Datum reset (ECDIS)

A datum reset, or datum offset, in ECDIS corrects a position so that GNSS fixes referenced to WGS-84 align with chart data surveyed on an older local datum. Most ENCs are referenced to WGS-84, but where source survey data is on a different geodetic datum a horizontal shift remains between the satellite position and the charted feature. The officer applies the offset noted on the chart or measured against a known feature so own ship plots in the correct place. Misapplying or forgetting the offset puts the ship hundreds of meters from where the display shows. Modern ENCs reduce the need, but the function and the discipline of checking position against radar and visual marks remain.