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One of the most common sources of confusion in marine navigation is a chart that shows your GPS position slightly offset from where you clearly are. The cause is almost always a datum mismatch.
What a datum is. A datum is a mathematical model of the Earth’s shape used as the basis for a chart. WGS84 is the datum used by GPS satellites and all modern electronic charts. Older paper charts and some older electronic charts use different datums — NAD27 in North America, OSGB36 in the UK, ED50 in Europe.
The practical effect. If your GPS uses WGS84 (all modern GPS devices do) and your chart uses NAD27, your position can appear offset by up to 100 metres. On a coastal chart at a 1:50,000 scale, 100 metres is a significant distance — enough to put you aground on a chart when you are in the middle of a channel.
How to check. Look for “Datum: WGS84” or “Datum: NAD27” in the chart notes, usually in the title block. On your chartplotter, confirm the GPS datum is set to WGS84. If the chart uses a different datum, the chartplotter has a setting to apply an offset correction — look for Datum or Position Format in the GPS settings.
d3kOS and datums. d3kOS receives GPS position from the NMEA 2000 bus in WGS84 coordinates, which is correct for all modern charts. If your position display in d3kOS appears offset from your actual position, the issue is in the GPS antenna or the NMEA 2000 GPS device, not in d3kOS.
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