Not a lot in Annex 14, viz
2.2 Aerodrome Reference Point
2.2.1 An aerodrome reference point shall be established for an aerodrome.
2.2.2 The aerodrome reference point shall be located near the initial or planned geometric centre of an aerodrome and shall normally remain where first established.
2.2.3 The position of the aerodrome reference point shall be measured and be reported to the aeronautical information services authority in degrees, minutes and seconds.
CAP168 defines the ARP as the geographical centre of the aerodrome and the centre of its traffic zone where an ATZ is established. This is normally taken to be the centre of the longest instrument runway.
VA