How to Use a Compass with a USGS Topographic Map

Transcription

1 How to Use a Compass with a USGS Topographic Map METHOD #1: (these directions assume your orienting arrow lines up with the North indicator on your compass dial, meaning the compass has not been adjusted for declination). A) Obtain the local magnetic declination for the area represented on your map. At the bottom of every USGS map is a diagram that displays the difference & direction between true north (represented as a star), grid north (abbreviated as GN ), and magnetic north (abbreviated as MN ). Magnetic declination is the number of degrees and direction between true north and magnetic north. Because declination varies over time, it is advisable to get a reasonably current figure. If your USGS map is more than 15 years old (the declination date appears in the diagram), here s an easy-to-use website that gives you only the information you need: If magnetic north is east of true north, the local declination is positive. If magnetic north is west of true north, the local declination is negative. B) Draw a line on the map that connects your starting point with the destination (your map bearing ). Extend the line all the way through the map border (the neat line ). C) Distance yourself from any nearby metal such as keys, belt buckle, desk, car, fence, etc. Place the compass on the map so the needle s pivot point is directly over the intersection of your map bearing and neat line. D) Rotate the dial until compass ring north agrees with map north. Read your map bearing from the compass dial. Make sure the bearing agrees with your direction of travel for example, if you intend to travel due east, the bearing is 90 degrees, not 270 degrees. E) Do this step mentally don t turn the compass dial. If the local declination is positive, then subtract the declination amount from the bearing you just derived. If the local declination is negative, then add the declination amount to the bearing you just derived. F) Turn the compass dial until the figure you calculated in step E lines up with the index line.

2 G) Lift the compass off the map, and with the direction of travel arrow pointing directly away from you, rotate your body and the compass all in one motion until the red magnetic needle overlays the orienting arrow. H) Site a landmark along this bearing, and proceed to it. Repeat this step until you reach your destination. METHOD #2: (these directions assume your orienting arrow lines up with the North indicator on your compass dial, meaning the compass has not been adjusted for declination). A) Obtain the local magnetic declination for the area represented on your map. At the bottom of every USGS map is a diagram that displays the difference & direction between true north (represented as a star), grid north (abbreviated as GN ), and magnetic north (abbreviated as MN ). Magnetic declination is the number of degrees and direction between true north and magnetic north. Because declination varies over time, it is advisable to get a reasonably current figure. If your USGS map is more than 15 years old (the declination date appears in the diagram), here s an easy-to-use website that gives you only the information you need: If magnetic north is east of true north, the local declination is positive If magnetic north is west of true north, the local declination is negative. B) Draw a line on the map that connects your starting point with the destination (your map bearing ). C) Distance yourself from any nearby metal such as keys, belt buckle, desk, car, fence etc. D) Place the compass on the map so the baseplate is parallel to the line you drew. Make sure the direction of travel arrow points to your destination. E) Rotate the dial until compass ring north agrees with map north. Do not move the compass when you rotate the dial. F) Remove the compass from the map and, with the direction of travel arrow pointing directly away from you, rotate your body and the compass all in one motion until the red magnetic needle overlays the orienting arrow. G) If local declination is positive, then subtract the declination amount (turn the dial clockwise). If local declination is negative, then add the declination amount (turn the dial counter-clockwise).

3 H) Again, with the direction of travel arrow pointing directly away from you, rotate your body and compass all in one motion until the red magnetic needle overlays the orienting arrow. Site a landmark along this direction of travel and proceed to it. Repeat this step until you reach your destination. ADJUSTABLE DECLINATION COMPASSES: A compass with adjustable declination allows you to rotate the orienting arrow independently of the compass dial. If you have such a compass, you may calculate your map bearing without adding or subtracting the amount of local magnetic declination. To calibrate your compass in this fashion, rotate the inner liquid capsule (or turn the screw with the key) until the orienting arrow deviates from the compass ring s north indicator by the amount & direction of the local magnetic declination. For example, if local declination is 10 degrees east of true north, rotate the inner liquid capsule (or turn the screw with the key) until the orienting arrow points to 10 degrees east. If using Method #1, you do not need to add or subtract as indicated in step E; just make sure compass ring north (not the orienting arrow) agrees with map north as directed in step D If using Method #2, skip steps G and H; again, be sure compass ring north (not the orienting arrow) agrees with map north as directed in step E. HOW TO GET YOUR MAP AND COMPASS FACING THE SAME DIRECTION: Place the compass on the map so the baseplate parallels the north-to-south map neat line. Rotate the dial until compass ring North agrees with map North. Add or subtract the amount needed to adjust for local magnetic declination (subtract if local declination is positive, add if local declination is negative); if your adjustable declination compass is already calibrated for local declination, you don t need to add or subtract, just make sure compass ring North (not the orienting arrow) agrees with map North. Holding the map and compass steadily (the baseplate should still be on the north-to-south map neat line), rotate the map and compass all in one motion until the red magnetic needle overlays the orienting arrow. Again, make sure there is no interference from metal when you perform this (ex: rebar in concrete). Your map and your compass are now oriented to true north. Compare the physical features around you with your map to help derive your location on the map.. HOW SHOULD I SET MY GPS & COMPASS? Let s assume the local declination is 10 degrees east of true north, and the waypoint you wish to navigate to is directly north of your current location. If using a compass without adjustable declination, set your GPS to magnetic north. Your GPS will indicate a bearing of 350 degrees must be followed to reach the waypoint. Dial 350 degrees on your compass. With the direction-of-travel arrow pointed

