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A map projection is a way of representing an area of the globe on a flat surface. It involves the transfer of meridians and parallels from the spherical surface of the globe to a flat surface. Because the earth has a compound-curvature geometric form, the surface cannot lie flat without serious distortion, therefore it is impossible to make a perfect map projection. Certain geometric surfaces such as the cone and the cylinder can be cut lengthwise and unrolled to make a flat sheet, and these can be used to create projections of varying accuracy. All, however, are distorted in some way.
The degree and kinds of distortion vary with the projection, the scale, and the area being mapped. Some projections are suited for large mainly north-south areas, others for mainly east-west areas, and others for areas oblique to the equator. Other projections are severely distorted in small scale maps but accurate in large scale maps.
To choose the best map projection for the task, first determine which attributes must be accurately displayed. This chapter should give you a basic knowledge of the properties of commonly used projections (United States Geological Survey, n.d.).