Which factor can affect the accuracy of a magnetic compass?

A magnetic compass's accuracy can be significantly affected by nearby electrical equipment. This is because such equipment can generate electromagnetic fields that may distort the magnetic field around the compass. Consequently, when the compass is influenced by these electromagnetic fields, it can result in erroneous readings, leading to navigational errors.

For instance, instruments like radios, generators, and other electronic devices can create interference that alters the compass's indication. It's important for navigators to be aware of these influences and to take appropriate measures, such as compensating for the magnetic deviation caused by nearby equipment.

While vessel speed, weather conditions, and altitude may have some impact on certain navigational aids, they do not directly create significant distortion in the magnetic field as electromagnetic interference does. Thus, the presence of electrical equipment is a critical factor in determining compass accuracy.