The present invention comprises a device for determining the roll orientation of a body with respect to a local fixed coordinate system or a predetermined reference vector. The device uses a measurement of an external magnetic field, such as the Earth's magnetic field, to determine a roll orientation reference with respect to the field or an uncompensated roll orientation. The uncompensated roll orientation is then adjusted according to a bias angle, such as an angular difference between the external magnetic field and a local fixed coordinate system, to determine the roll orientation of the device with respect to the local fixed coordinate system or a compensated roll angle.