Chart Datum is an arbitrary plane below which low water tide seldom falls. It is a datum to which sounding on a published hydrographic chart are reduced. All the bathymetric contours are referred to this datum. The tidal predictions and tidal levels given in the tide tables are also given with relation to the same datum. Chart datum in India had been fixed at Indian Spring Low Water Mark (ISLW) during nineteenth century. For this, sufficiently long tidal observations had been made at various locations. During the Spring tide, low water mark had been measured and fixed as chart datum at this location
All indian charts are based on Everest datum. Check MS notice 1/2004
gps positioning datum is wgs84 which cosiders d earth as a sphere and based on mean sea level.
but local surveys are based on some local reference datum which are different from wgs84. that shift can be calculated and applied aproximately..
chart datums are lowest astronomical tide and mean lower low water.
u wud b lucky if u face capt sareen for your 2 mate orals.........he want only and only ROR...........restricted visibility is his favourite.........