When mapping an archaeological site, a compass is used to determine the positions of the ruins or other features. In order to take accurate readings, it is essential to have a good quality compass. You can expect to pay upwards of $50 US dollars for a good compass. They can be purchased in stores that sell sporting goods or outdoor equipment. They can also be ordered on-line through the internet.
If at all possible, you might try to find a few friends who have compasses and experiment with them before making a decision as to what features you desire most in a compass and which ones you can live without. These things usually become apparent right away.
Some compasses are more sensitive than others. On the archaeological dig I participated on in Arizona, we had one compass that reacted whenever it was near any object containing metal... like a mechanical pencil. This made it nearly impossible to use for recording angles. It also reacted to the rocks on which the drawing board was placed. These may have had some metalurgical property to them. The needle of this particular compass would spin wildly in the presence of metallic objects. When you look for a compass, you might try placing several objects near the compass like a pen or a key. In particular, it might be a good idea to make certain it does not react to the mechanical pencils you'll be using to draw lines with.
Another nice "luxury item" on this compass is that it has clear rubberized "feet" on the underside of the straight edge. These feet help to stablize the compass and keep it from sliding around. Working in the field, I had a board with my drawing taped to it balanced on rocks. Keeping the compass aligned north was quite a job!
Look over your compass carefully. I found a one brand of compasses that had some, but not all, of their declination scales (used to read angled slopes) were installed upside down so that the numbers appeared backwards.
Turn the dial around. It should move smoothly, but not loosly. If it is "sticky" try turning it a few times to loosen it up. Try several dials and look one that moves smoothly when turned, but stays firmly in place when positioned.
Next look at the compass needle. Make sure you are standing clear of electronic equipment which might attract the needle. Set the compass on a flat, level surface like a table. Turn the compass slowly in a circle and watch the needle. The needle should be float freely in the compass and point consistantly towards north, despite the compass being turned by your hand.
You might also want to consider buying a pouch or case to protect your compass. After investing in your compass, you'll want to keep it protected from dirt and scratches!
Now that you've chosen your compass, move to my article on Taking Compass Readings
Images on this page are from the Silva Compass website.