Terrain mask is a term used in both aviation

Aviation

Aviation is the design, development, production, operation, and use of aircraft, especially heavier-than-air aircraft. Aviation is derived from avis, the Latin word for bird.-History:...

 (primarily military) and GPS equipment.

A terrain mask refers to the natural curvature of the earth. It is important as a means of avoiding active radar

Radar

Radar is an object-detection system which uses radio waves to determine the range, altitude, direction, or speed of objects. It can be used to detect aircraft, ships, spacecraft, guided missiles, motor vehicles, weather formations, and terrain. The radar dish or antenna transmits pulses of radio...

 by positioning the aircraft

Aircraft

An aircraft is a vehicle that is able to fly by gaining support from the air, or, in general, the atmosphere of a planet. An aircraft counters the force of gravity by using either static lift or by using the dynamic lift of an airfoil, or in a few cases the downward thrust from jet engines.Although...

 so there is natural earth hiding it from the radio waves sent from the radar system. For instance, suppose a radar transmitter is installed on a hill to monitor an area. If there was a sufficiently deep nearby valley, an aircraft flying near the bottom would not be detected because the radar would be absorbed by the top of the valley and not reach the aircraft. This concept works even on flat terrain, although only at longer ranges, due to the curvature of the earth.

When using GPS equipment, terrain masking relates to multipath effects of satellite

Satellite

In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....

 signals.

Terrain masking

Terrain masking is most commonly used in aerial combat, is the act of flying at extremely low elevations

Nap-of-the-earth

Nap-of-the-earth is a type of very low-altitude flight course used by military aircraft to avoid enemy detection and attack in a high-threat environment....

 upon normally hilly or mountainous terrain to minimize exposure time to anti-air

Anti-aircraft warfare

NATO defines air defence as "all measures designed to nullify or reduce the effectiveness of hostile air action." They include ground and air based weapon systems, associated sensor systems, command and control arrangements and passive measures. It may be to protect naval, ground and air forces...

weaponry.

Although this sounds counter-intuitive (an aircraft flying at lower altitudes will be easier to hear and see by those on the ground, not to mention the difficulty of navigating the aircraft through an otherwise complex course), if the terrain is hilly, the aircraft can simply maneuver around a hill and the anti-air weaponry will lose sight with the aircraft. If the aircraft was flying at a much higher altitude, the advantage of getting out of the weapon's line of sight could never be obtained.