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Map & Compass: Adjust for declination & orient the map

Backcountry navigation is an art. The basics can be learned quickly, like dead reckoning, reading a map, and using an altimeter watch. But extensive practice is necessary to seamlessly and flawlessly apply these skills in the field, especially when under duress or in challenging situations, like off-trail in a heavily forested area with rolling hills.

In the first two instructional videos, I focused on a single aspect of navigation: how to use a compass.

Magnetic declination

Watch the video, starting at 0:16

Before you start spinning the bezel and baseplate, you must understand magnetic declination. Or, really, you just need to know its effect: that the magnetic needle on your compass does not point at True North, i.e., the geographic North Pole, or where Santa Claus lives. Instead, it aligns with the Earth’s magnetic field, in a direction known as Magnetic North.

A topographic map will specify the angular difference of True North and Magnetic North. In the US, the difference is essentially zero when near the Mississippi River — Minneapolis, St. Louis, New Orleans, etc. In this zone, the magnetic needle still aligns with the Earth’s magnetic field, but simultaneously it points at True North, too. When east of the Mississippi (e.g., Boston), the needle pulls west to the river, and True North is to the east. When west of the Mississippi (e.g., Denver), the needle pulls east towards the river, and True North is to the west.

Don’t get this wrong, or your bearings will be incorrect by twice the declination. If you live in Seattle, where declination is 15 degrees East, that would be a 30-degree error!

Adjust for declination

Watch the video, starting at 2:50

An inexpensive baseplate compass like the Silva Start 1-2-3 will be non-adjustable. When finding or transferring bearings, you must manually account for the declination.

For regular compass use, I recommend an adjustable compass; the Suunto M-3 D Leader is a good option, but I like the Suunto M-3 Global because of its more forgiving needle. With an adjustable compass, the orienting arrow (“the shed”) can be rotated relative to the bezel by twisting a small screw on the back of the compass.

Observe the difference below. Both compasses are pointing north. I live in Boulder, Colorado, where the declination is 8 degrees East.

A non-adjustable (left) and adjustable compass (right), both pointing towards True North. With the non-adjustable compass, the needle is positioned to the right/east of the orienting arrow ("the shed"), whereas with the adjustable compass the needle is positioned directly over it.

A non-adjustable (left) and adjustable compass (right), both pointing towards True North. With the non-adjustable compass, the needle is positioned to the right/east of the orienting arrow (“the shed”), whereas with the adjustable compass the needle is positioned directly over it.

Orient the map

Watch the video, starting at 3:40

By orienting the map, you align the spatial features on the map with those in the field. In general, it makes much clearer where you have come from and where you need to go. For example, suppose you reach a trail junction where the signage has fallen over. Assuming you know where you are on the map, you can orient the map and identify which trail is which.

To orient the map, first:

  • Line up the edge of the compass with the edge of the map, or
  • Line up the meridian lines (the parallel lines inside the bezel) with a north/south grid line on the map

This latter option is a cheat since grid lines have their own declination value. (They are oriented to Grid North, which is not necessarily the same as True North or Magnetic North). But the value so small as to be functionally irrelevant, at least for backpacking.

Your compass bezel need not be rotated to zero degrees (True North), but conceptually it may help.

Next, rotate your entire body until “red is in the shed.” (Or, if you are using a non-adjustable compass, until the needle is at the correct declination.)

An oriented map, done in a few different ways. Normally I rotate the bezel to TN/0 degrees, then line up the edge of my compass (or the meridian lines inside the bezel) with the edge of the map. As a cheat, you can use grid lines on the map, but this will be slightly less accurate.

An oriented map, done in a few different ways. Normally I rotate the bezel to TN/0 degrees, then line up the edge of my compass (or the meridian lines inside the bezel) with the edge of the map. As a cheat, you can use grid lines on the map, but this will be slightly less accurate.


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5 Responses to Map & Compass: Adjust for declination & orient the map

  1. scotty July 16, 2016 at 11:15 pm #

    Any chance that backpack in the intro is the one you’ve been working on? When might we get the details?

    • Andrew Skurka July 17, 2016 at 6:14 am #

      Yep, that’s it. The one in the videos is a prototype, and a few tweaks will be made before production. It’s a Spring 2017 product. Here’s the press release about it:

      The Sierra Designs Flex Capacitor 40-60 backpack features a unique gusset system that allows the circumference to quickly expand from 40 to 60 liters with the adjustment of a few straps, providing a more stable and comfortable load carry. This multi-day pack features a unique “Y-FLEX” suspension that’s designed to be as lightweight as possible, allowing redundant materials to be removed without sacrificing comfort or stability. The pre-bent vertical stay transfers the pack weight into the waist belt, while providing space from the back for maximum ventilation. Other features include EVA foam padding, stretch mesh side pockets and a removable hydration sleeve. The Flex Capacitor will be available in 2 torso sizes: small/medium (17”-19″) and medium/large (19”-21″). MSRP: $189.95

  2. Sheila July 24, 2016 at 3:38 pm #

    Nice timing, Andrew! I am just about to brush up on my navigation skills for a big walk I’m planning, so this is very timely and a nice addition to the reading up I’ll be doing. Your website is really excellent for all the information it offers and I will be using several of your recipes during the walk, although I’ll have to adapt some slightly for the ingredients I can find in Australia. Looking forward to the next videos.

  3. Dhaval August 7, 2016 at 2:14 pm #

    When is second part coming in this navigation skills series? eagerly waiting….

    • Andrew Skurka August 7, 2016 at 7:27 pm #

      Tomorrow, I believe. The SD team was at a big tradeshow last week and the office was pretty quiet.

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