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It can be expressed as right, left, straight ahead, and so forth; but then the question arises, "To the right of what? It explains in detail how to determine the grid and the magnetic azimuths with the use of the protractor and the compass. It explains the use of some field-expedient methods to find directions, the declination diagram, and the conversion of azimuths from grid to magnetic and vice versa.
It also includes some advanced aspects of map reading, such as intersection, resection, modified resection, and polar plots. Directions are expressed as units of angular measure. Another unit of measure, the mil abbreviated , is used mainly in artillery, tank, and mortar gunnery. The mil expresses the size of an angle formed when a circle is divided into 6, angles, with the vertex of the angles at the center of the circle. A relationship can be established between degrees and mils. A circle equals mils divided by degrees, or To convert degrees to mils, multiply degrees by The grad is a metric unit of measure found on some foreign maps.
To express direction as a unit of angular measure, there must be a starting point or zero measure and a point of reference These two points designate the base or reference line. There are three base lines— true north, magnetic north, and grid north. The most commonly used are magnetic and grid. True North. All lines of longitude are true north lines.
True north is usually represented by a star Figure Figure Three norths. Magnetic North. 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 Figure Magnetic readings are obtained with magnetic instruments, such as lensatic and M2 compasses.
Grid North. The north that is established by using the vertical grid lines on the map. Grid north may be symbolized by the letters GN or the letter "y" Figure 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 from which the azimuth originates is the center of an imaginary circle Figure This circle is divided into degrees or mils Appendix G.
Origin of azimuth circle. Back Azimuth. A back azimuth is the opposite direction of an azimuth. It is comparable to doing "about face. Back azimuth. A simple mathematical mistake could cause disastrous consequences. Magnetic Azimuth. The magnetic azimuth is determined by using magnetic instruments, such as lensatic and M2 compasses. Refer to Chapter 9, paragraph 4 , for details. Field-Expedient Methods.
Several field-expedient methods to determine direction are discussed in Chapter 9, paragraph 5. A protractor is used to measure the angle between grid north and the drawn line, and this measured azimuth is the grid azimuth Figure Measuring an azimuth. If a mistake is made and the reading is taken from the ending point, the grid azimuth will be opposite, thus causing the user to go in the wrong direction.
All of them divide the circle into units of angular measure, and each has a scale around the outer edge and an index mark. The index mark is the center of the protractor circle from which all directions are measured. Types of protractors. The military protractor, GTA , contains two scales: one in degrees inner scale and one in mils outer scale.
This protractor represents the azimuth circle. The degree scale is graduated from 0 to degrees; each tick mark on the degree scale represents one degree. A line from 0 to degrees is called the base line of the protractor. Where the base line intersects the horizontal line, between 90 and degrees, is the index or center of the protractor Figure Military protractor. When using the protractor, the base line is always oriented parallel to a north-south grid line. See paragraph This is the grid direction line azimuth.
Distance does not change an accurately measured azimuth. Plotting an azimuth on the map. To obtain an accurate reading with the protractor to the nearest degree or 10 mils , there are two techniques to check that the base line of the protractor is parallel to a north-south grid line. The user should be able to determine whether the initial azimuth reading was correct.
Count the number of degrees from the 0-degree mark at the top of the protractor to this north-south grid line and then count the number of degrees from the degree mark at the bottom of the protractor to this same grid line.
If the two counts are equal, the protractor is properly aligned. If you have a map and a compass, the one of most interest to you will be between magnetic and grid north. The declination diagram Figure shows the angular relationship, represented by prongs, among grid, magnetic, and true norths. While the relative positions of the prongs are correct, they are seldom plotted to scale. Do not use the diagram to measure a numerical value. This value will be written in the map margin in both degrees and mils beside the diagram.
Declination diagrams. A declination diagram is a part of the information in the lower margin on most larger maps. On medium-scale maps, the declination information is shown by a note in the map margin. Grid-Magnetic Angle. The G-M angle value is the angular size that exists between grid north and magnetic north. It is an arc, indicated by a dashed line, that connects the grid-north and magnetic-north prongs.
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. 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. Simply refer to the conversion notes that appear in conjunction with the diagram explaining the use of the G-M angle Figure 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 north-grid prong.
In some cases, there are no declination conversion notes on the margin of the map; it is necessary to convert from one type of declination to another. A magnetic compass gives a magnetic azimuth; but in order to plot this line on a gridded map, the magnetic azimuth value must be changed to grid azimuth. The declination diagram is used for these conversions. Always align this line with the vertical lines on a map Figure Declination diagram with arbitrary line.
Draw a magnetic prong from the apex of the grid-north line in the desired direction Figure Draw an arc from the grid prong to the magnetic prong and place the value of the G-M angle Figure A glance at the completed diagram shows whether the given azimuth or the desired azimuth is greater, and thus whether the known difference between the two must be added or subtracted. Remember, there are no negative azimuths on the azimuth circle.
Since 0 degree is the same as degrees, then 2 degrees is the same as degrees. This is because 2 degrees and 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.
Converting to grid azimuth. Converting to magnetic azimuth. Converting to a magnetic azimuth when the G-M angle is greater. Converting to a grid azimuth on a map. Converting to a magnetic azimuth on a map.
FM 3-25.26 Map Reading and Land Navigation