GPS & compass question...

[Steve Pickford]

First of all, I admit to knowing very little about compasses and not much more about GPS apart from the basics.

Question: Am I correct in believing that a compass calibrated for the northern hemisphere will be inaccurate in the southern hemisphere?

If so, does the same apply to the electronic compasses found on many GPS systems?

What's the most versatile modern GPS unit currently available and what South American maps are available, specifically Argentina & Chile? I'm thinking of somethin like the Garmin Oregon 450t.

[T.REX63]

Originally Posted by Steve Pickford

...
Question: Am I correct in believing that a compass calibrated for the northern hemisphere will be inaccurate in the southern hemisphere?

During my flight training days, I never heard that it would make a difference concerning accuracy if you're in the northern or southern hemisphere, or the need to "re-calibrate" a compass.

Originally Posted by Steve Pickford

If so, does the same apply to the electronic compasses found on many GPS systems?

An "electronic" compass displayed on any GPS, it is strictly satellite driven, i.e. it should be accurate.

Cheers,

[Selous]

After 22 years in Army, relying on maps compass & bearings, (only from 2000 for gps)

Re Compass/ Maps
The only thing I would say is you have to remember there is True North & Magnetic north all maps should tell you the off set from North to Magnetic it is a few degrees /mills per year left or right to true North.
but you would not be far out really.
never heard rubbish like southern / Northern Hypersphere compasses North is North South is South!

I still use a compass as a aid to map reading & I would suggest a Silva compass The Army teach on these the basics before we go on to prismatic also professional Orienteering use Silva compasses.

May be I am un-trusting but I would always have a map & compass when as a in case so i can plot where I am (when off the bike).

would like to point out Pilots Marine & Air are taught to trust there compasses / instruments.

hope this helps

btw if you go to the OS web site they show you how to map read & use a compass

[beddhist]

If you look at nautical charts and some topo maps you will see that deviations from true North are indicated. This varies from place to place. I don't know whether being in either hemisphere has anything to do with it. You can't calibrate your compass for that, you have to be aware of the deviation in any particular location and adjust your readings for it.

Originally Posted by T.REX63

An "electronic" compass displayed on any GPS, it is strictly satellite driven, i.e. it should be accurate.

My Garmin 60CSx manual doesn't quite agree with that:

The compass ring is an electronic compass that functions like a magnetic compass when you are stationary. When you are moving and reach a pre-set speed, it uses data from the GPS receiver to maintain your heading. When you stop ... it again operates like a magnetic compass.

When you are moving the accuracy probably doesn't matter that much, because you can't level it to read the bearing anyway (unless on a boat).

Free maps: there are lots out there, just use any search engine. Beware, there are quite a few dud ones out there, including some with malware. Then there is OSM.

Which GPS? wars rage about that one, akin to the 'which bike?' wars. Try these links:

Motorcycle-Navigation.net - Motorcycle GPS, GPS Motorcycle Touring, Motorcycle Technology, Motorcycle Gadgets Powered By VBulletin
ANATOLY IVANOV / PROSE / GARMIN DAKOTA VS GARMIN OREGON GPS COMPARISON REVIEW
http://garmindakota.wikispaces.com/
http://garminoregon.wikispaces.com/Colorado+vs+Oregon
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Note that Anatoly says that screen readability on all 3 models he reviews is not as good in bright light as the old 60C series. I already find mine quite hard to read. Garmin seems to have noticed that, too and has reduced the resolution in the Dakota.

Many late Garmin models have problems with buggy firmware from what you will read on some reviews and forums.

[peekay]

Hi, unfortunately some of the previous responses here are not completely correct.

1. Compass accuracy in the northern and southern hemispheres

Steve is right. A typical compass optimized to the northern hemisphere will be less accurate in the southern hemisphere. I know this may seem surprising to some.

The reason is magnetic dip (inclination). Magnetic lines run nearly parallel to the earth's surface near the equator, but curve downwards near the poles. So, the north-end of a magnetic compass needle will want to point "downwards" in the northern hemisphere, but will want to point "upwards" in the southern hemisphere.

This dip angle may cause the compass needle to "drag" within the compass housing, compromising its accuracy. The compass needle must freely "float" within the compass housing for it to work well.

Hence precision compass needles are actually balanced to work in specific regions. In the northern hemisphere, compass needles are made slightly longer or heavier on their "south" end to counter this tendency for the "north" end to "dip down".

