The earth’s graticule and Projections 119
albers’s
Conic equal area
The conic equal-area projection (Figure 6.32) is also much used for the United States,
having standard parallels at 29° 30’ and 45° 30’. Despite being an equivalent projec-
tion, shapes are represented well, and for the United States the maximum linear error
is only 1.25%.
Other
Forms of the Conic
It is possible to place the cone on a small circle that is not a parallel. If a cone is placed
so that its axis does not coincide with that of the globe, oblique and transverse aspect
conic projections are formed.
An interesting variation of conic projections is the
polyconic. It might be rea-
soned that, if a single cone produces one zone of best representation, and a secant
cone two zones, it might be possible to create more zones by a series of standard par-
allels. If one visualizes a series of cones of differing sizes, each touching a different
parallel, then one could “peel off” these different standard parallels and place them
together along a central meridian (Figure 6.33). To eliminate the gaps between the
different parallels, it is necessary to introduce some stretching, but along the central
meridian where the strips touch there is a north–south zone of good representation.
When extended over the entire earth, this projection is very distorted, as can be seen
in Figure 6.32, but for narrow areas of great north–south extent it is very accurate.
The polyconic was the projection used for USGS topographic maps until recently.
