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How-To

Exploring the Compass app in the iPhone 3GS

The new Compass app in the iPhone 3GS is very handy. But have you wondered why it's necessary, given that the iPhone already has a GPS system? And what about that setting that shows Magnetic North or True North? Here's a quick explanation.

Compass app

Compass App. Touch i to set True or Magnetic North 

I am going to simplify things here to get on with the Apple related discussion, so I'm going to leave out a lot of the scientific details. But I'll give some references for those who want to know more. On with it...

The Earth has a molten iron core that rotates along with the Earth and acts like a magnet. The magnetic field of the Earth has a magnetic North Pole and South Pole. For complex reasons, these are not at the true geographic North and South Poles. Also due to the magnetization of near surface rocks, the magnetic field lines, the direction a magnetized compass aligns with, varies with position on the globe. As a result, at any given point on the planet, there is a difference angle between True North and Magnetic North. This angle is called the Magnetic Declination. The magnetic North Pole is just north of Canada and moves slightly in time. (40 km/year.)

Magnetic field lines

Iso (constant) magnetic field lines for planet Earth

Given that we know our position, in latitude and longitude, it's possible to calculate this Magnetic Declination. It can vary from 0 to 30 degrees here in North America. So, knowing the direction that a magnetic compass points to, there is a calculation that tells you the offset, East or West, and the angle to correct, to obtain True Geographic North.

The National Geographic Data Center, a division of NOAA, the National Oceanic and Atmospheric Administration, has a Web page that allows you do that calculation.

For example, for ZIP code 80120 in Denver, here's where a magnetic compass would point.

Littleton, CO

Littleton Coiorado, about 9 degrees East offset from True North

That angle computed above is what the iPhone calculates to convert from Magnetic North to True North. Some traditional, physical magnetic compasses can also dial in that adjustment because, most of the time, what we really want is True North in order to navigate on maps.

Previous compass apps on the iPhone and iPhone 3G utilized the fact that the user is in motion. If you move far enough, fast enough, the GPS coordinates can create a vector, a direction in which you are moving. That can be displayed on a pseudo-compass app. The problem is, if you don't keep moving, or don't move fast enough, it's hard to calculate the vector, and so users have been generally dissatisfied with the previous compass apps that do it all in software and utilize the motion obtained from GPS coordinates.

And now you know all about Apple's Compass app in the iPhone 3GS, how it augments the GPS system, how the app works and why it's better than the previous generation of 3rd party compass apps on the iPhone.

10 comments from the community.

You can post your own below.

Dave Strickler said:

For those that really like using the GPS, the compass is a missing “coordinate”. For instance, if you’ve stopped on your bike path to check out the Google Map of the surrounding areas, it’s useful to have the map have a clue as to which direction you are pointing at that moment. GPS coordinates are a great start, but as pointed out above, only if you’re moving. The compass completes the puzzle when you’re not moving.

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Lee Dronick said:

As an old school (pre GPS) ship’s navigator I wonder about the wisdom of putting an advertisement for “Super Strong Rare Earth Magnets” close aboard to the graphic for the compass.

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Lee Dronick said:

For those that really like using the GPS, the compass is a missing “coordinate”. For instance, if you’ve stopped on your bike path to check out the Google Map of the surrounding areas, it’s useful to have the map have a clue as to which direction you are pointing at that moment. GPS coordinates are a great start, but as pointed out above, only if you’re moving. The compass completes the puzzle when you’re not moving.

I can determine north using the sun, time of day, and such, but I still have this app on my iPhone Compass Free A few years ago I went into the ER and spent a week in the hospital. I got lost with the gurney getting spun around and all, I lost north. I was in a lather and kept asking which way was north. No one knew and didn’t care until an older doctor pointed the direction and I settled down. After a few days when I was well enough to walk a bit and got to look out a window I found out the doctor was just practicing medicine because he was no navigator. It was drummed into me to always no where you are in the world and which way is north.

Don’t discount the importance of magnetic variation. If you don’t take that into account you can get into trouble.

Gotta run and do some physical exercise, I think that this morning I will box my compass.

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roger said:

I’m a wildland firefighter.  Having both a GPS and true-north corrected compass in a single package like the iPhone is awesome.  You need to know where you are, and where you’re supposed to go, and the iPhone with a topographic map will help you get there.  My only concern would be battery life.  You wouldn’t want your compass to crap out in the middle of a wildfire…..

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John Martellaro said:

Roger: Check out the Mophie Juice Pack series.
http://www.mophie.com/SearchResults.asp?Cat=21
-JM

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roger said:

Roger: Check out the Mophie Juice Pack series.
http://www.mophie.com/SearchResults.asp?Cat=21
-JM

John, thanks!  This solves my problem.

Roger

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Michael A. said:

I want to know why the compass has to be calibrated by moving the phone in a figure 8. A friend told me his car navigation system’s compass had to be calibrated by driving in a figure 8. And when I do calibrate it does it matter if the phone is pointing in the same direction the whole time I’m moving it or do I have to twist and turn the phone so it’s always facing the direction in which it’s moving?

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Jeff Gamet said:

Roger: Check out the Mophie Juice Pack series.
-JM
John, thanks!  This solves my problem.
Roger

Hey Roger -

I’ll throw in my endorsement for the Mophie Juice Pack as well. I relied on mine every day during Macworld Expo this year, and I couldn’t have done my live on-location Twitter commentary for the iPhone 3GS launch without one.

Jeff

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John Martellaro said:

Michael A:  It may be because car compasses are embedded in a lot of metal, and the figure 8 calibrates out any bias.  Just a guess.

-JM

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Lee Dronick said:

ichael A:  It may be because car compasses are embedded in a lot of metal, and the figure 8 calibrates out any bias.  Just a guess.

When adjusting the ship’s magnetic compass for “deviation,” the errors caused by the metal hull and/or cargo, you put the ship on a series of headings/directions. Adjustments are made by inserting magnets into trays or tubes as appropriate to that particular binnacle and moving the quadrantal spheres, the metal balls you see in this photo of a binnacle.

It can get rather complicated depending on cargo, movement of big gun turrets, degaussing on or off and a number of other factors.

I have the manual for magnetic adjustment somewhere in my box of stuff out in the garage.

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