Frequently, instead of wanting to give all parts of a map equal importance, we wish to emphasize a certain part of it but yet show the relationship of that part to the surrounding regions.
Many early maps emphasized a region merely by placing it in the center of the map. Some of these were medieval maps called mappaemundi or "maps of the world." In the western world, Jerusalem was often placed in the center of a circular map, signifying a philosophical unity of the known world. Some simpler mappaemundi are often called "T-0" maps, with seas shaped like a T within an enclosing 0, surrounding and dividing the three known continents of Asia, Europe, and Africa (Figure 6-1). As these allegorical maps were replaced by maps representing geographical regions more realistically, various map projections were used to emphasize the regions.
In the 20th century, emphasis of a region is often still accomplished by placing it in the center of a map, as we shall see later. Emphasis is also frequently achieved by including two maps, the larger showing the region of special interest. In one corner of the larger map is placed a smaller map that shows an extended region at a much smaller scale. The portion of the smaller map contained in the larger one is emphasized by a heavy outline or a special color or shading. For example, a larger more detailed map of the Great lakes may include a small inset map outlining North America, with the Great Lakes highlighted (Figure 6-2).
Emphasis Using the Orthographic Projection
Emphasizing an area of special interest in the midst of its surroundings can also be accomplished by using a single projection instead of two separate maps. The Orthographic projection is sometimes appropriate for this purpose. This very old projection shows a hemisphere in much the way that a globe appears to us. It is more suitable for emphasizing regions that make up a substantial part of the hemisphere, such as North America, because the surrounding regions that are to be shown should lie near the edges of the hemisphere, in this case one centered on the Great Lakes (Figure 6-3). It is not very satisfactory for emphasizing, say, the relatively small Great Lakes themselves.
The Orthographic is one of the group of azimuthal projections. When displaying regions that are nearly circular, members of this group generally have a more suitable arrangement of distortion than corresponding conic and cylindrical projections. Since highlighted regions often tend to be more circular than lengthwise, it is appropriate to consider other azimuthal projections for enlarging the central region of a map.
Zooming in With Perspective Views
One choice to consider is the general perspective view of the globe. The Orthographic is a view from an infinite distance, but by bringing in the viewpoint to a few thousand or a few hundred kilometers from the surface of the earth, we can get other perspectives, like that of a camera viewing earth from space (Figure 6-4). When astronauts photographed portions of the earth during the Apollo and Gemini missions, they were in effect using a tilted form of a perspective projection, with the regions that surround the points of interest fading into the horizon.
Newer Projections That Enlarge the Center
About thirty years ago, Swedish geographer Torsten Hägerstrand used a specially tapered azimuthal projection to show migration patterns near a Swedish town. This type of projection, called a Logarithmic Azimuthal, can be used to enlarge a circular region of almost any size, with the surrounding region subordinated to an adjustable extent. The effect can resemble a "fish-eye" photograph (Figure 6-5).
Recently, another type of projection has been developed. The series, called "Magnifying-Glass" Azimuthal projections, includes several different forms. One type consists of a circular region of any chosen extent and at true area scale, surrounded by a region also at a constant area scale that is smaller by a predetermined amount than the scale in the central portion. Thus, it is as if a round magnifying glass were held over one part of an equal-area map so that the user can also see the surrounding regions outside the magnifying glass, but at a smaller scale. With an actual glass, some of the map would be hidden under the edge of the glass, but with this projection the entire surrounding region can be seen (Figure 6-6). Other possibilities include the use of the Azimuthal Equidistant projection instead of the Azimuthal Equal-Area, or tapering the surrounding regions instead of maintaining a constant scale (Figure 6-7).
Chapters: 1 2 3 4 5 6 7 8 9 10 11 12 Contents