earth’s surface is projected onto a cylinder, and theconic, in which the surface is projected onto a cone.A third method is the gnomonic method, in which theearth’s surface is projected onto a plane placed tangentto a particular point. For a polar gnomonic chart, thispoint is one of the earth’s geographical poles.MERCATOR PROJECTIONTo grasp the concept of Mercator projection,imagine the earth to be a glass sphere with a stronglight at the center. Imagine, also, that the geographicalmeridians and parallels are inscribed as lines on thesphere at a given interval (for example, every 15degrees). Now imagine a paper cylinder placedaround the sphere, tangent to the equator, as shown infigure 9-11. The shadow images of the meridians willappear on the paper as equally spaced, parallel, verti-cal lines. The shadow images of the parallels willlikewise 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 ofthe equator, the line of tangency) increases.Figure 9-11.—Mercator projection.You can see that there are two elements ofdistortion here, each of which progressively increaseswith latitude. One is the fact that the meridians, whichon the earth itself converge at each of the poles, areparallel (and therefore equidistant) for their entirelength on the cylinder. The other is the fact that theparallels, which are actually equidistant on the sphereitself, become progressively farther apart as latitudeincreases.These two elements produce the familiardistortion that is characteristic of a Mercator map ofthe 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 continentalUnited States, which has an area (excluding Alaska)of about 2,973,776 square miles.Figure 9-12 shows the meridians and parallels at15-degree intervals of the earth’s surface on a Merca-tor projection. Note that the parallels extend only to80 degrees north and south. Because the cylinder hasno ends, Mercator projection of regions in latitudeshigher 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 and75°N is the same on the ground, these distances aremuch different on a Mercator projection. Still anothercharacteristic to note is the fact that a meridian isperpendicular to all parallels it intersects and that allthe meridians are parallel to each other.Transverse Mercator ProjectionOn a Mercator projection the cylinder is placedtangent to the earth’s central parallel, the equator. OnFigure 9-12.—Meridians and parallels on a Mercatorprojection.9-11