MSL101L04 Basic Map Reading SR.pdf lesson 4.pdf

To explain further true north is defined as a line

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and grid. To explain further: True north is defined as a line from a point on the earth’s surface to the North Pole. All lines of longitude are true north lines. True north is usually represented by a star. Magnetic north is the direction to the north magnetic pole, as indicated by the north-seeking needle of a magnetic instrument. The magnetic north is usually symbolized by a line ending with half of an arrowhead. Magnetic readings are obtained with instruments such as the lensatic and M2 compasses. It has been proven that the geomagnetic poles migrate over time. This means that the effect on the declination diagram varies depending on location. The declination diagram for Fort Richardson, Alaska in 1993 was 23° E, but by 2013 it was 18° E—a five degree change. The declination diagram for Fort Benning, Georgia in 1993 was 2° W, but by 2013 it was 4° W—a difference of two degrees. The National Oceanic and Atmospheric Administration (NOAA) provides a magnetic field calculator web site to calculate the declination using the current international geomagnetic reference field model. The website address is: http://www.ngdc.noaa.gov/geomag-web/?id=declinationFormId#declination .
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Chapter 6 6-2 TC 3-25.26 15 November 2013 Grid north is established by using the vertical grid lines on the map. Grid north may be symbolized by the letters GN or the letter “y.” CAUTION Check the map for the date; see Chapter 3 for location of map information. Declination diagrams older than 20 years may provide magnetic readings which are unreliable when converting True or Grid North with the Magnetic North, or the opposite. Figure 6-1. Three norths AZIMUTHS 6-3. An azimuth is defined as a horizontal angle measured clockwise from a north base line. This north base line could be true north, magnetic north, or grid north. The azimuth is the most common military method to express direction. When using an azimuth, the point where the azimuth originates is the center of
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Directions 15 November 2013 TC 3-25.26 6-3 an imaginary circle. (See Figure 6-2.) This circle is divided into 360 degrees, or 6400 mils. Other azimuths are: Back azimuth . This is the opposite direction of an azimuth. It is comparable to doing an “about face.” To obtain a back azimuth from an azimuth, add 180 degrees if the azimuth is 180 degrees or less; subtract 180 degrees if the azimuth is 180 degrees or more. (See Figure 6-3.) The back azimuth of 180 degrees may be stated as 0 degrees or 360 degrees. For mils, if the azimuth is less than 3200 mils, add 3200 mils; if the azimuth is more than 3200 mils, subtract 3200 mils. Figure 6-2. Origin of azimuth circle Figure 6-3. Back azimuth calculation with azimuth less than 180 degrees
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Chapter 6 6-4 TC 3-25.26 15 November 2013 WARNING When converting azimuths into back azimuths, extreme care should be exercised when adding or subtracting the 180 degrees. A simple mathematical mistake could cause disastrous consequences.
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