Going from the known (line To convert degrees to mils, multiply degrees by 17.78. c. Grad. The first step to converting magnetic azimuth to grid azimuth is to determine the declination angle. kq_! Thus to convert from a magnetic bearing to a Grid North reference you would subtract 8 23. Converting to a magnetic azimuth when the G-M angle is greater. This sign convention is not necessarily the same for all contractors and should be clearly communicated and understood before drilling begins. The degree scale is graduated from 0 to 360 degrees; each tick mark on the degree scale represents one degree. This value will be written in the map margin (in both degrees and mils) beside the diagram. Using a protractor, draw a line for the back azimuth on the map from the known position back toward your unknown position. When using an azimuth, the point from which the azimuth originates is the center of an imaginary circle (Figure 6-2). Figure 6-18. (1) Orient the map on a flat surface by the terrain association method. (3) Measure the magnetic azimuth to one of the known positions from your location using a compass. In the jpg below, you can see the declination window at 6 oclock on the compass, or specifically, at Zero degrees. Figure 6-19. YMMV. Using knowledge point #3 above, the correct result will be the As the request is for the back azimuth, then what is wanted is I'll be doing it. GM Angle= 7W 2. If you are just navigating to hunt or recreate I would recommend not getting wrapped around the axle on this. The 0- or 360-degree mark is always toward the top or north on the map and the 90 mark is to the right. The magnetic azimuth is determined by using magnetic instruments, such as lensatic and M2 compasses. stream A protractor is used to measure the angle between grid north and the drawn line, and this measured azimuth is the grid azimuth (Figure 6-4). Resection is the method of locating ones position on a map by determining the grid azimuth to at least two well-defined locations that can be pinpointed on the map. Contact Us, Privacy & Legal Statements | Copyright Information How do you convert Magnetic North to grid north? Most well proposals are generated from rectangular coordinates derived from the UTM or local grid system. <> If you know who Lars Ulrich is, you can convert your azimuths. %%EOF On medium-scale maps, the declination information is shown by a note in the map margin. Remember, there are no negative azimuths on the azimuth circle. SOUTH IS 180. Being in the right place at the prescribed time is necessary to successfully accomplish military missions. When using the map and compass method (Figure 6-18). Draw an arrow for Grid North using an exaggerated angle east or west of True North showing the grid convergence angle (be sure of the sign convention of the grid convergence value used, for example does a west convergence angle put Grid North west of True North or visa versa?). H=k0w14}slPZHZ:u_-XC]}>2FD8\70}@YC"/QRpP>eEF^fxFh Going from the known (line (4) Convert the magnetic azimuth to a grid azimuth. This is a direct result of the geodetic north lines converging. Known= 270 mag 3. endobj In the last post onMagnetic Declination, the impact it has on azimuth/bearing accuracy was described. (5) Convert the grid azimuth to a back azimuth. NORTH IS 0/360 AZIMUTH EAST IS 90. To convert a magnetic azimuth to grid azimuth, subtract G-M angle. If you have a magnetic azimuth of 270 degrees, and the G-M angle is 8 degrees, your grid azimuth will be 262 degrees. However, the sum of the geodetic azimuths (in green) at station Fink and station D266 is not 360 degrees. The mil expresses the size of an angle formed when a circle is divided into 6,400 angles, with the vertex of the angles at the center of the circle. A circle equals 6400 mils divided by 360 degrees, or 17.78 mils per degree. You must log in or register to reply here. The G-M angle is important to the map reader/land navigator because azimuths translated between map and ground will be in error by the size of the declination angle if not adjusted for it. b. In this example, the known azimuth is 270 magnetic. hhKY__SJj oRdBe Using LARS, convert to a grid azimuth. In Colorado, the central meridian is 105 degrees, 30 minutes West Longitude, as shown here, on the left you have station D266 and on the right, station Fink. The azimuth is the most common military method to express direction. I never got lost and would find all the points giving by getting close and following the trails to them. 100 - 80 = 20 Susan is 20 degrees off course. %PDF-1.4 Resection with straightedge. endstream %PDF-1.5 b. JavaScript is disabled. Conversely, if its West, Magnetic North is West of your location. is the correct back azimuth. Please note that the sum of the grid azimuths (in red) at station Fink and station D266 is 360 degrees. A simple mathematical mistake could cause disastrous consequences. GN 4 3 1. "'pGX. Notice that, in both cases, that grid north is exactly parallel with the central meridian. ]od`l`hPh``` " K053NRhU@$4 ;3t0$202EV1/bera`9~H20D(Uf TO GN 4 3 1. Converting to a grid azimuth when the G-M angle is greater. 