When the cylinder is used as a surface to project the entire
World on to a single map, significant distortion occurs at
the higher latitudes, where the parallels become further apart,
and the poles cannot be shown. The famous Mercator projection,
is the best known example of this class and one of the earliest
of all projections (circa 1569).
Planar or Azimuthal Projection
With the planar projection, a portion of the Earth's surface is
transformed from a perspective point to a flat surface. In the polar case,
the parallels are represented by a system of concentric circles
sharing a common point of origin from which radiate the meridians, spaced
at true angles. This projection shows true direction only between the
centre point and other locations on the map.
Although these projections are most often used to map polar regions,
they may be centred anywhere on the Earth's surface. The gnomonic
is one type of planar projection on which any great circle appears
as a straight line. A great circle is the circle created when a plane
cuts the Earth through its centre. This term is most often used in the
expression, "great circle route", which is the shortest path between
two points on the Earth. This information is most useful in air navigation,
because aircraft usually travel along great circle routes. This explains
why aircraft flying from Toronto or Montréal to Japan fly near
the North Pole!
The AZIMUTHAL family of projections, also called ZENITHAL or PLANAR,
is produced by transforming the Earth's surface onto a plane. Members
of the family are distinguished from each other by the different perspective
points used to construct them. For the GNOMONIC projection, the perspective
point (like a source of light rays), is the centre of the Earth. For the
STEREOGRAPHIC this point is the opposite pole to the point of tangency,
and for the ORTHOGRAPHIC the perspective point is an infinite point in
space on the opposite side of the Earth.
[D] Click for larger version, 18 KB Figure3. Drawing of The Azimuthal Family of Projections
Other Projections
The pseudoconic and pseudocylindrical projections are both
constructed in the same manner as their unprefixed counterparts, except,
they both have curved meridians instead of straight ones.
Modified projections are versions of a projection to which changes
have been made to reduce or modify the pattern of distortion, or to add
more standard parallels.
Many other projections, some of which are in common use, cannot be easily
related to one of the three developable geometric forms. These can be
classed as individual or unique projections. Examples of
this group are the Bacon Globular, Peirce Quincuncial, Armadillo, Adams
World in a Square I, and Van der Grinten I, II, III, or IV.
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Properties of Map
Projections |
The Challenge: The Earth is a spheroid,
and the best way to represent it is with a globe. This scale
model retains all of the desired properties necessary to produce
the perfect map: area, distance, direction, and shape are
all accurately represented. However, when this spheroid is
projected on to a flat map, all these properties cannot be
retained simultaneously. In fact, each projection is a compromise,
showing some properties accurately, while at the same time,
allowing others to be distorted. The extent to which these
properties are preserved, provides another method of classifying
projections.
Despite the problems related to distortion, all projections do retain
one important feature, that of positional accuracy. By transforming the
graticule (a gridded reference network of latitude and longitude
lines, encompassing the globe) to a map, the spatial relationship between
points on both surfaces is maintained.
The Factor of Scale: Our interest in the
significant properties of map projections begins with map
scale. A small-scale map portrays a large area and a large-scale
map portrays a small area of the Earth. If the area to be
mapped is small (only a few square kilometres, as for example,
a county, township or city), then the occurrence of error
that results from projecting the curved surface of the Earth,
to the flat surface of the map, is negligible. In relation
to the surface of the entire Earth, a small area is conceptually
as flat as the sheet of paper on which we wish to represent
it. Only when larger areas of the Earth are to be mapped,
such as provinces, countries or continents, do the following
properties play a more important role in the selection of
projections.
A map projection is said to be equal-area or equivalent
if it portrays areas over the entire map so that they retain
the same proportional relationship to the areas on the Earth
they represent. The creation of this projection results in
shapes and angles being greatly distorted. This distortion
increases with distance away from the point of origin.
A projection which is equidistant maintains constant
scale (i.e., true distance), only from the centre of the projection
or along great circles (meridians), passing through this point.
For example, a planar equidistant projection centred on Montréal,
would show the correct distance to any other location on the
map, from Montréal only. This property is accomplished
at the expense of distorting area and direction.
A projection is azimuthal or zenithal when
angles or compass directions from one central point are shown
correctly to all other points on the map. However, to achieve
this property, shapes, distances and areas are badly distorted.
A map projection is conformal, (also known as orthomorphic
or equiangular) when all angles at any point are preserved.
Or, the scale at any point is the same in every direction.
Lines of latitude and longitude intersect at right angles,
and shapes are maintained for small areas. However, in the
process of projection, the size of large areas is distorted.
The following table shows which pairs of properties can be combined in
one projection:
Table 1. Which pairs of properties can be
combined in one projection?
Which pairs of properties can be
combined in one projection?
Equal-area |
- |
no |
yes |
no |
Equidistant |
no |
- |
yes |
no |
Azimuthal |
yes |
yes |
- |
yes |
Conformal |
no |
no |
yes |
- |
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World Map Projections |
There are many map projections in use that do not possess any of the
desired properties mentioned above. However, they are still suitable for
certain applications and, indeed, may be very useful if a compromise is
reached and a number of properties are reasonably preserved.
Those projections that succeed in showing the entire World on one map,
often encounter serious problems of distortion. World projections, by
their nature, usually distort regions shown at the extremes of the projection.
To improve the depiction of these distorted areas, "interrupted"
forms, splitting the projection into gores, have been developed.
Following this approach, many landmasses (or oceans), can have their own
central meridian, resulting in true shapes or conformality in each region
of the projected map.
