Mapmaking - The Science of Navigation - page 106

Near contact points, distortion is minimal. (Is it clear to you that near such
tangent points, the distortions that are generated by projection are mini-
mized? If not, read the ''Local Projections'' sidebar.) Thus, conical projec-
tions produce a strip of map with minimal distortion. Consider again fig-
ure 4.5b. Let us say that the cone angle is chosen so that the contact circle
with the globe corresponds to, say, the 50th parallel north. It so happens
that much of the populated region of Canada occupies land that runs just
north of the 49th parallel—the long border with the United States. So, for
places like the border region of southern Canada that stretch long dis-
tances east-west and yet are limited in north-south extent, conical projec-
tions are good news.
CYLINDRICAL PROJECTIONS
Projections from a sphere onto a cylinder (fig. 4.5c) are as many and as
varied as projections onto a cone or a plane. We might project from the
center of the globe onto a cylinder, for example, or we might project as in
figure 4.7, which shows the cylindrical equal area projection , created by
Lambert in 1742. The global map that results from this projection is shown
in figure 4.8. It looks distorted—and is distorted, but no more so than
some other, more familiar projections. It distorts almost everything but has
the advantage that it is equal-area everywhere. However, because it is so
strange, it is little used today.
There are dozens of other cylindrical projections, the most famous of
which is the Mercator projection . This projection was very popular with
navigators in the Age of Sail because it has one very desirable property:
straight lines drawn on the map represent courses of constant compass
direction. Such a course is known as a rhumb line or a loxodrome and was
the natural course for a ship of centuries past to follow because in those
days navigators had few tools other than a magnetic compass to guide
them. A straight line toward the destination on a Mercator-projection map
would indicate the true course to be followed. If, for example, a straight
line drawn from a ship's current position to the desired destination indi-
cated that the ship should set a northwesterly course, all the navigator had
to do was point his vessel to the northwest, as indicated by his compass.
Following this course would send him to his desired destination. Clearly, it
was very important in those days to have a method of navigation that was
both simple and reliable.
The Mercator projection is the only map projection with this straight-
line loxodrome property. Meridians and parallels are straight on a Merca-