MSL101L04 Basic Map Reading SR.pdf lesson 4.pdf

# Grid convergence an arc indicated by a dashed line

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Grid convergence . An arc indicated by a dashed line connects the prongs for true north and grid north. The value of the angle for the center of the sheet is given to the nearest full minute, with its equivalent to the nearest mil. These data are shown in the form of a grid-convergence note. Conversion. There is an angular difference between the grid north and the magnetic north. Since the location of magnetic north does not correspond exactly with the grid-north lines on the maps, a conversion from magnetic to grid or vice versa is needed. CAUTION Declination diagrams older than 20 years may provide magnetic readings which are unreliable when converting True or Grid North with a Magnetic North, or the opposite. N OTES 6-12. When notes are furnished, simply refer to the conversion notes that appear in conjunction with the diagram explaining the use of the G-M angle. One note provides instructions for converting magnetic azimuth to grid azimuth; the other, for converting grid azimuth to magnetic azimuth. The conversion (add or subtract) is governed by the direction of the magnetic-north prong relative to that of the grid-north prong. C ONVERSION OF GRID AND MAGNETIC AZIMUTHS 6-13. A magnetic compass gives a magnetic azimuth, but in order to plot this line on a gridded map, the magnetic azimuth value is changed to grid azimuth. The opposite process is done for converting a grid azimuth to a magnetic azimuth. The declination diagram is used for these conversions.

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Chapter 6 6-10 TC 3-25.26 15 November 2013 6-14. Conversion of grid azimuths to magnetic azimuths and vice versa, depend on whether one is converting easterly or westerly G-M angles, the degree of declination change, and whether one is going from a magnetic azimuth to grid azimuth, or the opposite. (See Figure 6-8 and Table 6-1.) To do this— From an easterly magnetic azimuth to grid azimuth, one would add. To go from a grid azimuth to magnetic azimuth, one would subtract. (See Table 6-1.) From a westerly magnetic azimuth to a grid azimuth, one would subtract. To go from a grid azimuth to a magnetic azimuth, one would add. (See Table 6-1.) Figure 6-8. Declination diagrams 6-15. There are no negative azimuths on the azimuth circle. Since 0 degree is the same as 360 degrees, then 2 degrees is the same as 362 degrees. This is because 2 degrees and 362 degrees are located at the same point on the azimuth circle. The grid azimuth can now be converted into a magnetic azimuth because the grid azimuth is now larger than the G-M angle. (See Table 6-1.)
15 November 2013 TC 3-25.26 9-1 Chapter 9 Elevation and Relief The elevation of points on the ground and the relief of an area affect movement, positioning, and sometimes the effectiveness of military units. Soldiers know how to determine locations of points, measure distances and azimuths, and identify symbols on a map. They also determine the elevation and relief of areas on standard military maps. To do this, Soldier’s first understand how the mapmaker indicated the elevation and relief on the map.

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

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