4 directly away from you, turn your body & compass in one motion until the red magnetic needle overlays the orienting arrow. Site a landmark along that bearing, and proceed. You will be traveling directly north toward the waypoint. If using a compass with adjustable declination, set your GPS to true north. Your GPS will indicate a bearing of zero degrees must be followed to reach the waypoint. Adjust the declination on your compass so the orienting arrow points to 10 degrees east. Dial zero degrees on your compass. With the direction-of-travel arrow pointed directly away from you, turn your body & compass in one motion until the red magnetic needle overlays the orienting arrow. Site a landmark along that bearing, and proceed. You will be traveling directly north toward the waypoint TO DETERMINE PERCENT OF SLOPE AND ANGLE OF SLOPE: Percent of slope is determined by dividing the amount of elevation change by the amount of horizontal distance covered (sometimes referred to as the rise divided by the run ), and then multiplying the result by 100. The run assumes you re traveling on an idealized flat surface it does not account for the actual distance traveled once elevation change is factored in. Example: let s assume your climb gains 1,000 feet in altitude (the rise) and the horizontal distance as measured on the map is 2,000 feet (the run). 1,000 divided by 2,000 equals.5 Multiply.5 by 100 to derive percent of slope: 50% Example: let s assume your climb gains 500 feet in altitude (the rise) and the horizontal distance as measured on the map is 3,000 feet (the run). 500 divided by 3,000 equals.166 Multiply.166 by 100 to derive percent of slope: 16.6% Example: let s assume your climb gains 700 feet in altitude (the rise) and the horizontal distance as measured on the map is 500 feet (the run). 700 divided by 500 equals 1.4 Multiply 1.4 by 100 to derive percent of slope: 140%

5 Angle of slope represents the angle that s formed between the run (remember it s an idealized flat surface that ignores elevation change) and your climb s angular deviation from that idealized flat surface. To calculate this, you divide the rise divided by the run, and then obtain the inverse tangent of the result. Example: let s assume your climb gains 1,000 feet in altitude (the rise) and the horizontal distance as measured on the map is 2,000 feet (the run). 1,000 divided by 2,000 equals.5 Press the INV button on your calculator (sometimes called 2 nd function) Press the TAN button on your calculator Your angle of slope is 26.5 degrees Example: let s assume your climb gains 1,000 feet in altitude (the rise) and the horizontal distance as measured on the map is 1,000 feet (the run). 1,000 divided by 1,000 equals 1 Press the INV button on your calculator (sometimes called 2 nd function) Press the TAN button on your calculator Your angle of slope is 45 degrees WHAT DIRECTION AM I FACING? In this case, you know the aspect, but you don t know the number of degrees. If using a compass without adjustable declination, make sure the direction of travel of arrow is pointing directly away from you. Now rotate the compass dial until the red magnetic needle overlays the orienting arrow. Observe the reading at the index line. If local magnetic declination is positive, then add the necessary amount. If the local declination is negative, then subtract the necessary amount. The number at the index line after adding or subtracting is the true direction you are facing. If using a compass with adjustable declination, make sure the direction of travel arrow is pointing directly away from you. If you haven t done so already, adjust the declination so the orienting arrow deviates from the compass ring s north indicator by the amount & direction of local magnetic declination. Turn the compass dial until the red magnetic needle overlays the orienting arrow. The number at the index line is the true direction you are facing.

6 HOW DO I POINT MYSELF TOWARD A SPECIFIC DIRECTION? In this case, you know the number of degrees, but don t know what aspect to face (what direction should I face?). Let s assume you wish to face true north (zero degrees). Compass without adjustable declination: turn the compass dial until zero is at the index line. If local declination is positive, subtract this amount from zero on the compass ring (turn the dial clockwise). If local declination is negative, add this amount to zero on the compass ring (turn the dial counter-clockwise). With the direction of travel arrow pointing directly away from you, rotate your body and compass together in one motion until the red magnetic needle overlays the orienting arrow. You are now facing true north. Compass with adjustable declination: turn the compass dial to zero. If you haven t done so already, adjust the declination so the orienting arrow deviates from the compass ring s north indicator by the amount & direction of local magnetic declination With the direction of travel arrow point directly away from you, rotate your body and compass together in one motion until the red magnetic needle overlays the orienting arrow. You are now facing true north. WHY AM I ADDING FOR ONE SITUATION, BUT SUBTRACTING FOR ANOTHER? Let s assume the local magnetic declination is 10 degrees east of true north (a positive declination). Therefore, the needle always points to 10 degrees. Let s also assume that although you don t know what direction you re facing, it coincidentally happens to be true north. If you rotate the dial until the red magnetic needle overlays the orienting arrow (implying, What direction am I facing? ), the reading at the index line will be 350 degrees; you must add 10 degrees to the 350 to determine the true direction you are facing. Conversely, if you dial zero on the compass (implying What direction should I face to be pointed toward true north? ), then rotate your body and compass together in one motion until the red magnetic needle overlays the orienting arrow, the direction you ll face is 10 degrees, even though zero is dialed at the index line. You must subtract 10 degrees by turning the dial clock-wise to a reading of 350 degrees, and then rotate your body & compass again until the red magnetic needle overlays the orienting arrow. If you re in an area where magnetic declination is positive, here s an easy way to remember: WHAT DIRECTION AM I FACING? WHAT DIRECTION SHOULD I FACE? A = ADD S = SUBTRACT