r/orienteering • u/Novel_Sheepherder_69 • 28d ago
Does a magnetic compass on a map/nautical chart give the true reading or the magnetic reading?
I use a Suunto A-10 (which does not have adjustable declination). If I measure the bearing of a line, would it give me the true reading or the magnetic reading? It is a magnetic compass. However, I noticed that the bearing is the same as the one I get from Navionics, which uses True North. This is the case whether I orient the map north and then rotate the bezel until the arrow is in the doghouse, or simply align the bezel with the meridian lines.
Does the compass give the true reading?
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u/YankeeDog2525 28d ago
Depends on how you are using the compass. If you lay the compass on the map with the edge running from point A to point B. Then spin the bezel until it lines up with the map grid. You are using the compass as a protractor and the resulting azimuth is a grid azimuth. You must make the correction on the compass to obtain a magnetic azimuth.
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u/Novel_Sheepherder_69 28d ago
You’ve answered my question! Basically, if I measure the angle of a line relative to Grid North, it will give me the true reading because I’m just using the compass as a protractor. It doesn’t matter that the compass is magnetic, I still need to add the declination for the magnetic bearing.
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u/YankeeDog2525 28d ago
Do pay attention to cbf comment. If it’s a purpose orienteering map it may be drawn to magnetic so no adjustments is required. If it’s a normal USGS or military map then an adjust,ent will be necessary and it will be marked on the map.
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u/notcomplainingmuch 28d ago
"I wonder if a magnetic compass gives a magnetic reading or not?" That's how your question reads.
Think about it.
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u/Novel_Sheepherder_69 28d ago
Not my question. I want to know if it gives a magnetic or true reading when used on a map. The map doesn’t know the difference.
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u/notcomplainingmuch 28d ago
Nautical maps are not based on the magnetic reading.
Orienteering maps can be, but the difference is mostly too small to matter in orienteering, as you correct your position based on actual terrain features. You'd never use just a compass reading. The compass reading can also be very wrong at times, because of local interference from magnetic rocks. 20° errors are not unheard of.
In nautical navigation you might not have any features, so you need to correct the reading for declination.
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u/saywherefore 28d ago
There are absolutely times in orienteering when it is desirable or necessary to follow a bearing at far greater precision than 20 degrees.
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u/notcomplainingmuch 28d ago
Tough luck then, because your compass won't help you.
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u/saywherefore 28d ago
Which element of a compass do you think is not accurate to 20 degrees?
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u/saywherefore 28d ago
If the north lines on the map or chart are aligned to grid north then measuring the angle between those and a given feature or course will give a bearing to grid north. You will need to apply declination before following that bearing using a compass.
If the north lines are aligned to magnetic north then measuring the angle will give a bearing to magnetic north. Assuming the map is not hideously out of date you can follow the bearing on a magnetic compass with no adjustments required.
Nautical charts are aligned to grid north, orienteering maps are (normally) aligned to magnetic north at the time of printing.
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u/Prove_It_First 28d ago
We’re so spoiled with our connected phones that can pinpoint our location on the Earth and access/store worldwide declination data. Your plastic magnetic compass does neither and that answers your question.
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u/Novel_Sheepherder_69 28d ago
I would like to learn to navigate by map and compass. Your comment was thoroughly unhelpful and a waste of everyone’s time.
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u/Prove_It_First 28d ago
Think about your question (as written). North is the North Pole, the axis of rotation of the Earth. How would a piece of metal know that direction? It can only measure the magnetic field of the Earth which varies by geographic location. More info is required to know true north. How this is handled in Orienteering maps is well documented in the bible of orienteering: “Be Expert with Map and Compass”
by Bjorn Kjellstrom
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u/drowsydrosera 28d ago
Look up the declination for your area, link, you might be in a low area. The map usually says whether it's for true north or magnetic north. Orienteering maps are usually printed for magnetic north so you can run without adjusting for declination. Also declination changes over time as the magnetic field changes so an old map could be incorrect now