UNDERSTANDING MAP PROJECTION PART 1

GENERAL CONCEPT OF MAP PROJECTION

Whether you treat the earth as a sphere or a spheroid, you must transform its three-dimensional surface to create a flat map sheet. This mathematical
transformation is commonly referred to as a map projection.

One easy way to understand how map projections alter spatial properties is to visualize shining a light through the earth onto a surface, called the projection surface. Imagine the earth’s surface is clear with the graticule drawn on it. Wrap a piece of paper around the earth. A light at the center of the earth will cast the shadows of the graticule onto the piece of paper. You can now unwrap the paper and lay it flat. The shape of the graticule on the flat paper is very different than on the earth. The map projection has distorted the graticule.

A spheroid can’t be flattened to a plane any easier than a piece of orange peel can be flattened—it will rip. Representing the earth’s surface in two dimensions causes distortion in the shape, area, distance, or direction of the data. A map projection uses mathematical formulas to relate spherical coordinates on the globe to flat,
planar coordinates. Different projections cause different types of distortions. Some projections are designed to minimize the distortion of one or two of the data’s characteristics. A projection could maintain the area of a feature but alter its shape. In the graphic below, data near the poles is stretched.

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