Some manufacturers produce up to five different versions of each compass optimized for various regions of the world.

Other designs allow the needle to freely float even at high dip angles, usually by having another mechanical axis to allow the needle to level itself within a deep housing. Often the needle is actually a free-floating "ball". Marine and aviation compasses tend work this way.

Suunto Global is an example of a field compass designed to work well anywhere. Compare that to the Suunto Clipper, which must be balanced for either the northern or southern hemisphere.

2. Electronic compass in GPS receivers

There are two different kinds of azimuth information to consider.

When you are moving, virtually any GPS receiver can calculate your heading based on your ground track in relation to the satellites. You must be moving and have adequate satellite lock for the heading to be accurate. As soon as you stop, the heading becomes unreliable.

Many (but not all) GPS receivers also have a built-in electronic compass. An electronic compass determines cardinal direction based on the local magnetic field, much like a physical compass. An electronic compass still work even when you're not moving, and does not depend on any satellites. Most electronic compass designs require the unit to be held at a specific angle, unless it has tilt-compensation built-in.

Neither type above are subject to "magnetic dip" because with an electronic compass there is no physical needle to "drag" on the compass housing. Therefore electronic compasses work equally well in any hemisphere, and even when located very close to the poles.

3. Magnetic Declination vs Deviation

Magnetic Declination (not Deviation, see below) is the difference between magnetic north and true north. Many charts show the amount of declination in different sections of the chart, which can be considerable. Aeronautical charts used by pilots, for example, have isogonic lines showing areas of equal magnetic declination (also called magnetic variation).

Magnetic Deviation is the error to the compass caused by nearby metals / magnetic fields. For example, a nearby electric motor can deviate a compass from its true reading. In fact, the compass housing / mount itself might deflect the compass needle. A compass correction card are often mandated for critical applications (such as in aircraft, etc.)

Probably more than you'd like to know about compasses.

[Steve Pickford]

Thanks for the info, I'd heard of declination but was unsure of what it was or if it was relevant to my query.

I'll pass the info re: South America mapping over to my partner on who's behalf I posted in the first place.

[Old Git Ray]

100% agree with PEEKAYs post above.

I taught navigation to my fellow soldiers prior to landing on the Falkland/Malinas Islands in 1982.
Our compasses had to be used at an angle (tilted) as the needle wanted to point to the magnetic north pole wich at the time was somewhere north of Iceland. (Actually Canada's Ellesmere Island)
If you think about it, this region is literally underneath you when you are stood in Southern South America.
Also in the UK the "error" is to the West and in Chile it will be virtually straight North. Both of these are currently heading Westward as the pole is currently moving at 60km/year towards Siberia !

And this also depended upon which grid system you were using - there were two on the maps we we were given !!

In short - the compasses will work but not as they do in the North.

Good luck.

[ozhanu]

for garmin gpsmap 60csx: two types of compass available for this reciever. one is digital compass. this should be calibrated for every 100 km, according to manual. other compass is only active when you are moving and doesn't need any calibration at all.

map and mechanic compass is what you need if you really want to travel out of civilisation.

[Threewheelbonnie]

All useful and correct stuff, but maybe a question, are we really worried about this sort of accuracy on the road? Military navigation is about being able to tell people with howitzers and bombs about places you'd like left in a less flatened condition while they go to town on some unfortunate blokes who picked the wrong side half a mile away. Aviation navigation is about finding a 3 km runway on a 10km island, 10000 km from your starting point in a typhoon. Even in a desert environment you'd be mad to base your journey on finding a petrol drum in the middle of the Sahara or waiting for the archeologists to find your body. You use a compass to identify features on a map and make sure you aren't wasteing fuel going miles and miles off course. The sort of errors we are talking here are going to be maybe a litre of petrol over the sort of distances a bike can do. If you are working to such a small margin of error, what are you going to do when something mechanical goes wrong?

Practically, I think you need to simply be aware that this exists and not bin the compass because it looks to be off.

Andy

[peekay]

If you stay on well-signed roads, and only use your compass to double-check your general direction, then maybe an inaccurate compass wont make a difference.

I grew up hiking and often did solo mountaineering. In the winter, if you make a navigational mistake, you could put yourself at great risk. Yet between declination, deviation, and needle-drag errors, a compass could be off by 45 degrees or more!

Look at it this way: one could buy a brand new, globally-balanced precision compass for around $40. Why take the risk?

Get a proper compass, or a GPS, or both.