221.Cylindrical Projection. -In this projection it is considered that a cylinder is wrapped around the globe touching at the equator. The pointson the globe are projected on to the cylinder by lines drawn from the cen- ter of the globe through each point to the surface of the cylinder. Thus the meridians become straight lines always the same distance apart; and the parallels of latitude are also straight lines, but their distances apart will increase with the latitude. The poles themselves, being in the diam- eter of the cylinder, lie at an infinite distance from the equator.
When such a cylinder is unrolled it forms a skeleton map on which can be plotted places whose latitude and longitude are known. The directions north and south will be up and down the map, and east and west to the right and left. This cylindrical projection causes a degree of latitude to vary from about 1/360 of the earth"s circumference at the equator to infinity at the poles; and a degree of longitude, which near the poles has almost no length, is made to have a length everywhere equal to that of a degree on the equator. Thus passing from the equator toward the poles, the areas of surfaces on the earth are increased when represented on this projection, but the increase east and west and the increase north and south are not equal. This causes the shape of the portions of the earth farthest from the equator to be much distorted.
The Mercator projection is the most commonly used of all projections.
It is a simple modification of the cylindrical, in which the exaggeration north and south is made equal to that east and west. In this projection the polar regions are greatly enlarged. This explains why Greenland, which on the globe is of comparatively small size, when seen on the ordinary map of the world is half the size of North America. The great advantage of this projection is that the meridians and parallels are both represented by straight lines. A navigator can thus at any time find his course by drawing a straight line joining the places between which he is sailing. This is why most nautical charts are constructed on this projection. But to geographers this projection is not of as great value as some others since the shapes of the land masses are so much distorted.
222.Stereographic Projection. -Of the hemispherical projectionsprobably the best for study is the stereographic. This, or a slight modifi- cation of it, is the projection upon which are constructed the hemispheri-cal maps usually seen. In it a plane is considered as held tangent at a cer- tain point on the globe and from a point on the globe directly opposite the point of tangency, lines are drawn to the plane through the intersec- tions of the parallels of latitude and longitude. Through these projected intersections the meridians and parallels of latitude are drawn.In this projection, places near the point of tangency have their outlines correctly reproduced, but the farther away a place is from this point, the greater the distortion. This distortion, however, is never as great as that at the north and south in the cylindrical or Mercator projections. In the stereographic projection, however, the directions north and south and east and west must be traced on a curved line, thus making it much more difficult to tell at a glance the direction of one place from another. It is not possible on this projection to show more than one half the earth"s surface on a single map.