Throughout the world, various datums are used to provide position information on maps and charts. The most common datum is WGS84 and used on most charts in US waters. However, it is not uncommon for other Hydrographic offices to produce their charts in a local datum. The result of this is that positions indicated on these charts may differ from your GPS - since most GPS units transmit their position in the WGS84 datum.
Because of these discrepancies, care should always be exercised when navigating on non-WGS84 charts.
Electronic chart plotting software products that fully support Maptech's BSB chart format (such as Maptech's Offshore Navigator) should automatically adjust for shifts in datum. Thus, the vessel's position will be shown in its correct position (although the latitude/longitude values may differ - since the values on the chart will be displayed in the local datum, while the GPS remains in WGS84.)
However, some software products, such as older editions of Nobeltec's Visual Navigation Suite, do not properly adjust for the local datum. Instead, these products incorrectly assume that all charts are produced in the WGS84 datum. Please use extreme caution when using these charts with these products - the vessel's on-screen position will not accurately reflect its true position in the water. In short, this situation makes any non-WGS84 chart unsuitable for navigational purposes in products that do not fully support the BSB chart format.
On occasion, Maptech will produce a BSB chart that is not in WGS84 datum, and can not be adjusted from the local datum. Most of these charts are found in the Mediterranean. Electronic chart plotting software that is fully BSB compatible should display a warning when these charts are shown.
Please check with the manufacturer of your electronic chart plotting software to ensure that they fully support the BSB format in regards to non-WGS84 charts. Some companies may offer updated software that is fully compliant. In the meanwhile, all of our chart products include Maptech's Offshore Navigator - which (when used properly in conjunction with the datum warnings and datum calculator) can be used to safely navigate across all BSB format charts.