3 0 obj<>>>/MediaBox[0 0 612 792]>> Draw a polar diagram showing True North at 0 degrees azimuth (12 o'clock). (b) Place the protractor on the map with the index mark at the center of mass of the known point and the base line parallel to a north-south grid line. or (2) To plot an azimuth from a known point on a map (Figure 6-7), (a) Convert the azimuth from magnetic to grid, if necessary. The College of Earth and Mineral Sciences is committed to making its websites accessible to all users, and welcomes comments or suggestions on access improvements. This is accomplished by ADDING 180 degrees to the grid azimuth. To convert this UTM Grid North direction to a True North direction, you must apply the grid convergence to the calculated UTM Grid North direction. hbbd``b`@) Dl;@D#` n%"~1Ll %` ( For map based orienteering, where you can Terrain Assoicate, maybe it matters less. Now, if you want to convert a MAZ to GAZ using the example above (in that you have a MAZ of 180, simply add the 15 degrees declination to the 180, and you have a correct GAZ. The direction to the north magnetic pole, as indicated by the north-seeking needle of a magnetic instrument. You see over here on the right at station Fink, the convergence is plus 17.2 arcseconds. (2) Locate and mark your position on the map. azimuth and 2) 180 degrees either added or subtracted will yield (6) Move to a second known point and repeat steps 1, 2, 3, 4, and 5. (c) Make a mark on the map at the desired azimuth. In 1896, Henry Cabot Lodge warned, Your email address will not be published. The most accurate method to determine local declination is to measure the magnetic field with a magnetic transit. *3"0sp=Tq-#wS^v\qC50-h "9Y`l/&w&;O:XT Z,UtBy5tl_nHyUB{&Nqe#RT2,Z`MG1 d If you plot your grid azimuth on a map and it reads (for example), 180 degrees (GAZ), and you know your local declination is 15 degrees East, and you wish to convert your GAZ to a MAZ, simply subtract the Declination of 15 degrees and youll have a 165 MAZ, or magnetic azimuth that you can use with your compass. WebIn this example, the known azimuth is 270 magnetic. What is the cast of surname sable in maharashtra? (1) Add an easterly G-M angle to a magnetic azimuth to obtain a grid azimuth. WebThe GM angle is used to convert magnetic azimuth to grid azimuth and vice versa: Grid azimuth - determined with a protractor and is measured from grid north. It also may be convenient to construct a G-M angle conversion table on the margin of the map. on a hundredths grid 7 hundredths is 7; but seven tenths is 0.7 which is 70 hundredths. This is easy to remember. <> a. endobj \ P]0oY_&@*rX +Fd$Jc&ym?krede QqMwz%l(Yy 3i%[\ V. (7) The intersection of the lines is your location. Intersection, using a straightedge. { yBeDS#5HXM5B^*`Flm5^TQR!j.AzS!OZhkO\%[mk|lq#N. 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. Find out how to get your bearings with this beginner-friendly tutorial and discover some of our favorite things nearby!. The reason you need to subtract a value from grid to get magnetic is that there are actually multiple norths. Figure 6-9. None of your azimuths need conversion until such time as you use a different compass one that cant be adjusted similarly. Azimuth in Surveying You should have written down your Grid Azimuth (GAZ). (d) Read the value of the angle from the scale; this is the grid azimuth from point A to point B (Figure 6-4). True <>/ProcSet[/PDF/Text/ImageB/ImageC/ImageI] >>/MediaBox[ 0 0 612 792] /Contents 10 0 R/Group<>/Tabs/S/StructParents 1>> be Northeast relatively.) To start, what is your goal? xUM0#hWgK G!