Chartwork

COMPASS ERROR

Both deviation and variation must be taken into account when correcting compass error.

COMPASS VARIATION

Definition:

Variation is the difference between true north and magnetic north measured in degrees and minutes E or W. The chart shows the variation for a certain year, as variation changes over time.

To correct for variation:

You take the bearing and either add or subtract the variation to the bearing depending on which conversion you are doing.

MAGNETIC TO TRUE

If the variation is EAST you will need to ADD.

If the variation is WEST you will need to SUBTRACT.

TRUE TO MAGNETIC

If the variation is EAST you will need to SUBTRACT

If the variation is WEST you will need to ADD

To find change in variation since date of publication of the chart:

Variation is continually changing due to movement of the magnetic poles of the earth. This is noted inside the compass rose in brackets as a change in minutes east or west per year. You need to find the difference in years between the date shown on the chart and the current date. Multiply the change in variation by the difference in years. This is then added or subtracted from the original variation marked.

Change In Variation

Variation

Add/Subtract

West

Add

East

Add

West

East

Subtract

East

West

Subtract

Use this table to find whether the change in variation needs to be added or subtracted to find total current variation.

COMPASS DEVIATION

Definition

Deviation is a compass error due to immediate surroundings. Compasses are affected by objects made of iron, nickel and cobalt. These are all materials that change their surrounding magnetic field.

Correction for deviation

Correcting for deviation is done in the same way as variation except deviation does not change constantly like variation. Boats made of steel, with lots of electrical equipment or steel cargo have correction magnets either side of the compass to correct for deviation. Adjustment of these magnets is known as compass swinging.

Set and Drift Example

1. you have a start point (Cott Beach)
2. you have a mid point or fix point (1/2 way across)
2. you have an end point (1M off Rotto)
3.you have a course to steer and a speed to do to get there. (say 270 degrees and 6 knots) (to the mid point)

You take ONE fix at Cott beach and mark the chart. Identify your destination and mark the chart. (you find that Rotto is 270 degrees and 12 Miles away. You make a fix point 1/2 way. (6 miles off the beach).

After 1 hr steaming at 6kn heading 270 degrees you take a fix. From this fix point, draw a line to the mid point. (1/2 way mark). Measure this distance and angle. If for example the line between is 1 mile long and in the direction of 045 degrees (projects NE of the fix point), this means that you have a SET of 045 degrees and at 1 knot.

To arrive at Rotto on the end point you must compensate for this set and drift.

Transfer this set and drift line onto the opposite side of your destination point. Draw a line from your PRESENT POSITION to the end of the reverse set and drift line on final position.

Your new course would be 259 degrees and a distance to go of 7.55 miles. (I plotted this out) So you are steering South of West to compensate for the NE set.
So...if you wanted to end up right on this spot it would take 1 hr 15 min. Plot it out... should work!

(The strangest thing about set and drift is that sometimes you don't actually point the V/l bow at the destination. In shyte Wx, the bow can be up to 45 degrees away!!! Even 89 degrees is in theory possible, but in the words of Blake, at this stage "go home".)

YOU DO NOT ALTER YOUR COURSE OR SPEED TO GET TO THE 1/2 way FIX POINT. This chasing a waypoint has come about with the advent of GPS and constant position info. If you use set and drift method you must be aware of what is to the leeward of your intended track!!!! The further from your fix positoin the more possible error creeps in to your position and you could run up on a reef even though you had followed your course and speed exactly!