Pressure Error Correction is quite straightforward.
The altimeter is "fed" with static pressure - it is, after all, an aneroid barometer.
Accurate pressure sensing depends on the position of the static source/port on the aeroplane. Various aerodynamic effects may cause the sensed pressure to be in error.
This error is termed position or pressure error.
By design, this error will be minimal when the aeroplane is in the clean configuration, but when dirty the error may be magnified.
This is the primary reason why on most jet transports the static ports and standby ports will be found on the front fuselage somewhere below the office windows.
Maybe this seemples explanation will help.
