Map Projections - Spatial Mathematics: Theory and Practice through Mapping - page 199

features. One reason for this difficulty is that these models are visually static.
Indeed, in Tissot models, one sees only simple country-level boundary distor-
tion in association with ellipse elongation—at the scale of the full globe it is
difficult to see more.
9.5 Some projection characteristics
With a clearer view of visualizing distortion at the global scale, we consider
once again a few common projection characteristics and associated distortion
issues.
• Conformal ( Figure 9.3 ) . A conformal projection maintains shape
fidelity in local (small) areas. No map preserves the shape of global
(large) areas. In Figure 9.3 , the circular shape is maintained; only
area is distorted. The Mercator projection is a conformal projection.
• Equal area ( Figure 9.4 ) . An equal-area projection maintains area
in such a way that a small circle covering one part of the map con-
tains the same amount of mapped area as another small circle of the
same size placed elsewhere on the map. In Figure 9.3 , Greenland
appears larger than Brazil; in Figure 9.4 , the opposite is true. It
is easy to visualize the problem when the Tissot circles appear on
Figure 9.3 : Clearly, the equal-area characteristic does not apply to
the Mercator projection! However, it does in the Mollweide projection
where all the circles have equivalent area (but not shape). Meridians
are halves of ellipses on the Mollweide. The meridians at 90 degrees
east and west longitude form a circle. The Equator is a horizontal
line perpendicular to a central meridian of one-half the length of the
Equator. Generally, equal-area maps are good for making landmass
comparisons.
• Equidistant ( Figure 9.5 ). An equidistant projection preserves the dis-
tance from a standard point or pair of points (line). The sinusoidal
projection of Figure 9.5 is also an equal-area projection. In addition,
it is an equidistant projection because distances are preserved along
parallels. Meridians are halves of sine waves; as with the Mollweide,
the central meridian is half as long as the Equator. No map can show
the distance correctly between all points on the map.
9.6 Pseudo or miscellaneous projections
To make the image of the map look better to the casual eye, by minimizing
angular and areal distortion, pseudocylindrical projections may offer a view
of the Earth with evenly spaced straight parallels and curved meridians (usu-
ally equally spaced). The Mollweide ( Figure 9.4 ), sinusoidal ( Figure 9.5 ), and
Robinson projections ( Figure 9.6 ) serve as examples. Generally, bending the