Sunday, January 19, 2014

Features on a topo map - Declination

Introduction

Declination is the difference between the direction of the earths north-south pole axis and the magnetic reading of a compass. The reading of a compass will vary from location to location around the world because of differences in the magnetic field of the earth.

Since the magnetic field moves within the earth, the declination will also change over time for a given area. Any time you are in a new area, check your declination and adjust for it.

Note: Declination is the same concept as variation in marine navigation terminology.

Declination is important when you are navigating by map because most maps are drawn referencing physical north instead of magnetic north. If you try to navigate from a map using the physical north reference you will be off by the amount of deviation. A 10 or 20 degree error in navigation has huge consequences.

declination example

The example above shows the problem with declination. You are at the center of this compass and you are trying to walk to the edge of the lake (#1). The distance is approximately 1.5km. If you ignore the declination and travel the same distance you will miss the lake by approximately 1/2 km and you will never intersect with it.

Declination will normally be shown on the map with the following symbol, or something similar:


This symbol shows:

  • GN - Grid north - The orientation of the grid on the map
  • TN - True north - Physical north-south access
  • MN - Magnetic north - the direction the compass points to as north
MN usually gives the number of degrees difference. Sometimes another number is also given, which is the change in declination over time. The example above shows the declination as just over 18 degrees west from the grid. If no date or annual change is indicated you can assume that the declination shown is current. Sometimes the map will have a publish date and the amount of change per year. In the example above the change is 1.4' decrease per year. 

Compass declination settingHow do you correct for declination? 


Compass adjustment

Most good compasses will have an adjustment for setting declination. Usually this setting will be a scale on either side of the North heading with a number of degrees change. It can usually be adjusted either by sliding the base plate of the compass rose in the direction required or, in better compasses, with a small screw in the back of the compass rose that you can turn to move the setting.

The easiest way to verify the setting is by aligning your compass with the declination diagram on the map to ensure that you have adjusted it in the right direction.


Manual

If your compass does not have a declination setting you can manually calculate your bearings, as long as you are careful and don't forget to do the math.

Declination can be calculated as follows:

  • If the declination is EAST, subtract that number from your bearing
  • If the declination is WEST, add that number to your bearing

The mnemonic you can use to remember is:
East is least (subtract), West is best (add)

Deviation

One important concept that isn't often discussed in orienteering as often as marine navigation is deviation. Deviation is the concept that magnetic materials will modify the direction that your compass points. In marine navigation this is a real problem because things like boats have a lot of metal that can cause deviation errors. 

In land navigation the important thing to remember is that it is not uncommon to have metal with you in the woods. Good examples are knives and watches. Either of these can significantly impact the accuracy of your compass. Rather than try to adjust for the deviation, remove all sources of magnetism from yourself before using your compass. 

NOTE: Many new backpacks have neat magnetic water tube holders. These are a huge source of deviation and should not be used if you are navigating by compass.