(If you want to learn the basics of using CalTopo, start with this tutorial video.)

Magnetic declination is the angular difference between true north (North Pole) , and magnetic north, which is where the red end of the compass needle points. This changes depending on where you are in the world, and it also changes over time. If you’re using an older map, the declination printed in the margin might well be incorrect.

(If you want the full scoop on declination, watch my video here.)

So, whenever you're using a compass, it's important to know the correct declination for your local area. One good way to do this is to check the website magnetic-declination.com, as we cover in this article.

Here's another way. A helpful little feature I just discovered in the best backcountry mapping software, CalTopo, is hiding in plain sight at the bottom of the CalTopo screen - the correct magnetic declination for (I think) the center of your screen.

(I've been a fanatic CalTopo user for many years and never noticed this before today. Who knows how long it's been there and I just never saw it? =^)

Here are a few examples. (Depending on your web browser, it might look slightly different than this; I’m using Chrome on an iMac.)

Declination for Seattle, about 15° east.

Boston? About 15° west declination. (Don't worry, that won't help you in Boston, you're still going to get lost driving there. =^)

How about New Zealand, a country with some pretty extreme compass declination because it's fairly close to the south magnetic pole? 24 degrees east.

Finally: If you print a map in CalTopo, the correct declination is shown in a diagram on the bottom right of the PDF file.