Static Pressure
Static pressure is measured through a number of vents, situated at aerodynamically neutral points on the aircraft fuselage. Vents are sited on either side of the fuselage and feed into a common tube; this has the effect of cancelling out to some extent errors arising from the position of the vents.
A combination of careful vent siting and accurate calibration reduces errors to an acceptable degree.
Commercial aircraft have at least two completely independent static systems to provide redundancy in the case of system failure.
Static vents are often plugged when the aircraft is parked for more than a short period of time to reduce the chance of blockage or contamination. Vents may be electrically heated to prevent blockage by ice.