With the advent of GPS systems in cars and even in cell phones (and some cameras), you wouldn't think there would be much need for a compass when you're traveling. And I have to admit, if you already have an iPhone with the mapping & compass widget (lucky you), then you already have a good compass in your pocket. But for those of us who haven't advanced to that level yet, there are still many photographic uses for an inexpensive pocket compass--especially if you're traveling in an unfamiliar area.
The primary use of a compass, of course, is just to know which direction the rest of the world is relative to your direction of travel. And using a compass for this basic concept is extremely simple. Since the needle on a compass always points north, if you turn until the needle until it is overlaying the "N" (or turn the dial on an adjustable face compass) then north, south, east and west will all be very obvious to you.
This is pretty important information, for example, if you are leaving Paris and want to drive to the Loire Valley, which is (roughly) south of the city. It's even more important if you decide to get off the nicely marked French highways and take some scenic back roads. Unless your car does have GPS navigation, without a compass you'll be left to follow the sun or trust the directions of French signs--which, as you can see from the shot here, can be a bit intimidating even in a small town.
But an even more important reason for photographers to have a compass handy is to keep track of where the sun rises and sets. This may be pretty obvious at sunrise or sunset, but at midday, you won't have a clue. If you want to photograph a harbor at sunset, for example, it would be nice if you could scout locations during the day knowing approximately where the sun will set. You could ask a local, of course, but good luck finding one that actually knows where the sun sets or which direction is west! You, on the other hand, with your pocket compass, will know in an instant where the sun will hit the horizon; just locate north and then read west off of the compass face. Sunrise? Just find east.
There are lots of good websites and books on using a compass, just do a search on "orienteering" or "basic compass use" and you'll find one. This page has good basic instructions for using a compass. Remember, even if your car has GPS, unless it's portable and you can take it with you, a compass is still a great thing to have handy. And besides, there is no battery in a simple compass, so it will never go dead.
There are many more fun and practical uses for a compass and I think every photographer should know how to use one and always keep one in their bag. By the way, as a side note, I'm shocked by how many maps that are available online have no compass rose--the indicators of direction that should be on all maps. Be sure when you print out any map from an online source that you choose one that has compass directions included.
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