There are two kinds of official electronic charts commonly available:
1. Vector or Electronic Navigational Charts (ENC)
2. Raster Navigational Charts (RNC)
Vector Charts - are compiled by attributing to each and every chart feature a set of values, and each chart feature is stored in a layered digital database. Storage in a database allows the chart data to be displayed as a seamless chart, while layering enables field of data that are not required at the time to be removed from display to reduce chart clutter.
Chart features can be interrogated to display additional information about charted objects.
The inherent "intelligence" of vectorised charts allows three dimensional "route safety zone monitoring". Chart depth contours and air draught clearances around the ship can be monitored automatically, both while the route is being planned and while the ship is on passage. Alarms will be triggered automatically if a safety zone around a ship s breached.
An international standard for vector charts has been finalised by IHO (S-57 Edition 3) and vector charts complying with this standard produced by or on the authority of a national hydrographic office are known as electronic navigational charts (ENC).
Raster Charts - are exact copies of paper charts and are produced by digital scanning techniques. Information on raster charts cannot be layered and the move from one chart to another will not be seamless. Raster charts have to be individually selected and displayed.
Raster charts have no inherent "intelligence". The chart data itself cannot trigger automatic alarms wthout the addition of user-inserted information that has been entered manually during route planning.
Without selecting different scale charts, the look ahead capability using raster charts may be limited, causing some inconvenience when determining the identity of distant objects. Datums and projections may differ between raster charts and care must be taken to take account of such differences.
A facsimile of a paper chart originated by or distributed on the authority of a national hydrographic office is known as a raster navigational charts (RNC).
Extracted from : Bridge Procedures Guide, 4th Ed.