(l9Rq[L1yfzYjz`W^7]>o_vz{a}Kt;8X 9ngA>A))(&J%qWI H-5dEC{,O^fa< nY&j!5bA w}C|3nq{ceUxc0P }*%9M4.e$$""U rQ1md6G+ a :mB>8a/7Z's?9hd-VyH"a.9QEhd?SGq9~*OO.fM&:CEY) You Brilliant people, who have, Below, we explain the trend for those non-Spanish speaking/understanding viewers Thankfully, @babycakes607 explained the trend, In William Shakespeares Romeo and Juliet the character Mercutio is prosaic about love and considers, Hostility to immigrants isnt new to the United States. WebSelect and give grid coordinates for three points on the map; list azimuths to be followed in the construction of a line through each point. How do you calculate Grid North to magnetic north? Declination angles or GM angles are the difference between these two alignments, so you add or subtract your angle as needed to get them to match up, so that you can plot a point on a map, and then shoot an azimuth to it with a compass. 0 %%EOF a. Known= 270 mag 3. It also includes some advanced aspects of map reading, such as intersection, resection, modified resection, and polar plots. f. The location of the user is where the line crosses the linear feature. Its a familiarity thing for me. Thatll be covered in one of the next posts on this subject. %PDF-1.5 % Therefore, there is a value, shown here in blue, called convergence. The angle of correction is the angle between the meridians of longitude and the Northings of the grid system at a specified point. (5) Draw a line on the map from your position on this grid azimuth. This courseware module is offered as part of the Repository of Open and Affordable Materials at Penn State. A line from any point on the earths surface to the north pole. Depending on what your doing it can matter a lot. 4 0 obj <>stream Draw an arc from the grid prong to the magnetic prong and place the value of the G-M angle (Figure 6-9). a. The instructor informed us that here in the US it really wasn't relevant. This protractor represents the azimuth circle. How to convert magnetic azimuth to grid azimuth. This is a direct result of the grid north lines being parallel with each other. To convert a grid azimuth to a magnetic azimuth, subtract the G-M angle from the grid azimuth. Personally, I prefer maps that use the MGRS (Military Grid Reference System) which is similar to the UTM, but scaled in 1:25,000 & 1:50,000, which is what I learned in during active duty. WebConverting azimuths to quadrant bearings or vice versa is easy. 10 0 obj <>stream (b) To use a magnetic azimuth in the field with a compass, first change the grid azimuth plotted on a map to a magnetic azimuth (Figure 6-11). WebTo convert a magnetic azimuth to a grid azimuth, add the value of the G-M angle to the magnetic azimuth. Left Add, Right Subtract. Luckily, with the use of This method can be used at night. ^z].;@ Webc. Magnetic North. All lines of longitude are true north lines. To determine your GAZ, you should use one of these, known as a protractor. Using it is pretty simple: Place the center over the point on the map youre using to determine which azimuth youre going to travel on, square the protractor to the maps grid, place a small tick mark in the center of the protractor (they usually have a hole for that purpose), and use some sort of straight edge to line up the center point and the azimuth (use the INNER RING of degrees, not the outer ring), place a tick mark, move the protractor out of the way, and draw a straight line from the origin point to the azimuth line. Swarovski ATC Compact Spotting Scope Review, Helping Mule Deer Thrive: Muley Fanatic Foundation, Growing Big Mule Deer with Wildlife Prof Randy Larsen. 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. When Magnetic North lies to the west of True North, the magnetic declination is said to be westerly, and if to the east, easterly. What is the Grid Azimuth? Is Grid North the same as magnetic north? A rule to remember when solving such problems is this: Nomatter where the azimuth line points, the angle to it is always measured clockwise from the reference direction (base line). The difference between magnetic north and grid north is often referred to as grid magnetic angle and it not only varies from place to place, but changes with time. You need to know that if your declination is East, it means that Magnetic North is East of your location. Grid North - based on a specific coordinate system and is used to define the relative direction of a line. Its the little black pincer looking thing. hb```$O eah``dFg9-GbEi|x Who's going to be the first hunting 'celebrity' or Youtube personality to put their foot down and stop using cellular cameras? H[k0e0Zu(fW9QKq:.LJ)~OBhmUEz]@F|;xv?H'F07@F{d;F#: F8hMg$/[?Y% FC4s~ujTzF8[4ZOEOi[nkJ . True North. True north is usually represented by a star (Figure 6-1). Do you have pictures of Gracie Thompson from the movie Gracie's choice? Going from the h[k0`+namhZ*Cbo?N6 ^cXODeQ GJp2iE"JO!