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be plotted easily with the Mercator projection because a course with
constant azimuth (compass direction) is shown as a straight line.
The Mercator is a regular cylindrical projection (the cylinder axis
passes through the north and south poles). Meridians of longitude are

shown as equally spaced vertical lines, intersected at right angles by
straight horizontal parallels. The spacing between parallels increases
away from the Equator to produce a conformal projection. The scale is

true along the equator for a tangent Mercator projection, which has a
natural origin at the equator. Assigning a different latitude of natural
origin produces an intersecting cylindrical projection with two
standard parallels (with true scale) equidistant from the equator.

The poleward increase in spacing of parallels produces great
distortions of area in high-latitude regions. In fact, the y coordinate for
the poles is infinity, so maps using the Mercator projection rarely

extend poleward of 75 degrees latitude. The Mercator projection
remains in common use on nautical charts. Because scale distortion is
minor near the equator, it also is a suitable conformal projection for

equatorial regions.

Transverse Mercator Projection

Transverse Mercator projection of North America with central
meridian at 100° west longitude.
The Transverse Mercator projection is a conformal cylindrical

projection with the cylinder rotated 90 degrees with respect to the
regular Mercator projection. The cylinder is tangent to a central
meridian of longitude around its entire circumference. The central
meridian and equator are straight lines, but all other meridians and

parallels are complex curves.
Scale is constant along any meridian. Scale change along parallels
is insignificant near the central meridian, but increases rapidly away

from it, so the Transverse Mercator projection is useful only for
narrow bands along the central meridian. It forms the basis for the
Universal Transverse Mercator Coordinate System, and is primarily

used for large-scale (1:24,000 to 1:250,000) quadrangle maps. The
central meridian can be mapped at true scale (Central Scale parameter
= 1.0), or at a slightly reduced constant scale (for example, the value

0.9996 used in the UTM system). In the latter case a pair of meridians
bracketing the central one maintain true scale, and the mean scale for
the entire map is closer to the true scale.

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