Projection Standards
Cartographers are challenged to represent 3-dimensional information (the earth's surface)
in two dimensions (a piece of paper). One solution to this problem is to use a globe
rather than a plane. Globes provide the smallest distortion of features, however globes
are expensive, difficult to mass produce, bulky, difficult to use to measure distances and
features, and the user is unable to see objects on both sides of a globe at the same time.
However, the earth is not flat. Therefore, in order to depict a three-dimensional object on
a flat plane (i.e. a piece of paper), you must project the object. Projections need to
accurately portray the surface of the earth in terms of:
- Shape
- Area
- Distance
- Direction
Why are there so many projections?
When three dimensions are reduced to two, one or more of these characteristics will be
sacrificed. Therefore, projections are created and used to accurately portray
characteristics that are most important. For example...
Conformal Projections
Preserve local shapes
To preserve individual angles describing the spatial relationships, a conformal projection must show
graticule lines intersecting at 90-degree angles on the map
The drawback is that the area enclosed by a series of arcs may be greatly distorted in the process
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Equal-Area Projections
Retain all areas at the same scale - all other properties are distorted
In some instances, especially maps of smaller regions, shapes are not
obviously distorted, and distinguishing an equal area projection from a
conformal projection may prove difficult unless documented or measured
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Equidistant Projections
Preserve distance between certain points - scale is not correct except along specific lines which are
based upon which projection is used
No projection is equidistant between all points on a map
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True-Direction Projections
The shortest route between two points on a curved surface such as the earth is along the spherical
equivalent of a straight line on a flat surface - called the great circle
True direction - or azimuthal - projections maintain some of the great circle arcs
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How can I choose a map projection?
The purpose of the map, its scale, and the geographic extent of the mapped area dictate the selection of a
map projection.
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Purpose
Navigation
Road Maps
Thematic Maps
Thematic Maps
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Key Feature
True direction
Equidistant
Conformal (preserves shape)
Equal-area
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Example
Mercator
Azimuthal
Lambert, Mercator
Cylindrical, Albers
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What happens if I mix projections?
The simple answer is that the data will not overlay, as shown in the following figure.
I have heard of Universal Transverse Mercator (UTM). What is that about?
UTM is a specialized application of the Transverse Mercator projection. The globe is
divided into 60 north and south zones with each having its own central meridian. The
origin for each zone is its Central Meridian and the Equator. To eliminate negative
coordinates, the coordinate system alters the coordinate values at the origin. The value
given to the central meridian is the false easting, and the value assigned to the equator is
the false northing. A false easting of 500,000 meters is applied. A north zone has a false
northing of zero, while a south zone has a false northing of 10,000,000 meters.
Properties
- Shape - UTM is conformal - shapes are preserved in small areas
- Area - minimal distortion of larger shapes occurs within the same zone
- Distance - local angles are true
- Direction - Scale is constant along the central meridian, but at a scale factor of 0.9996 to reduce
lateral distortion within each zone
Limitations
- Designed for a scale error not exceeding 0.1 percent within each zone
- Error and distortion increase for regions that span more than one UTM zone - UTM is not designed for
areas that span more than a few zones
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