MSL101L04 Basic Map Reading SR.pdf lesson 4.pdf

# This possibility can be eliminated by using an

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using the system, they can interpret the message and find our location. This possibility can be eliminated by using an authorized low-level numerical code to express locations. AR 380-40 outlines the procedures for obtaining authorized codes.

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15 November 2013 TC 3-25.26 5-1 Chapter 5 Scale and Distance A map is a scaled graphic representation of a portion of the earth’s surface. The scale of the map permits the user to convert distance on the map to distance on the ground, or vice versa. The ability to determine distance on a map, as well as on the earth’s surface, is an important factor in planning and executing military missions. REPRESENTATIVE FRACTION 5-1. The numerical scale of a map indicates the relationship of distance measured on a map and the corresponding distance on the ground. This scale is usually written as a fraction and is called the representative fraction. The RF is always written with the map distance as 1, and is independent of a unit of measure. (It could be yards, meters, inches, or something else.) An RF of 1/50,000 or 1:50,000 means that one unit of measure on the map is equal to 50,000 units of the same measure on the ground. 5-2. The ground distance between two points is determined by measuring between the same two points on the map and then multiplying the map measurement by the denominator of the RF or scale. (See Figure 5-1.) EXAMPLE: The map scale is 1:50,000 making RF = 1/50,000. The map distance from point A to point B is 5 units. 5 x 50,000 = 250,000 units of ground distance. Figure 5-1. Converting map distance to ground distance 5-3. Since the distance on most maps is marked in meters and the RF is expressed in this unit of measurement, in most cases a brief description of the metric system is needed. In the metric system, the standard unit of measurement is the meter. It is multiplied this way: 1 m contains 100 centimeters (cm). 100 m is a regular football field plus 10 m.
Chapter 5 5-2 TC 3-25.26 15 November 2013 1000 m is 1 km. 10,000 m is 10 km. Note. Appendix E contains the units of measure conversion tables. 5-4. The situation may arise when a map or sketch has no RF or scale. To be able to determine ground distance on such a map, the RF is determined. There are two ways to do this: Comparison with ground distance. Measure the distance between two points on the map— map distance (MD). Determine the horizontal distance between these same two points on the ground—ground distance (GD). Use the RF formula and remember that RF is in the general form: RF = 1 = MD X GD Both the MD and the GD is in the same unit of measure, and the MD is reduced to 1. EXAMPLE: MD = 4.32 cm GD = 2.16 km (216,000 cm) RF = 1 = 4.32 or 216,000 = 50,000 X 216,000 4.32 therefore RF = 1 / 50,000 or 1:50,000 Comparison with another map of the same area that has an RF. Select two points on the map with the unknown RF, and measure the MD between them. Locate those same two points on the map that has the known RF, and measure the MD between them. Using the RF for this map, determine GD, which is the same for both maps. Using the GD and the MD from the first map, determine the RF using the formula: RF = 1 = MD X GD 5-5.

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• Fall '16
• Cartography, Geographic coordinate system, Topographic map, Universal Transverse Mercator coordinate system

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