GPS is context, not the whole of navigation
The GPS in the iPhone gives you your position — latitude and longitude — at any time. On board that is enormously useful: as a starting point for the course, as the timestamped location in the logbook, as a cross-check against classic navigation. But GPS does not replace an understanding of course, current and tides — it complements it.
How accurate is GPS really?
Under an open sky the accuracy is often within a few metres. But it varies with conditions:
- Clear view of the sky (open sea) → good.
- Narrow bays, high banks, below deck → worse, because satellites are obscured.
- An accuracy figure (sometimes shown as HDOP or in metres) tells you how much you can trust the position.
This scatter is also why an anchor alarm needs a radius with a reserve: even at a fixed berth the measured position “wanders” a little.
GPS in the interplay
- Course: the current position is the starting point from which you calculate the course.
- Logbook: every logbook entry gets position and time automatically — handy for reconstructing a voyage.
- Plausibility: a terrestrial bearing (for example Light on the horizon) held against the GPS position exposes errors on both sides.
Your position stays with you
GPS reception is passive: the device only listens to the satellites and needs no mobile network for it. NauticCalc uses the position locally — it is not sent to a cloud, no live tracking, no accounts. That is good for privacy and works even where there is no reception (see Offline navigation).
Common mistakes
- Blind reliance on the GPS position without a cross-check.
- Ignoring accuracy: a metre value of 30 is not a pinpoint position.
- Power-saving mode, which slows down the fix.
- Confusing GPS course with compass course — one is over ground, the other through the water.
Conclusion
GPS on board is a strong helper: a quick position for course and logbook, a good cross-check against classic navigation. Whoever reads the varying accuracy correctly and does not take the position blindly gains confidence — and keeps control over their own data, because everything stays local.
Read more: GPS position · Logbook · Offline at sea