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chapter 6
The earth’s graticule
and Projections
“. . . but then I wonder what Latitude or Longitude I’ve got to?”
(Alice had not the slightest
idea what Latitude was, or Longitude
either, but she thought they were nice grand words to say.)
—L
ewis
C
arroll
,
Alice in
Wonderland (1865)
the eARth’s gRAtIcUle
the size and shape of the earth
Many aspects of mapmaking would be greatly simplified if the earth were a plane
surface. However, the earth is not flat, as was realized early in the history of cartog-
raphy. Attempts were made even by the early Greeks to represent the actual surface
with as little distortion as possible.
The true figure of the earth is not a regular shape like a sphere or an ellipsoid, but
a lumpy solid called a
geoid, which simply means “earth-shaped figure.” As Table 6.1
shows, the polar radius is roughly 13 miles less than the equatorial radius. This does
not seem like a great difference (on a 12-inch globe the difference is only 0.019 inches
or 0.05 centimeter) but for large-scale maps, such as topographic maps or navigation
charts, the difference is significant. In addition to flattening of the poles there are
other smaller irregularities.
Geodesists, who deal mathematically with the shape of the earth, must consider
actual earth measurements, as must surveyors and cartographers who make topo-
graphic maps and GIS specialists who make large-scale city maps and the like. For
most thematic mapping, however, we are not concerned with the irregularities of the
earth’s shape, since we commonly make maps of very small areas or small-scale maps
for which geodetic accuracy is not a major issue.
