Wilderness Navigation #5 - Measuring a Bearing from a Map - croc.org

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• published: 09 Feb 2016
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One of the main times when map and compass are used together is when you want to measure a bearing between two points on a map. It;s easy to do, but there are also two common mistakes you want to be sure and avoid. Here's an example of when you might need to do this. Let's say you’re camped at this trail junction. The one problem with the camp is that there's no water source nearby. You can see from your map that there's a small lake about a half mile away. Get out your map, measure the bearing from your camp to the lake, and then walk there to get water. Two important things to note when we do this. Anytime you're using your map and compass together, in this case measuring a bearing, or as we seen video number six plotting a bearing, We’re completely ignoring the magnetic needle. And, we’re completely ignoring the orienteering arrow. We're essentially using a compass as a protractor, and so we’re only interested in the numbers around the dial. First, I'm going to show you the correct way to do it, and then I'm going to demonstrate two commonly made errors. Get out your map. Locate your current position, and where you want to go. In this case, we know our current position is the trail junction here, and objective is the lake, here. Get out your compass, and put one side edge on a line between your current location and your objective. It doesn’t matter what edge you use. Be sure the direction of travel arrow is facing toward your objective. With one hand, firmly hold the baseplate of the compass down onto your map so then compass doesn't move. With the other hand, rotate the compass dial until “N" or north, is pointed to the north end of the map. Many compasses have parallel lines in the center of the dial, known as meridian lines. You can use these meridian lines to fine-tune the adjustment of your dial, by aligning them to any printed north-south grid lines that might be on your map. Lift up your compass, and look at the bearing. On this compass, the Sunnto M3, this mark is indicated with the small green triangle, right here. In this case, we can see our bearing is about 120°. That's it. Now you can walk on your this bearing until you get to the lake. To do this, hold the compass flat in front of you, and rotate your body and the compass together until red is in the shed. Look up from your compass, pick out a distinct landscape feature that’s on your bearing, and walk to it. Repeat until you get to your destination We cover that skill in Lesson 4. =========================== Now, I'll demonstrate TWO COMMON MISTAKES that will result in a bearing that's 180° wrong. Mistake number one is pointing the direction of travel arrow toward yourself, rather than your objective. If we try measuring the bearing from camp to the lake with the compass held backward, here's what happens. When I do this, you can see that it gives us a wrong bearing of 300°, and not the correct one of 120 degrees. My favorite compass, the Suunto M3, has a distinct curved end. This is the end that goes in your hand or on your current position on the map and is a big help in avoiding this common mistake. Mistake number two is rotating the compass dial so South is pointing at the top of the map, rather than north. Let's measure the bearing for camp to the lake, doing this. If I do this, again, I have a wrong bearing of 300°, and not the correct one of 120 degrees. ============================= I find a great way to avoid this mistake is before you ever get your compass out make a rough estimate of what the bearing should be. Just by looking at the map, we can tell the general direction we need to go to get to the lake is Southeast. draw NSEW cross line on map, If we remember that east is 90°, and south is 180°, we know that the bearing we're measuring has to be between 90 and 180. If you want to get fancy, you could add 45° to 90 and get a bearing at 135 which should be pretty darn close. If we use this quick estimation method before we ever get out a compass, it can be a good way to check your work for any errors. Thanks for watching. Please visit our website at www.croc.org for more videos on wilderness navigation and orienteering.