GPS vs the Nautical Chart: the Two Datums
The word 'datum' does two different jobs at sea. Horizontally, it names the reference frame a position sits in — your GPS speaks WGS84, while older charts and surveys used local datums that can shift the same point 100–400 metres. Vertically, it names where zero depth is — chart datum, set at Lowest Astronomical Tide (LAT) on most charts or Mean Lower Low Water (MLLW) on US ones — so charted depths are close to the least water you'll ever see, and the tide adds from there.
Datum #1 — horizontal: which grid your position sits in
A latitude and longitude is meaningless until you say which reference frame it's measured in, and that frame is the horizontal datum. Your GPS receiver computes positions in WGS84 (strictly, in frames aligned with it), and every modern official chart is compiled on WGS84 or a sibling. But the world's chart portfolio is old: plenty of charts in circulation — and nearly all positions in older pilots, wreck registers and dive guides — were surveyed on local datums built for one region: ED50 around Europe and the Mediterranean, NAD27 in the Americas, Tokyo Datum in East Asia, OSGB36 under Britain's land maps. The same physical rock can differ by 100–400 metres between frames — famously about 200 m between WGS84 and ED50 in the Mediterranean.
Charts confess this in the title block. Look for “Positions refer to WGS84” — or, on older sheets, a satellite-derived positions note giving the shift to apply (“positions should be moved 0.06′ south…”). The failure mode is silent: plot a WGS84 GPS position on an unshifted old-datum chart and everything looks plausible while being consistently wrong — the harbour entrance 150 m from where the chart draws it. Britain's land-grid version of this story, with the mathematics, is in WGS84 vs OSGB36.
Datum #2 — vertical: where zero depth is
The second datum has nothing to do with position. Every depth on a chart is measured down from chart datum — a chosen 'zero water' level. On charts following the international convention (the UK and most of the world) chart datum sits at Lowest Astronomical Tide: the lowest level the tide can reach under any normal astronomical alignment. US charts use Mean Lower Low Water — the average of each day's lower low tide — a slightly higher zero. Either way the logic is conservative: a charted depth of 2.0 m means about the least water you should ever find there, and the day's tide height (from the tide tables, measured above the same datum) gets added on top. Actual depth ≈ charted depth + height of tide.
Heights flip the reference: bridge clearances and overhead cables are measured from a high datum (Highest Astronomical Tide or Mean High Water Springs), so they're also worst-case. And your echo sounder joins in with its own zero — the transducer's depth or the waterline, depending on setup — which is why sounder, chart and tide table can only be reconciled when you know all three zeros. The tide side of this story continues in tides explained.
Making GPS and chart agree
Three habits close the gap. Read the title block of any chart you navigate on — datum, units (metres or the older fathoms and feet), and any position-shift note; it's one paragraph and it's the chart telling you how to lie to it correctly. Match the datum setting: chartplotters and handhelds can output positions in datums other than WGS84 for exactly this reason — set the receiver to the chart's datum (or apply the printed shift) when working on an old sheet. Verify against reality once: put the boat alongside a charted feature — a pier head, a beacon — and confirm the plotted position lands on it; a constant offset announces a datum problem immediately.
Electronic charts mostly dissolve datum #1 — official ENCs are WGS84 — but they inherit every sounding and its vertical datum from the paper survey behind them, some of which is a century old. The datum lesson generalizes beyond the sea: any time a position 'moves' between two maps, the first suspect is the frame, not the fix. That's as true for a dive site as for a harbour entrance.