When mapping an archaeological site, a compass is used to determine the positions of the ruins. 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.
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.
Images on this page are from the Silva Compass website.