Positions obtained from Global Navigation Satellite Systems, such as GPS, are normally referred to WGS84 Datum. In many parts of the world, however, charts were originally produced on a variety of local datums for which the shift to WGS84 may be unknown or unreliable. ENCs derived from such charts may not be accurately referred to WGS84 Datum. This can result in small but noticeable positional differences where adjoining cells have been shifted by slightly different amounts.
A number of Indonesia (ID) and UK (GB) ENCs are derived from charts that cannot be accurately referred to WGS84 Datum. The differences between satellite-derived positions and positions on these cells cannot be accurately determined; the estimated values of the differences for these cells are detailed in the Information attribute of Caution Areas thus: "Positions in this region lie within ± nn metres of WGS84 Datum". Mariners are warned that these differences may be significant to navigation and are therefore advised to confirm GPS positions shown in the chart display using alternative navigational techniques, particularly when closing the shore or navigating in the vicinity of dangers. Such cells contain a warning encoded as the name of a Sea Area feature covering the cell thus: "This chart cannot be accurately referenced to WGS84 Datum; see caution message". They are intended for use with this warning continuously displayed and should not be used otherwise.