Compasses don’t point towards true north, the geographic north pole. Instead, they roughly point towards the magnetic north pole, which is well south of the geographic north pole in Canada. But since the lines of magnetic force aren’t uniform, the actual direction a magnetic compass points (“magnetic north”) can be slightly different than the true bearing towards the magnetic north pole. The degree difference between true north and magnetic north is the “magnetic declination”, and you need that number to correct the reading from a magnetic compass to the true direction. Here’s a NOAA map showing the lines of constant magnetic declination (Figure 1):
Magnetic declination can be positive (red) or negative (blue); you add the declination to your compass bearing to get your true bearing. Some more advanced compasses come with an adjustable dial that lets you set this magnetic declination correction, so that you can get your true bearing directly from the compass.
Here are two apps whose main function to give you that magnetic declination value. One provides it only for your current location as determined by GPS, while the other lets you select any location on the earth, and gives you the magnetic declination (plus other info) for that spot.
Application Name: Earth Geomagnetic Field
Description: Calculates magnetic declination/parameters for your current location; lists GPS position info.
Publisher’s website: Loopbot
Cost: Free; ad-supported
Version/date reviewed: v.1.0 / 9-6-10
Phone/OS: Droid X / Android 2.1
Android market link
Android Market (browser)
Figure 2: Earth Geomagnetic Field only gives you the magnetic declination for your current location, and needs the GPS to be enabled in order to get that.
Figure 3: Once you have the position, and press a “calculate” button, you’ll get a simple text listing of the magnetic declination at top, along with the inclination of the magnetic field (how far it tips vertically), and various components of its strength.
Other Issues:
– Would be useful to have the ability to select a different location.
Final thoughts:
If you need the magnetic declination value for your current location to several decimal places in the simplest way possible, this could be a useful app. If you only need it to the nearest degree, some compass apps (like Snaptic’s excellent compass app) also calculate and display the declination value, albeit with lower precision, and offer additional functionality.
Application Name: Magnetic Declination
Description: Calculates magnetic declination/parameters for any location on Earth; looks up GTOPO30 altitude as well.
Publisher’s website: Green Leaf
Cost: Free; ad-supported
Version/date reviewed: v.1.2 / 9-6-10
Phone/OS: Droid X / Android 2.1
Android market link (mobile app only)
Android Market (browser)
Magnetic Declination uses an alternate approach: select a location in a Google Maps interface, and get the magnetic field parameters for that spot.
Figure 4: GPS will fire up, and you’ll get a red “X” showing your current location. There doesn’t seem to be an easy way to pull up the magnetic parameters for your GPS position right away. The idea here is that you tap on the screen, and get the magnetic parameters for that location. But at this zoom level, it’s impossible to tap accurately, and there are no zoom buttons visible. Zoom buttons will appear if you tap and drag on the screen. Don’t just tap, though, because …
Figure 5: … you’ll immediately get the info screen popping up for that location, with magnetic parameters for the spot you tapped on(e.g. magnetic declination is –7.008023 degrees here), regardless of whether you wanted it for this position. And the zoom buttons are inoperative with this window open , and disappear quite quickly. Takes a while to figure out that you shouldn’t tap once until you’re fully zoomed in on the spot of interest
Figure 6: Once you’ve zoomed in closer to your desired location, and tap on the spot, you’ll get full magnetic parameters for it …
Figure 7: … and closing that info window will show the spot you tapped on with the red “X”. The altitude value comes from the GTOPO30 dataset, which has a 1-km spatial resolution; take it with a grain of salt. There are applications I’ll cover soon with better altitude resolution.
The buttons at the bottom take you to websites with info about the Android program functions used, the GTOPO30 dataset, and magnetic field info from NOAA.
Final thoughts:
I wish the interface wasn’t so clunky, but the Magnetic Declination app does the job it’s supposed to. I guess my ultimate judgment of utility for these two apps is that I have the Magnetic Declinationapp on my phone permanently, but not the Earth Geomagnetic Field app.