GSGS series 4228. First Edition. Italian Istituto Geografico Militare map dated 1916 and photolithographed by the British Ordnance Survey in 1943. The meridians are based on the meridian of Rome, which is 12 degrees, 27 minutes and 7.1 seconds east of Greenwich. The grid on British map series of Yugoslavia, which adjoin or overlap this series, is computed on the geographicals to the GREEK datum. To convert GREEK geographicals to the ITALIAN datum, upon which the grid of this map is based, subtract 2.5 seconds from the latitudes, and add 10.5 seconds to the longitudes. Contours at 25 metres (V.I.). North Italy Grid.