1. n. [Drilling]

Toolface angle used for near-vertical wells. Magnetic toolface is the angle, or azimuth, of the borehole survey instrument within the wellbore measured clockwise relative to magnetic north and in the plane perpendicular to the wellbore axis; the north, east, south and west directions have magnetic toolface angles of 0°, 90°, 180° and 270°, respectively. Magnetic toolface may be corrected to reference either grid north or true north.

See: gravity toolface,  toolface