What is speed over ground?
This page opens in knots and measures distance in nautical miles, because that's how the water works. You get speed over ground, course over ground, the distance you've run and a trace of your track, all from your own receiver with nothing sent anywhere.
Speed over ground or through the water?
This is the distinction that matters most on a boat, and one a car driver never has to think about.
Your log, the paddlewheel through the hull, measures speed through the water. GPS measures speed over ground. On a lake with no current they agree. Anywhere with a tide they don't, and the difference is the current.
Both are real and both are useful. Speed through water tells you how the boat is performing and is what you trim sails to. Speed over ground tells you when you'll arrive, and it's what you navigate by.
Course over ground isn't your heading
The course reading shows where you're actually going. Your compass shows where the bow is pointing. With a beam tide or a strong wind those differ by the leeway and set, sometimes by twenty degrees or more. That angle is your crab angle, and pointing the bow at a distant mark is a reliable way to miss it.
Why GPS works so well at sea
Open water is the best environment satellite positioning gets. Nothing blocks the sky, nothing reflects the signal, and boat speeds are slow enough that one reading a second loses nothing. Expect accuracy under ±5 m and an unusually steady number, better than anything you'll get on land.
- Below decks is different. A steel or aluminium hull blocks satellites almost completely. GRP or wood is much more forgiving.
- Cockpit bulkheads can cut your view of the sky in half. Somewhere with a clear view up is worth finding.
- Spray and rain don't affect the signal, only the phone. A waterproof case is the concern, not reception.
Speed and distance at sea
A knot is one nautical mile per hour, and a nautical mile is one minute of latitude. That's why the unit survives. Cruise at 6 knots and you cover 6 minutes of latitude every hour, with no conversion needed. For anything else, use the knots to km/h converter.
| Knots | km/h | mph | Miles per hour of chart |
|---|---|---|---|
| 1 | 1.85 | 1.15 | 1 |
| 3 | 5.6 | 3.5 | 3 |
| 5 | 9.3 | 5.8 | 5 |
| 6 | 11.1 | 6.9 | 6 |
| 8 | 14.8 | 9.2 | 8 |
| 12 | 22.2 | 13.8 | 12 |
| 20 | 37.0 | 23.0 | 20 |
| 35 | 64.8 | 40.3 | 35 |
This is not a navigation instrument
It shows speed and course and that's all. No chart, no depth, no AIS, no alarms, and it has no idea where the rocks are. A phone isn't something to rely on offshore. Use it alongside your proper instruments, not instead of them.
Frequently asked questions
What is the difference between SOG and STW?
Speed over ground is your speed relative to the seabed, which is what GPS measures. Speed through water is what your log measures. The difference between them is the current.
Why does my log disagree with this?
Usually a current. It can also be a fouled impeller or a log that was never calibrated. If they agree in slack water and disagree on the tide, it's the current.
Does it work with no phone signal?
Yes. GPS is receive-only and needs no mobile network. The page saves itself to your phone on the first visit, so it loads out of range too.
Can I use it below decks?
On a steel or aluminium boat, no. The hull blocks the satellites. On GRP or wood it often works, though it's better on deck.
Is the distance in nautical miles?
Yes, whenever the unit is knots. Switch to km/h or mph and distance follows to kilometres or miles.