Examples of World Map Projections:
[D] Click for larger version, 19 KB Figure 4. Goode's Homolosine Equal-area Projection. (with the oceans interrupted to show the continents)
[D] Click for larger version, 16 KB Figure 5. Miller Cylindrical World Map Projection
[D] Click for larger version, 15 KB Figure 6. Eckert IV Equal-area World Map Projectio
[D] Click for larger version, 11 KB Figure 7. Sinusoidal Equal-area World Map Projection
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Edge Matching |
A situation worth noting in regard to map projections, and their properties,
is the edge matching of adjacent regions. This problem is frequently encountered
by cartographers and map readers, particularly when dealing with maps
in a series. In order to fit two or more separate maps exactly along their
edges, a number of parameters must be maintained:
1. the maps must be constructed with the same projection;
2. they must be at the same scale;
3. they must have the same standard parallels; and
4. they should be based on the same ellipsoid reference datum
(in other words, longitude and latitude are calculated from
an ellipsoid, such as the reference surface NAD83 - North
American Datum)
The Transverse Mercator projection, which lends itself to
edge-matching operations, is commonly used for map series,
such as the 1:50 000 and 1:250 000 scale National Topographic
System (NTS), produced by Geomatics Canada. Other factors
influencing the accuracy of edge matching are: the instability
of map paper when exposed to changes in temperature and humidity
and errors of cartographic drafting or surveying.
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Choosing a Map Projection |
The selection of the best map projection depends upon the purpose for
which the map is to be used. For navigation, correct directions
are important; on road maps, accurate distances are of the
major concern and on thematic maps (depicting area-related
data), the correct size and shape of regions is important.
Other considerations in choosing the best projection are the
extent and location of the area to be mapped. With reference
to map extent, the larger the area being mapped the more significant
is the curved surface of the Earth and, therefore, the greater
the distortion of the desirable properties. For map location,
the following conventions can be applied: for low-latitude
regions, use cylindrical projections; for middle-latitude
areas, use conical; and for polar regions, use planar.
The following table provides a summary of more common map projections,
their properties and use:
Table 2. Summary of More Common Map Projections,
Their Properties and Use
Summary of More Common Map Projections,
Their Properties and Use
Mercator |
cylindrical |
conformal true
direction* |
World*, equatorial,
east-west extent, large and medium scale |
navigation
large scale map series, USGS** maps |
Transverse Mercator |
cylindrical |
conformal |
continents/
oceans, equatorial/ mid-latitude, north-south extent,
large and medium scale |
topographic
large scale map series, N.T.S.
and USGS maps |
Lambert conformal
conic |
conic |
conformal
true direction* |
continents/
oceans, equatorial/ mid-latitude, east-west extent,
large and medium scale |
mapping countries
of Canada and USA,
National Atlas of Canada 5th edition, IMW |
Azimuthal equidistant |
planar |
equidistant*
true direction* |
World*, hemisphere,
equatorial/ mid-latitude, continents/ oceans, regions/seas,
polar, large scale* |
navigation,
topographic large scale map series, USGS maps |
Lambert azimuthal
equal-area |
planar |
equal area
true direction |
hemisphere,
continents/ oceans, equatorial/ mid-latitude, polar |
navigation,
thematic, Geomatics Canada North America reference
map, USGS
maps |
Polyconic |
conic |
equidistant* |
region/seas,
north-south extent, medium and large scale |
topographic
map series, USGS |
Stereographic |
planar |
conformal
true direction* |
polar, continents/
oceans, regions/seas, equatorial/ mid-latitude medium
and large scale |
navigation,
topographic, USGS
maps |
van der Grinten
I |
individual
or unique |
compromise |
World, equatorial,
east-west extent |
Geomatics Canada
World Map, USGS
maps |
Robinson |
pseudo- cylindrical |
compromise |
World |
thematic, reference
maps National Geographic |
Miller cylindrical |
cylindrical |
compromise |
World |
thematic, reference
maps, USGS
maps |
Eckert IV |
pseudo- cylindrical |
equal area |
World |
thematic, reference
maps |
Sinusoidal |
pseudo- cylindrical |
equal area |
World, continents/
oceans equatorial, north-south extent |
thematic, reference
maps, USGS
maps |
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* Limitations apply
** United States Geological Survey - the supplier of base
and thematic maps covering the United States of America.
The following maps show Canada projected in different ways:
[D] Click for larger version, 25 KB Figure 8. Gnomonic Azimuthal Projection
[D] Click for larger version, 27 KB Figure 9. Lambert Conformal Conic Projection
[D] Click for larger version, 31 KB Figure 10. Transverse Mercator Projection
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References |
Dana, Peter H. 1995. Map Projections.
The Geographer's Craft Project. Austin. Dept. of Geography,
University of Texas at Austin.
ESRI (Environmental Systems Research Institute, Inc.). 1994.
Map Projections, Georeferencing spatial data. Redlands,
California: ESRI.
Gersmehl, Philip J. 1991. The Language of Maps.
Pathways in Geography Series, title no. 1. Indiana, Pennsylvania:
Indiana University of Pennsylvania.
Greenhood, David. 1964. Mapping. Chicago: The University
of Chicago Press.
Pearson II, Frederick. 1990. Map Projection: Theory and
Applications. Boca Raton, Florida: CRC Press, Inc.
Raisz, Erwin. 1962. Principles of Cartography. New
York: McGraw-Hill Book Company.
Robinson, Arthur H., and Randall D. Sale. 1969. Elements
of Cartography. Third edition. New York: John Wiley &
Sons.
Snyder, John P., and Philip M. Voxland. 1989. An Album
of Map Projections. U.S. Geological Survey Professional
Paper 1453. Denver: United States Government Printing Office.
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