Check where on the airfield the elevation is specified. A 50 foot variation between one end of an 800m runway and the other is equates to a gradient of less than 1:50 - difficult to see by eye. If you are checking the altimeter "on chocks", and there is such a slope, it could account for the error.
In the UK the published
aerodrome elevation is the highest point on the manoeuvring area. As I said in my previous post above the altimeter check should be done on QNH (not QFE) with a knowledge of the exact elevation amsl of the ground on which the a/c is situated.
(Note - in some states, Greece for example I think, the aerodrome elevation is the height amsl of the aerodrome reference point - typically the centre of the main runway.