The real earth is round. Most maps are printed on flat pieces of paper. It is impossible to take a round object and flatten it out on a piece of paper without bending or stretching or tearing it in some way. The manner in which a curved earth is transferred to a flat piece of paper is called the "projection". There is no such thing as a perfect projection, they all create distortions that will produce an inaccurate map. There are three basic types of distortion.
1. Distortion in direction: Say you want to travel between two points on a map. The shortest distance between two points is a straight line, right? Not on most maps! Because of distortion, the direction between two points is inaccurate, and instead of a straight line is really a curving line. This can make navigation very confusing and complicated.
2. Distortion in area: geographic areas, like the size of a country, or an ocean, or a city, are shown on the map as either too big or too small.
3. Distortion in shape: geographic areas are either stretched out or pushed together, so that one end of a geographic area will look too big, and the other end may look too small.
Cartographers have devised a wide variety of projections by which they can represent the surface of the earth. The course that precedes this one considered projections themselves in more detail. Two important points need to be emphasized, however, in this course. First
the base projection used on the map dramatically influences the capabilities of that map. A map can not have the shape of small areas and the size of small areas both be correct. Thus, great care must be taken to ensure that the characteristics of the projection used in construction of a map MATCH the purpose or use to which the map is going to be put. Unfortunately, maps are not always made by cartographers, so these types of mistakes are all too common. The second point is that good maps should have the projection information provided for the user. For the topographic quadrangles, the USGS follows and publishes their standardized practices. The Cumberland map, though identifies a less standardized part of this information in the statement, "National Geodetic Vertical Datum of 1929." While their projections are fairly standardized, the datum base used has changed through time, so this new map, based on an older base, identifies what the datum was for this old base.
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