MSL101L04 Basic Map Reading SR.pdf lesson 4.pdf

# The distance east or west of a prime meridian to a

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Greenwich, England and is known as the Greenwich Meridian. The distance east or west of a prime meridian to a point is known as its longitude. Lines of longitude (meridians) run north- south but east-west distances are measured between them. (See Figures 4-1 and 4-2.)

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Grids 15 November 2013 TC 3-25.26 4-17 4-11. Grid coordinates divide the earth’s surface into 6-degrees-by-8-degrees quadrangles, covered with 100,000-m squares. The military grid reference of a point consists of the numbers and letters indicating where in the 100,000-m grid zone areas the point lies, plus the coordinates locating the point to the desired position within the 100,000-m square. The next step is to tie in the coordinates of the point with the larger areas. To do this, it is important to understand— Grid lines . The regularly spaced lines that make up the UTM and UPS grid on large-scale maps are divisions of the 100,000-m square. The lines are spaced at 10,000-m or 1000-m intervals. (See Figure 4-14.) Each of these lines is labeled at both ends of the map with its false easting or false northing value, showing its relation to the origin of the zone. Two digits of the values are printed in large type, and these same two digits appear at intervals along the grid lines on the face of the map. These are called the principal digits, and represent the 10,000 and 1000 digits of the grid value. They are of major importance to the map reader because they are the numbers used most often for referencing points. The smaller digits complete the UTM grid designation. Figure 4-14. Grid lines and principle digits
Chapter 4 4-18 TC 3-25.26 15 November 2013 Note. As an example, the first grid line north of the south-west corner of the Columbus map is labeled 3570000m N. This means its false northing (distance north of the equator) is 3,570,000 m. The principal digits, 70, identify the line for referencing points in the northerly direction. The smaller digits, 35, are part of the false coordinates and are rarely used. The last three digits, 000, of the value are omitted. Therefore, the first grid line east of the south-west corner is labeled 689000m E. The principal digits, 89, identify the line for referencing points in the easterly direction. (See Figure 4-15.) Figure 4-15. Southwest corner of the Columbus, Georgia, map

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Grids 15 November 2013 TC 3-25.26 4-19 Grid squares. The north-south and east-west grid lines intersect at 90 degrees, forming grid squares. Normally, the size of one of these grid squares on large-scale maps is 1000 m (1 km). Grid coordinate scales. The primary tool for plotting grid coordinates is the grid coordinate scale. This scale divides the grid square more accurately than can be done by estimation and the results are more consistent. When used correctly, it presents less chance for making errors. GTA 5-2-12 contains four types of coordinate scales. (See Figure 4-16.) Figure 4-16. Coordinate scales
Chapter 4 4-20 TC 3-25.26 15 November 2013 Note. The 1:50,000-scale (upper left in Figure 4-16) subdivides the 1000-m block into 10 major subdivisions, each equal to 100 m. Each 100-m block is then divided in half. Points falling between the graduations are estimated to the nearest 10 m for the fourth and eighth digits of the coordinates.

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

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