/^UgCF97_X]!A(pOXZ,|J;cT-Ve;TOwgRC1x6o1ruqeS{AuG_zPK$H,oZ xsc.mUL.w$& endobj the reverse (or inverse) of the azimuth, 223 degrees. There are 400grads in a circle (a 90-degree right angle equals 100 grads). xVr0}+$7-)NCRL^*Y_ Dd"]IQvta2wIqr dC;[yTPu&QiFc@Lh&%\IL me)_0XbHVz$cX\?h*L4KAWF!!T! x]Yo#7~`C?JAXHYd6@Aefl_]~{uOv:e#Y7evu (3) The G-M angle diagram should be constructed and used each time the conversion of azimuth is required. When using the protractor, the base line is always oriented parallel to a north-south grid line. They are certainly not parallel with one another and certainly not parallel with the central meridian. One does not typically work with directions exceeding 360 Using LARS, convert to a grid azimuth. The magnitude of the correction angle depends upon its location within the grid and its latitude. 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. (d) Determine the value of the G-M angle. But which equation yields the correct back azimuth? This circle is divided into 360 degrees or 6400 mils . endobj Then, what exactly is a grid azimuth? ^ni`H2pI35%S8lKXoboAJXT/k]3 @[J Is kanodia comes under schedule caste if no then which caste it is? When using the map and compass method. b. Mil. This is because 2 degrees and 362 degrees are located at the same point on the azimuth circle. Have students construct an azimuth-back azimuth line through each point until each end of the line hits a map margin. (3) Lay a straight edge on the map with one end at the users position (A) as a pivot point; then, rotate the straightedge until the unkown point is sighted along the edge. Military personnel need a way of expressing direction that is accurate, is adaptable to any part of the world, and has a common unit of measure. This north base line could be true north, magnetic north, or grid north. Its easy to find if youre back at home, but they can be tricky in unfamiliar territory! An azimuth is defined as a horizontal angle measured clockwise from a north base line. endobj How do you convert magnetic bearings to grid bearings? (c) Convert a grid azimuth to a magnetic azimuth when the G-M angle is greater than a grid azimuth (Figure 6-12). Magnetic readings are obtained with magnetic instruments, such as lensatic and M2 compasses. While this seems a simple task, misunderstandings surrounding the relationship between these references can cause a target to be missed. 2287 0 obj <>/Filter/FlateDecode/ID[<01E97BD82B4A724EA381572D5ADBF101><24C8F5536AE732469D3285819F05CE2D>]/Index[2282 15]/Info 2281 0 R/Length 48/Prev 360229/Root 2283 0 R/Size 2297/Type/XRef/W[1 2 1]>>stream That explanation on drawing lines on the map is definitely very useful. The Pennsylvania State University 2020, GPS and GNSS for Geospatial Professionals, Lesson 6: State Plane Coordinates and Heights, State Plane Coordinate Zones 1983, False Eastings and Scale, Lesson 7: Static Global Positioning System Surveying, Lesson 8: Real-Time Global Positioning System Surveying, Lesson 10: Global Navigation Satellite Systems and the Future, Repository of Open and Affordable Materials, Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, John A. Dutton Institute for Teaching and Learning Excellence, Department of Energy and Mineral Engineering, Department of Materials Science and Engineering, Department of Meteorology and Atmospheric Science, Earth and Environmental Systems Institute, Earth and Mineral SciencesEnergy Institute, iMPS in Renewable Energy and Sustainability Policy Program Office, BA in Energy and Sustainability Policy Program Office, 2217 Earth and Engineering Sciences Building, University Park, Pennsylvania 16802. The other choice, Ab = A + 180 = 223 + 180 = 403 degrees, and G.O.M.A when applying the magnetic declination of the geographical area youre in. Several field-expedient methods to determine direction are discussed in Chapter 9, paragraph 5. c. Grid Convergence. Since wells must be surveyed with sensors that reference direction to either Magnetic or True North, it will be necessary to convert between these references. Your current quad will be drastically differant from your 1950s quad. is, must be between zero and 360 degrees. The following elements must be present when using polar coordinates (Figure 6-21). Converting to a magnetic azimuth on a map. Theyre ok to use; theyre typically in 1:24,000 scale, and you need to ensure your map tools correspond in scale. The bottom line is this. 