Declination Deviation and Variation - what is the difference, by The Map Reading Company on YouTube. 18 min 17 s.
- Video length
- 18 min 17 s
- Time to do it
- 20 minutes
- Difficulty
- Easy
- Steps
- 9
What you need
Steps
Start by naming the three norths on your map: True North at the top of the map, Grid North along the vertical grid lines, and Magnetic North where your needle points.
Check the map margin for the declination diagram. It shows the angle between Grid North and Magnetic North for that sheet, plus the year it was calculated.
Note that Magnetic North drifts slowly each year. If the map is old, add or subtract the annual change printed in the margin to get the current angle.
Convert a grid bearing to a magnetic bearing by adding or subtracting the declination as the diagram tells you. Do the reverse when going from needle to map.
Treat the word variation as the same angle. Pilots and sailors use variation where land navigators say declination, so do not apply both.
Keep metal away from the compass. Watches, phones, belt buckles, car bodies and even some rocks can pull the needle off by several degrees.
Hold the compass the same way each time. Switching hands or letting the lanyard dangle can shift the reading slightly.
Test for deviation by taking the same bearing in two spots a few metres apart. If the needle swings, move away from the metal or object causing it.
Write your corrected bearing down before walking. A short note stops you mixing up the add and subtract step later.
Tips and common mistakes
Always read the declination diagram on the map you are using, since the angle changes with location and date.
Do not apply declination twice. Decide once whether you are converting to or from the needle.
Keep the compass flat and let the needle settle before reading the number.
If the needle behaves oddly, suspect nearby metal or a magnet in a phone case before blaming the compass.
Questions
Is declination the same as variation?
For most land navigation, yes. Both describe the angle between Magnetic North and True or Grid North. Pilots and sailors tend to say variation.
Why does my compass give a different bearing in each hand?
Your watch, ring or phone can pull the needle. That local error is deviation, and moving the metal away fixes it.
Do I add or subtract declination?
It depends on whether you go from map to field or field to map, and on whether the declination is east or west. Follow the diagram and write the rule down.
This guide was drafted from the video's details and general task knowledge. It has not been checked against the full video. Check the video and relevant product instructions before following a procedure. Report a correction.
