It should be noted that the components of a seismometer may affect a compass, and therefore true north should be established with the seismometer away from the compass.
Once you have your declination, you need to check whether it is positive or negative. If it is negative, magnetic north will be west of true north, if it is positive; magnetic north will be east of true north.
Declination on a compass can usually be set it one of two ways – either by rotating the compass ring, or manually adjusting the compass screw as shown below.
Finding North - declination set by rotating the compass ring
Step 1
Ensure the compass is flat and orient it so that magnetic north (needle) overlays with the orienting north arrow.
Step 2
If our declination is negative, this means that magnetic north is west of true north, for example a declination of -20° may also be called 20° west. If our declination was -20°, turn the ring on the compass 20° clockwise (or so that the south needle is pointing to 20° west declination) for the orienting arrow to point towards true north.
Alternatively, you can just rotate yourself and the compass (rather than the compass ring) until the south needle is pointing to 20° west declination. The orienting arrow will then be pointing to true north.
If our declination is positive, this means that magnetic north is east of true north, for example a declination of 20° may also be called 20° east. If our declination was 20°, turn the ring on the compass 20° anti-clockwise (or so that the south needle is pointing to 20° east declination) for the orienting arrow to point towards true north.
Alternatively, you can just rotate yourself and the compass (rather than the compass ring) until the south needle is pointing to 20° east declination. The orienting arrow will then be pointing to true north.
Finding North - declination set by using the adjusting screw
Some compasses will allow for manual adjustment of the orienting arrow using an adjustment screw to compensate for declination and can be done instead of the above. The steps below show how to use the adjustment screw to set a negative declination (west) but the same principles apply for setting a positive (east) declination.
Step 1
Ensure the compass is flat and orient it so that magnetic north (needle) overlays with the orienting north arrow.
Step 2
Turn the screw so that the bottom part of the orienting arrow is pointing towards 20° west declination (for 20° west or -20° declination).
Step 3
When you then hold your compass out, move around until the needle and the orienting arrow line up and then the directions on the compass ring will be the direction of travel. So true north is N on the compass ring.
