The aim of all subsequent navigation is to follow the chosen track as accurately as possible. Occasionally, the pilot may elect on one leg to follow a clearly visible feature on the ground such as a railway track, river, highway, or coast. When an aircraft is in flight, it is moving relative to the body of air through which it is flying; therefore maintaining an accurate ground track is not as easy as it might appear, unless there is no wind at all—a very rare occurrence. The pilot must adjust heading to compensate for the wind, in order to follow the ground track. Initially the pilot will calculate headings to fly for each leg of the trip prior to departure, using the forecast wind directions and speeds supplied by the meteorological authorities for the purpose. These figures are generally accurate and updated several times per day, but the unpredictable nature of the weather means that the pilot must be prepared to make further adjustments in flight. A general aviation (GA) pilot will often make use of either a flight computer – a type of slide rule – or a purpose-designed electronic navigational computer to calculate initial headings. The primary instrument of navigation is the magnetic compass. The needle or card aligns itself to magnetic north, which does not coincide with true north, so the pilot must also allow for this, called the magnetic variation (or declination). The variation that applies locally is also shown on the flight map. Once the pilot has calculated the actual headings required, the next step is to calculate the flight times for each leg. This is necessary to perform accurate dead reckoning. The pilot also needs to take into account the slower initial airspeed during climb to calculate the time to top of climb. It is also helpful to calculate the top of descent, or the point at which the pilot would plan to commence the descent for landing. The flight time will depend on both the desired cruising speed of the aircraft, and the wind – a tailwind will shorten flight times, a headwind will increase them. The flight computer has scales to help pilots compute these easily. The point of no return, sometimes referred to as the PNR, is the point on a flight at which a plane has just enough fuel, plus any mandatory reserve, to return to the airfield from which