Position error
Most aircraft have an inherent difference between (theoretical) calibrated airspeed (CAS) and the airspeed actually shown on the instrument (indicated airspeed, or IAS). This position error is mainly due to errors in sensing static pressure. It is usually not possible to find locations (positions) for the static ports on the aircraft which accurately sense static pressure at all speeds and angles of attack.
Because Bernoulli's principle states that total, or stagnation pressure is constant along a streamline, a pitot tube that is away from the boundary layer and approximately aligned with the local airflow should not cause much position error.
Position error is typically small (a few percent), and, for small planes, the IAS will be lower than CAS at slow speeds and higher than CAS at high speeds. A calibration chart is usually provided but rarely used.