The selection of an appropriate map projection includes arranging the projection's pattern of deformation, i.e., its pattern of scale changes, in some optimum way. When we envision the earth from space, we are above only one point on its surface, which we see essentially in plan and at a maximum scale, while all other points are seen at decreasing scales and in some increasing degree of obliquity out to the surrounding horizon line. Features there, if greatly exaggerated, could be seen only in profile. To represent this view of the earth, cartographers may utilize an oblique aspect of the Orthographic projection, a perspective projection derived from some specific altitude above the earth, or any oval or azimuthal projection that preserves some desired quality of the earth's surface.
Normally the chosen projection would be centered on some specific area of interest as described in the preceding two chapters. Scale decreases progressively outward and deformation increases from the centers to the peripheries of Orthographic and general perspective projections (Figure 7-1). The Orthographic projection views the earth from infinity, hence half the earth can be displayed on any one map, but perspective projections can be constructed from any elevation so that their limits extend from a great circle, as with the Orthographic, down to a very small circle centered on the point of interest. Other projection choices may allow viewing more than half of the earth. Consequently, the map designer can control two important aspects of the finished map: its direction of viewing and extent of coverage.
The Direction of Viewing
We often insist, unnecessarily, that maps be oriented with north at the top. There are good reasons for this, particularly when the area being presented is not a familiar one. But there are often compelling reasons to use other orientations. These may relate to the desire to match a known viewpoint, to force the viewer to consider a geographic relationship from another perspective, or to suggest a change of importance or value from one area to another.
To present a specific direction of viewing, the projection is rotated so that the direction of interest is at the top. The projection can also be cropped below the center point so as to place the horizon in its natural position at the top or far side of the "scene" (Figure 7-2).
If the projection is not cropped, the cartographer can place any lettering so that it is easily read in the preferred orientation. While this signals the intended orientation for viewing, it doesn't prevent a map user from rotating the map to a more familiar orientation so as to better appreciate the geographic area being viewed.
Because of the decrease of area scale away from the projection center, areas near the center can carry greater detail than those toward the horizon line. This, together with the change in perspective from orthogonal at the center to oblique at the periphery, suggests a parallel change in importance or significance for those areas away from the center. This effect can be used to imply a reduction in some phenomenon other than geographic area.
For example, a perspective view of North America from over a point in northwestern Europe could be used to suggest the difference in knowledge about the New World held by Europeans in the middle of the 16th century (Figure 7-3). The coastal areas were better known than the continental interior; but beyond some horizon, nothing was known -- the known world essentially disappeared from view. From such an image it is clear why early exploration focused on the various inlets and breaks in that coastline in search of waterways to the Orient.
The Extent of Coverage
The area covered in a view from space can be varied by changing the altitude above the
earth's surface at which the earth is being viewed. The lower the vantage point, the
smaller the area of coverage
(Figure 7-4); the higher the viewer's
altitude, the greater the coverage
(Figure 7-5). Being able to change the
altitude of the vantage point allows one to
illustrate such things as the increase in coverage or range of different communication
or transportation technologies, the hinterlands around various-sized urban areas,
or the surrounding regions of political or military hegemony of particular countries.
It is also possible to simulate a global view with an oblique aspect of any world projection that has a circular or oval perimeter. This might be done to preserve, for example, equivalence among areas in the projection (Figure 7-6), something that could only be done with very small areas in a perspective projection from a very low altitude.