A compass is a navigational instrument that shows directions in a frame of reference that is stationary relative to the surface of the Earth.
A current meter measures the speed and (in some cases) direction of fluid current, usually water current
A geologist's hammer, rock hammer, rock pick or geological pick is a hammer used for splitting and breaking rocks. In field geology, they are used to obtain a fresh surface of a rock in order to determine its composition, nature, mineralogy, history and field estimate of rock strength. In fossil collecting and mineral collecting, they are employed to break rocks with the aim of revealing fossils inside. Geologist's hammers are also sometimes used for scale in a photograph.
A quadcopter, also called a quadrotor helicopter, quadrotor, is a multirotor helicopter that is lifted and propelled by four rotors. Quadcopters are classified as rotorcraft, as opposed to fixed-wing aircraft, because their lift is generated by a set of rotors (vertically oriented propellers).
Unlike most helicopters, quadcopters use two sets of identical fixed pitched propellers; two clockwise (CW) and two counter-clockwise (CCW). These use variation of RPM to control lift and torque. Control of vehicle motion is achieved by altering the rotation rate of one or more rotor discs, thereby changing its torque load and thrust/lift characteristics.