MSL101L04 Basic Map Reading SR.pdf lesson 4.pdf

Occasionally it may be necessary to determine map

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Occasionally, it may be necessary to determine map distance from a known ground distance and the RF: MD = GD Denominator or RF Ground distance = 2200 m RF = 1:50,000 MD = 2200 m = 0.044 m 50,000 MD = 0.044 m X 100 (cm per meter) MD = 4.4 cm 5-6. When determining ground distance from a map, the scale of the map affects the accuracy. As the scale becomes smaller, the accuracy of measurement decreases because some of the features on the map are exaggerated so that they may be readily identified. GRAPHIC (BAR) SCALES 5-7. A graphic scale is a ruler printed on the map that is used to convert distances on the map to actual ground distances. The graphic scale is divided into two parts. To the right of the zero, the scale is marked in
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Scale and Distance 15 November 2013 TC 3-25.26 5-3 full units of measure and is called the primary scale. To the left of the zero, the scale is divided into tenths and is called the extension scale. Most maps have three or more graphic scales, each using a different unit of measure. (See Figure 5-2.) When using the graphic scale, be sure to use the correct scale for the unit of measure desired. Figure 5-2. Using a graphic (bar) scale 5-8. To determine the straight-line distance between two points on a map, lay a straight-edged piece of paper on the map so that the edge of the paper touches both points and extends past them. Make a tick mark on the edge of the paper at each point. (See Figure 5-3.) Figure 5-3. Transferring map distance to a paper strip
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Chapter 5 5-4 TC 3-25.26 15 November 2013 5-9. To convert the map distance to a measured ground distance, move the paper down to the graphic bar scale, and align the right tick mark (b) with a printed number in the primary scale so that the left tick mark (a) is in the extension scale. (See Figure 5-4.) The primary scale provides the whole unit distance, while the extension scale provides the divided scale used to determine smaller increments of measure. Figure 5-4. Measuring straight-line map distance 5-10. The primary scale is read from the zero mark to the right. Figure 5-4 shows the right tick mark (b) is aligned with the 3000-m mark in the primary scale with the left tick mark (a) in the extension scale, so the distance is at least 3000 m. To determine the distance between the two points to the nearest 100 m, look at the extension scale. The extension scale is numbered with zero at the right and increases to the left. (See Figure 5-4.) The first area on the extension scale represents 0 to 100 m. The second represents 100 to 200 m. Remember, the distance in the extension scale increases from right to left. 5-11. To determine the distance between the two points to the nearest 10 m, divide the distance inside the extension scale into tenths. In Figure 5-4, tick mark (a) falls within the 900 to 1000-m scale area. The location is approximate 5 tenths to the left of the 900 m start point. This is read as 950 m. Adding the distance of 3000 m determined in the primary scale to the 950 m determined by using the extension scale, the total distance between points (a) and (b) is 3950 m.
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