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. Doesn't matter if you're in the northern or southern hemisphere. Why Walden's rule not applicable to small size cations. This means how to measure a direction - or azimuth - on your map and 9 0 obj<>>>/MediaBox[0 0 612 792]>> Label the borehole azimuth with reference to each system. Copyright 2023 ScienceAlert.quest | Powered by Astra WordPress Theme. Determine the grid coordinates to the desired accuracy. Meridians converge on the surface of the Earth. (f) The inclusion of the true-north prong in relationship to the conversion is of little importance. Figure 6-13. one that is between zero and 360 degrees. e. Convert the grid azimuth to a back azimuth. PRECISION LIGHTWEIGHT GLOBAL POSITIONING SYSTEM RECEIVER, Map Reading and Land Navigation REFERENCES. There are two methods of intersection: the map and compass method and the straightedge method (Figures 6-16 and 6-17). b. Label the borehole azimuth with reference to each system. Azimuths are conventionally measured in a clockwise direction from north with 0 degrees corresponding with north, 90 degrees east and 180 degrees south. Proceed as follows: a. Orient the map using a compass or by terrain association. exceeds the range of the compass, so 403 degrees cannot be the 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. For your convenience, you can use a UTM Universal Transverse Mercator AKA USGS 7.5 minute map. grid north (uncountable) The direction northwards along the grid lines of a map projection. It is used to locate distant or inaccessible points or objects such as enemy targets and danger areas. How to convert magnetic azimuth to grid azimuth, The how to convert magnetic azimuth to grid azimuth is a question that may be asked in the context of Figure 6-7. (7) The location of the unknown position is where the lines cross on the map. More on that in the future. (1) With Notes. There is true north (the northern axis around which the earth rotates), magnetic north (the direction compasses point, which changes slightly every year) and grid north (the somewhat arbitrary point where all of your grid lines would converge if your map could extend all the way to the North Pole. 8 0 obj Figure 6-12. On a plane surface this would be the shortest distance between two points]. <> The shortest WebThe Azimuth Calculator is an incredibly simple tool you can use to determine the azimuth between two specific points on earth. WEST IS 270. ' vuxcwxXONIq$eC CRFRERH>QF4J#[~CO]U26#iB(V]vLa`'xLa1?Ro~& *!o$8EAbMZIU&G3V`o! Except where otherwise noted, content on this site is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License. Simply refer to the conversion notes that appear in conjunction with the diagram explaining the use of the G-M angle (Figure 6-8). endobj Most of the declinations are listed on each USGS quad but from an accuracy issue they change so what year is your quad. The north that is established by using the vertical grid lines on the map. The simplest way available to man right now is to get a compass that has an declination adjustable feature, such as the Sunnto MC-3. The terminal coordinates program may be used to find the coordinates on the Earth at some distance, given an azimuth and the starting coordinates.. a. easterly or westerly G-M angle. Intersection, using map and compass. 1 0 obj Apply the mathematical formula to convert a magnetic azimuth to a grid azimuth. The grad is divided into 100centesimal minutes (centigrads) and the minute into 100 centesimal seconds (milligrads). First, a word about grid maps. b. Determine the grid coordinates to the desired accuracy. _k6 u7PO3+i_yF(igS|pVGXW~o6zom6 8bl_FP)T iN5;SIlw-KKwj{kLx!:E"Ux++syH9Fj- 5V -PY.f;!eOa$*^ TRdNb8ds)Rjn9TKT{y&wN4# g (b) To use a magnetic azimuth in the field with a compass, change the grid azimuth plotted on a map to a magnetic azimuth (Figure 6-14). Where the base line intersects the horizontal line, between 90 and 270 degrees, is the index or center of the protractor (Figure 6-6). Resection with map and compass. hb```"?f" !E5r|x:s9DvD*\^b"/mKG@@al>;/UbD-m- 1?(RA)@'7$Z4 ER#cJCg((`R``A*x Remember, when taking a reading of a mountain then transfering it to your map, you must do the opposite of what you did when you took a reading from your map. <>>> Find a distant point that can be identified on the ground and on the map. The angle can be clockwise or counter clockwise from True North to Grid North. Figure 6-14. Web(a) To plot a magnetic azimuth on a map, first convert it to a grid azimuth (Figure 6-13). (6) The intersection of the lines on the map is your location. WebAdd the GM angle to convert a magnetic azimuth into a grid azimuth. dy|zn/_|\`dA/xo_vMS@"/9F}6DQ%u>.]l.ny,Vx~#oWCO1[KAVG9m4DyM*7vp`x~\^ ]S,6 Consider that if the object was magnetically due north of you (0 degrees magnetic), then physically it would have to be 12 degrees east of Grid North [Noun. Figure 6-11. c. Grid North. 