recently i am investigating case of grounding vessels. one of the finding is the paper chart used for passage planning using bessel 1841 spheroids, whilst the GPS system using WGS 1984 spheroids. The waypoints' coordinates planned in the paper chart then inserted into the GPS (exactly same GPS coordinate numbers). The navigating officer continuously maintain the ship track following waypoint setup in GPS not the paper chart. However the ship track went closer to a shallow reef and then grounded.

Why could two spheroid systems result in shifting of ship position?