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distortion might be sampled, in advance, so that the map user has an idea of
what map projection to choose and what not to choose.
9.3 Looking at maps and their underlying projections
The creation and classification of map projections is a broad field, and the
decision of which one to choose for any particular application or problem
is a complex one. Often it is helpful to have a set of visual cues in making
mental comparisons of map projections. Visualize the graticule of the globe.
Compare the grid arrangement of parallels and meridians on the map with
your mental image of the graticule arrangement on the globe. On the globe
graticule there are a number of simple geometric properties—are these pre-
served on the map you are looking at? On the globe:
• Parallels are spaced equal distances apart.
• The Equator is the longest parallel and is unique in that regard. It
is the only parallel representing a great circle on the sphere.
• Lengths of parallels, representing small circles, decrease as one
moves toward the poles. The pattern of decrease is symmetric on
either side of the Equator.
• All meridians are of equal length.
• They are halves of great circles extending through the north and
south geographic poles.
• They converge at the poles.
• At given latitude, meridians are evenly spaced.
• Parallels and meridians meet at right angles.
One commonly used method of classifying map projections groups them
by the type of surface onto which the graticule is theoretically projected
( Figure 9.1 ).
Cylindrical . Consider a light bulb in the center of a globe. Wrap a
sheet of paper around the globe, tangent to the Equator. Meridians
and parallels from the globe are projected onto the resulting cylinder
as straight, parallel lines. These sets of lines meet at right angles on
the cylinder, which becomes the map, just as they do on the globe.
The meridians on the projected cylindrical surface do not meet at the
poles, as they do on the sphere. Thus, maps made in this way are
increasingly stretched and distorted toward the poles. The left-hand
column of the figure ( Figure 9.1 ) shows the graticule projected to a
cylinder in “regular” position (with the axis of the cylinder passing
through the Earth's polar axis), to a cylinder in “transverse” position
(with the axis of the cylinder orthogonal to the Earth's polar axis—the
basis for the UTM, or “Universal Transverse Mercator” grid), and to a
cylinder in “oblique” position (with the axis of the cylinder inclined at
any other angle to the Earth's polar axis).
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