Quantcast MERCATOR  PROJECTION - 14070_185

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earth’s surface is projected onto a cylinder, and the conic, in which the surface is projected onto a cone. A third method is the gnomonic method, in which the earth’s  surface  is  projected  onto  a  plane  placed  tangent to a particular point. For a polar gnomonic chart, this point is one of the earth’s geographical poles. MERCATOR  PROJECTION To  grasp  the  concept  of  Mercator  projection, imagine the earth to be a glass sphere with a strong light at the center. Imagine, also, that the geographical meridians and parallels are inscribed as lines on the sphere  at  a  given  interval  (for  example,  every  15 degrees).   Now   imagine   a   paper   cylinder   placed around the sphere, tangent to the equator, as shown in figure  9-11.  The  shadow  images  of  the  meridians  will appear on the paper as equally spaced, parallel, verti- cal  lines.  The  shadow  images  of  the  parallels  will likewise  appear  as  straight  lines  running  perpendicu- lar to the shadow images of the meridians. The paral- lels are not actually equally spaced, however; instead, the  distance  between  adjacent  parallels  will  progres- sively  increase  as  latitude  (distance  north  or  south  of the equator, the line of tangency) increases. Figure  9-11.—Mercator  projection. You  can  see  that  there  are  two  elements  of distortion  here,  each  of  which  progressively  increases with latitude. One is the fact that the meridians, which on the earth itself converge at each of the poles, are parallel (and therefore equidistant) for their entire length on the cylinder. The other is the fact that the parallels, which are actually equidistant on the sphere itself,  become  progressively  farther  apart  as  latitude increases. These   two   elements   produce   the   familiar distortion that is characteristic of a Mercator map of the world. On such a map the island of Greenland, which has an area of only about 46,740 square miles, is  considerably  larger  in  outline  than  the  continental United States, which has an area (excluding Alaska) of about 2,973,776 square miles. Figure  9-12  shows  the  meridians  and  parallels  at 15-degree intervals of the earth’s surface on a Merca- tor projection. Note that the parallels extend only to 80 degrees north and south. Because the cylinder has no ends, Mercator projection of regions in latitudes higher than about 80 degrees is impossible. Note, too, that  although  the  distance  along  a  meridian  between (for example) 15°N and 30°N and between 60°N and 75°N is the same on the ground, these distances are much different on a Mercator projection. Still another characteristic to note is the fact that a meridian is perpendicular to all parallels it intersects and that all the meridians are parallel to each other. Transverse  Mercator  Projection On a Mercator projection the cylinder is placed tangent to the earth’s central parallel, the equator. On Figure 9-12.—Meridians and parallels on a Mercator projection. 9-11

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