This enables the Map Information Office generally to state whether or not an area has been photographed, and if so. from what source prints are procurable. The following are important government mapping agencies from which maps may be obtained directly:

Geological Survey, U. S. Department of Interior. Basic topographic maps of approximately one-half the United States. Key

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A Map Drawn on Mercator Projection, A "Developed" Projection.

1. The term "developed" is derived from' the method: a cylindrical or conical surface is

substituted for the plane of projection and then is "developed" or rolled out in a plane. The two types of projection most commonly used today are the Mercator and the polyconic.

2. The Mercator projection was first introduced in 1568 by Gerardus Mercator. a Flemish

lecturer on geography and astronomy. In the Mercator projection a tangent cylinder is employed. The meridians and parallels of latitude cut each other at right angles and are represented by straight lines.

3. The polyconic projection employs an infinite number of tangent cones. The starting

point for these cones is at the middle parallel or latitude of the area mapped.

4. See 267.

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Map Measuring Device.

This instrument is used to measure lines and distances on a map. The small wheel follows the line and the distance is recorded on the dial in inches or centimeters.