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Tuesday, August 27, 2013

Determining your Position with a Pocket Compass with a Declination Ring

Finding your Position using a Pocket Compass with a Declination Ring | Compass Knowledge Base: "Determining your Position with a Pocket Compass with a Declination Ring"

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Determining your Position with a Pocket Compass with a Declination Ringby Eric Newman

If you could ask for a single upgrade to a conventional pocket compass with a compass needle, it would be a movable declination ring.  In the tutorial on using a standard pocket compass with a compass needle, we found that taking a bearing was relatively easy, but if our map was only referenced to true north, the correction for readings referenced to magnetic north was cumbersome.

In this tutorial we will use a pocket watch style pocket compass with a movable declination ring, and a magnetized needle suspended over a compass rose marked 0 - 360 degrees, as shown in Figure 1.
If you do not have a compass rose on your map, then north is up, east to the right is 90 degrees, south is 180, and west is 270 degrees.  The problem is that these are probably referenced to true north, and your pocket watch style compass reads magnetic north.  The difference between true north and magnetic north is the local magnetic declination, which depends on your general area.  The magnetic declination in Los Angeles is 13 degrees east.  New York City is 13 degrees west.  If you are lucky enough to be lost just west of the Mississippi River on the agonic line where the declination is zero, then magnetic north and true north are the same, so your compass reads true north.  If you are not so lucky, you need a method to take compass bearings referenced to true north, even though your compass senses magnetic north.  As you have guessed by now, this is where the magnetic declination ring comes in.

You need to determine the declination in your area.  If you intend to use the declination rose or lines on your map, check the map's date for the yearly correction as magnetic north wanders over time.

Your new compass should have the declination ring's north exactly aligned to the compass rose north, as shown in Figures 1 - 3.  If you are in New York where the declination is 13 degrees west, rotate the declination ring 13 degrees clockwise so that north on the compass rose aligns with -13 degrees (equals 347 degrees) on the declination ring.  In the Los Angeles area with 13 degrees east declination, rotate the declination ring 13 degrees counterclockwise.  If your map has two concentric compass roses, one true and one magnetic, you see that we are mimicking these two roses on your compass.

An extremely useful feature of good quality compasses with declination rings is to have luminescent dots on the north end of the compass needle, and two that move with the Declination Ring.  This is invaluable using your compass in the dark.  See Figure 4.

Now you are ready to take headings.  Locate two or three landmarks that you can also locate on your map.  A landmark can be a water tank, church spire, radio antenna, or a distant hilltop.  Ideally, you want two landmarks about 90 degrees apart, or three landmarks about 120 degrees apart.  You do not want any two landmarks to be in a nearly identical heading, or 180 degrees apart.  It is also critical that you can clearly see the landmarks and locate them on your map.

Now find the magnetic bearing for the first landmark.  Face the direction of the landmark and hold your pocket compass just below eye level.  Make sure you hold the compass steady and level.  Slowly rotate the entire compass until the north end of the compass needle is exactly over the north marking on the compass rose.  The north end of the compass needle is usually marked red or has an arrow shape.  Align the compass needle to the compass rose, not the declination ring.  See Figure 2.

While you are holding the compass with the needle precisely aligned to the compass rose, sight the landmark across the center of the compass bearing and read the degree marking on the compass rose directly away from you.  Note this heading because you will need it to find your location on your map.  In Figure 3, we are sighting a bearing of 222 degrees, referenced to true north.
Figure 1 - Pocket Compass with Adjustable Declination Ring
Figure 1
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Figure 2 - Aligning the Compass with Adjustable Declination Ring to North
Figure 2
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Figure 3 - Taking a Sighting of 222 Degrees
Figure 3
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Figure 4 - Luminescent Dots on Compass Needle and Declination Ring
Figure 4
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Repeat this procedure with the next landmark and note this second angle.  For our example, the second landmark is in the direction of 186 degrees.  If you can identify a third landmark and determine its magnetic bearing, that is even better.

Now transfer your landmark headings directly onto your map.  Lay your map out flat and locate the compass rose or grid lines on the map.  Since you have already corrected your headings for true north, use the true north compass rose, or up on the map for true north.

You can either transfer a parallel line between your landmark and the map's compass rose, or you can place your pocket compass directly onto the map and transfer the landmark heading to the map's compass rose by moving the compass.  With the compass aligned to the map, you can use its compass rose to draw a line through the first landmark using the angle of the first landmark's bearing.  Repeat this with your other landmarks, drawing a line for each in the direction of its direction.
Refer to the topographic map in Figure 4 and note the 222 and 186 degree lines.  Where the two lines intersect is your location.  If you have three landmarks, consider your position to be in the triangle formed by the three lines.  The triangle should be fairly small.  If your triangle is large something is wrong.  You probably incorrectly identified a landmark, misread the heading, or did not draw the lines correctly.  This is a good reason to use three landmarks, as two lines from two landmarks will not show a problem.  The more landmarks you use, the more confidence you will have in your position.Figure 4 - Topo Map with sighting lines
Figure 4
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Now that you have your location, verify that it is reasonable.  Using the example in Figure 4, you should be near a reservoir and near the top of a ridge.  The ridge near you should be running roughly east-west and the closest shore of the reservoir should be southwest of you.

Copyright © 2010 Eric Newman * All rights reserved

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