Dive Site Coordinates, Done Properly
Log dive sites in decimal degrees on WGS84 (what every GPS and map app speaks natively), note whether the point marks the mooring, the entry or the feature itself, and record depth and datum alongside. When a saved reef seems to have 'moved', the reef didn't — the position was transcribed wrong, read in the wrong format, or plotted against a chart on a different datum.
One reef, four ways to write it
Every dive site position is one pair of numbers wearing four costumes. Decimal degrees (9.362800, 98.022200) is what GPS chips, phones and map apps speak natively — log in this, six decimals, and you can paste it anywhere. Degrees + decimal minutes (09° 21.768′ N, 098° 01.332′ E) is what charts, chartplotters and marine radio use — the format you'd read to a boat captain. DMS (9°21′46.1″N) survives in older guidebooks and park documents. And grid formats (UTM and friends) appear in scientific papers and some park permits. They're all the same point; the failures start when someone reads one format as another.
The classic: 9°21.768′ typed into an app as 9.21768° — the minutes read as decimals — puts you about 16 km from the reef. The reverse error is subtler and commoner: 9.3628° written down as 9°36.28′, off by roughly 20 km. Our DMS ⇄ decimal converters exist for exactly this seam, and the rule that prevents it costs one character: always write the format with the number — a degree sign and a minute mark, or a plain decimal point, never something ambiguous.
Mooring, entry, or the thing you came to see?
A 'dive site' is an area pretending to be a point. At a wreck, the mooring buoy, the bow and the debris field can span 200 metres; at a wall, the boat drops you up-current of the feature on purpose; at a shore site, the car park, the entry and the swim-through are three different marks. When you log a site, decide which point you're recording and say so: mooring (where a boat ties in), entry (where a diver gets in), or feature (the pinnacle, the anchor, the cavern mouth). Guides in our diving destinations pages label points exactly this way, and your logbook deserves the same discipline.
Depth belongs next to position — '−18 m at the plateau, −32 m at the sand' turns a dot into a dive plan. So does the datum note (WGS84, which your phone GPS already uses) and a one-word accuracy hint: a phone fix on a rocking boat deck under open sky is typically 3–10 m, which is plenty for finding a mooring and useless for relocating a lost camera. For collecting these into files you can keep and share, saved places and the GPX viewer handle the mechanics.
Why the reef 'moved' 200 metres
Reefs don't move; positions do, and always for one of three reasons. Transcription: a swapped digit in the minutes is 1.8 km; in the decimals of a minute, 18 m — most 'wrong' coordinates were copied by hand from a slate, a logbook or a forum post. Format confusion: the minutes-as-decimals errors above. Datum mismatch: a position lifted from an old chart or survey on a local datum (ED50 in the Mediterranean, NAD27 in the Americas, Tokyo Datum in parts of Asia) can sit 100–400 m from the same point expressed in WGS84 — the reef 'moved' because two maps disagree about where zero is. That third failure has its own article: GPS vs the nautical chart.
The sanity checks take seconds. Plot the point before you rely on it — a dive site on dry land or in 4,000 m of blue is a format error announcing itself. Compare against a second source. And when a position matters (a permit boundary, a search), keep the original string exactly as received alongside your converted version, so the error stays findable.
Sharing coordinates responsibly
Coordinates are power tools. Publishing the exact mark of a fragile site — a sea-fan garden, a cavern with limited air space, an unbuoyed wreck that anchors would destroy — can concentrate damage that dispersed ignorance used to prevent. Marine parks sometimes keep site positions deliberately vague, and local operators' livelihoods are built on knowledge that took years to earn; both are reasons to think before posting a six-decimal pin in a public forum. Sharing with a buddy, a club or a logbook is different from broadcasting.
Where sites are already public and moored — most of what our destination guides cover — precision helps everyone: accurate marks mean boats find moorings instead of dropping anchors, and divers surface where the pickup expects them. The rule of thumb: match the precision you publish to the protection the site has. Buoyed and managed → share freely; fragile and findable only by GPS → keep it in the logbook.