Crowd sourced magnetic compass app

I switched from using a Silva magnetic compass which can be set to use true bearings to a Santo which can not. Confusing at first to have to use magnetic. Found this NOAA app: CrowdMag

Gives the correct deviation (called variation at sea) for current time and location. It’s also helpful that the display shows true and magnetic directions as well as deviation. Makes it easier to visualize then trying to remember if the error is plus or minus.


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Compass display is hung on my cheap Samsung phone. Level sensor works but display stuck at 0 magnetic.

I had never carefully checked the accuracy of the compass in a phone app until just now. CrwodMag on my phone shows a 5 degree error compared to the $130 Suunto which as near as I can tell has no error. Or at least none that I can detect.


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I used to have a very nice English hand bearing compass with a prism, part of the handheld RDF in the clip below. Card was mounted four degrees off, constant error all around. Was great if you remembered that (I wrote it on the clear top). The receiver broke early. We just took the compass off and used that for decades, very handy indeed.

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I swear by my Suunto hand bearing compass for flagging land boundaries. I use the model graduated in quadrants so that the bearings read the same as the deeds and older surveys.

Suunto claims 1/2 degree of accuracy. I believe it.

I’d like to have an accurate pedometer app to replace my hip chain.

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I think that hand bearing compass was a sestrel compasses that was supplied with a wooden case that cold be mounted by the chart table.
Variation is the difference between magnetic north and true north and varies with the location. Deviation is that caused by the ship.
When swinging the compass the objective is to reduce the deviation to less than 3 degrees using magnets for permanent magnetism and soft iron ( Flinders bar and compass balls) for induced magnetism).

Sestrel sounds right. Sweet little compass. This one came supplied on the Seafix RDF, used a twist mount with two studs. Lovely metal bellows inside the bottom so it had no bubble. We never made a handle for it, just used it out of hand. Put a couple screws in the bulkhead to keep it on.

This is what I used it on (and for the dinghy as well, with a lanyard) between '70 and '05 roughly. They really did get the calibration wrong at the factory.

While I’m feeling sentimental, Electronic Laboratories of Poole, Dorset who made the Seafix also made the Seafarer MkII neon flasher echo sounder that I was absolutely in love with. Feet or fathoms just by changing motor speed, and virtually silent on the fathoms range. Was on its fourth neon lamp when I accidentally murdered it.

This is true of marine navigation but not necessary true in other contexts. For example the USGS (United States Geological Service), the agency that makes topo maps uses the term ‘declination’ for the error caused by the earth’s magnetic field.

This is from USGS website:

The deviation of the compass from true north is an angle called “declination” (or “magnetic declination”).”.

Are Topo maps terrestrial maps like those used by trampers? I have a compass mounted on a piece of Perspex that back in my hunting days you could rotate the ring on the compass to allow for the difference and align it with the bearing and fix your position on the map. These days I use it to align the satellite dish on my trailer.
Back in the day the biggest shipping company in New Zealand required their masters to swing their ships compasses once a year. A statute required a licensed compass adjuster to do it once every 3 years.
Entering a conspic object as a waypoint on a hand held GPS near the compass makes it a piece of cake these days.

All terrestrial maps use the terms the same as to the USGS topographical maps. Only aviation and maritime use ‘variation’ and ‘deviation’ the way they do. At least AFAIK.

Working on a property line project in the woods. GPS is sometimes useful but sometimes not accurate enough under the canopy.

The Silva compass I’ve been using did have a ring to rotate to give true bearings which makes things simple. For this project I switched to a Suunto hand-held compass which has better precision and accuracy then the Silva but only magnetic direction.

An issue arises however when using the GPS for a true bearing and only having the Suunto compass. I find myself stumbling through the woods, muttering under my breath about true virgins while wondering if I’m dumb enough to have to carry two compasses with me? At this point I think the answer is yes.


Try a Brunton Standard Transit, I have used them for decades in the field for geological and bio work, ajustable, tough and accurate The Suunto KB-14 hand compasses are good for some nav work and rough survey (they are very tough too), but don’t have the accuracy for accurate survey work that the Geo Brunton’s have.

The Brunton’s cost three times as much though
Francis-Barker M-73 military compasses are very good, tough, ajustable, heavy and very expensive, my two are over 40 years old now and apart from the tritium dimming, are still going fine.

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I thought about the Brunton, it’s $600 and the Suunto seems more than adequate for these short sections.

Here’s part of my map so far.

The property line in this case is not established (no surveyor needed if both owners agree) by using bearings and distances but instead by actual evidence on the ground like stonewalls or old sections of wire fence.

The road is an old dirt road. The grey vertical line is the line from the town’s map, the red line was an estimate from GPS, the waypoints were established using the compass. The old fence meanders a bit.

I can see WP 0 on google earth and it has a clear view of the sky. After that it’s mostly under heavy canopy. There are three more legs total of about 600 meters with one more clear view of the sky.

Yep, although the obviously-rotatable ring is so you can use the compass as a protractor, like a Breton plotter. The fancier models have a tiny screw you can adjust that offsets the orienting lines so that when you align them with the meridian lines on the chart/map they are offset by just the right amount to correct for declination/variation.

For land nav purposes, it’s simpler to ignore the true virgins and instead use the “LARS” rule. Left-Add Right-Subtract: look at the declination diagram, point to the tip of the (true/magnetic/grid/etc) arrow you have, and figure out which way you’d move your finger to get to the arrow you want.)


Grand-Ma Sucks, GMS, grid to magnetic subtract
My Grand-ma’s Arse, MGA, magnetic to grid add (Australian spelling, not considered particularly bad language here)

Australian Infantry, back in the day, bet it’s been cancelled and replaced nowday’s

LARS, never heard that, it’s a good one.

The rotatable ring is called a bezel (just looked it up). It also allows the compass to be set on the desired bearing. I realize now that’s what makes the Silva so easy to use compared to the Suunto. Once the bezel is set there’s no need to use numbers at all. Just put “red in the shed” and go.

The built-in adjuster for magnetic declination allows staying in true directions, no conversions. My Silva has one the Suunto does not.

The declination adjusting screw is on the bezel at 045 degrees, the adjusting tool is on the lanyard. Compass is set for -15 degrees (15 W)

Just looked and that is the model I have in the trailer where it is used solely to point the satellite dish at opus 2.
When I first got the trailer some years ago I overthought it and thought the chart that they gave you was the true azimuth and lined up the dish accordingly.

The Silva T-15 Ranger compass is very popular, it’s the one I use. There’s one on Amazon that looks similar called the Silva Ranger 2.0 for about $50.

As far as the true/magnetic, I think ‘LARS’ is the ticket. It works converting either way. Just have to visualize what the declination arrows look like.


Edit: According to this article the Silva compasses may not be of the same quality as before : Silva Ranger 15 CL Reviews - Trailspace