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 180-degree mark at the bottom of the protractor to this same grid line. Convergence is the difference between geodetic north and grid north. <> This angle is the difference between the north and the magnetic The consequence of not converting or adjusting for declination is not reaching your objective..or worse, getting hopelessly lost, which might be terminal, depending on your location and the scenario you find yourself in. 8 0 obj <>stream This is a direct result of the grid north lines being parallel with each other. GM Angle= 7W 2. WebPlease note that the sum of the grid azimuths (in red) at station Fink and station D266 is 360 degrees. (2) The user should re-read the azimuth between the azimuth and north-south grid line to check the initial azimuth. grid azimuth = geodetic azimuth How do you convert magnetic azimuth to grid azimuth? Ab = A + 180 For greater accuracy, the desired method of resection would be to use three or more well-defined locations. There are 180 140 = 40 degrees between the South and the point, therefore the quadrant bearing is S40E. ?101rT$ > ` For a better experience, please enable JavaScript in your browser before proceeding. to arrive at the starting point. This needs to be taken into account when navigating with a map and compass. If you're using a regular map with a declination diagram this will always work. 4 0 obj All the norths are parallel in the plane systems; this is, of course, unrealistic. Ab = A - 180 Magnetic Azimuth. A back azimuth is the opposite direction of an azimuth. In order to measure something, there must always be a starting point or zero measurement. Computer programs like GEOMAG use current magnetic models and calculate up-to-date local declination figures. WebTO CONVERT A MAGNETIC AZIMUTH TOA GRID AZIMUTH ADD G-M ANGLE 0-95 ro GIVE A STANDARD REFERENCE ON THIS SHEET TO NEAREST 100 METERS tifying Web1995 G-M ANGLE MILS) 5040 TO CONVERT A MAGNETIC AZIMUTH TOA GRID AZIMUTH ADD G-M ANGLE 0-95 ro GIVE A STANDARD REFERENCE ON THIS SHEET TO NEAREST 100 METERS tifying 100,000 meter point lies: ICAL grid line to LEFT LARGE figures labeling e top or bottom margin. xr0A{K;M64%X GqJ8u_~E ,E}\x6%aX@Bl8p Eight Digit: _____m 7) Convert a grid azimuth of 357 to a magnetic azimuth. There are three base lines true north, magnetic north, and grid north. Grid convergence as calculated by the directional software package is the angular difference in degrees between True North and UTM Grid North. We used the earlier GPS's called sluggers. Back Azimuth. NJUi eh)Sf1!S3"k0 o#fLE>;*Dn|rVUUlk uUP_HH(4\0y}[||] 7Ka4IWp7$A26mA{=3~lN ]L : The University of Akron, Ohio The most common unit of measure is the degree () with its subdivisions of minutes () and seconds (). (4) Convert the magnetic azimuth to grid azimuth. Determine the grid coordinates to the desired accuracy. (c) Convert a magnetic azimuth when the G-M angle is greater than the magnetic azimuth (Figure 6-15). Not sure your question can be answered. Underneath the compass is a small standard screw slot that an accompanying tool will adjust to whatever your declination is, East or West. Given this knowledge and the variable definitions Ab = Back a. All of them divide the circle into units of angular measure, and each has a scale around the outer edge and an index mark. Basically, once you determine what your Magnetic Declination is, you simply use the provided tool, adjust the declination reading to correspond with that number (either East or West) and use your compass on the map as well as when shooting your azimuth. Thus we ADD the variation (12 degrees) to give the Grid Bearing [A rhumb line appears as a straight line on a Mercator projection map. 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. The back azimuth of 180 degrees may be stated as 0degrees or 360 degrees. Suunto makes a good mirror compass that allows you to set it Silva makes one too, foresters use these types as it makes things a bit easier. Such procedure is important when working with a map for the first time. It is comparable to doing about face. To obtain a back azimuth from an azimuth, add 180degrees if the azimuth is 180 degrees or less, or subtract 180 degrees if the azimuth is 180degrees or more (Figure 6-3).
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