GPS Accuracy for Anchor Alarms Explained

The Key Insight: It's a Relative Measurement, Not an Absolute One

Start here, because it dissolves most of the worry. An anchor alarm does not need to know exactly where your boat is on the surface of the Earth. It needs to know one thing: how far you have moved from the point where you dropped the anchor. That is a relative measurement, and relative measurements are far more forgiving of GPS error than absolute ones.

Here is why that matters. Suppose your phone's position is off by 4 metres in some direction. When you set the alarm, it records your “anchor point” with that same 4-metre offset baked in. A few hours later it measures your position again — with a roughly similar offset, because the error sources change slowly. The two readings share most of their error, so the distance between them is much more accurate than either reading alone. A boat that has dragged 30 metres reads as having dragged roughly 30 metres, give or take a couple. The signal you care about towers over the noise.

How Accurate Is Phone GPS, Really?

Modern smartphones are far better at this than the “GPS” label suggests. Three things drive their accuracy:

• Multiple constellations. Your phone doesn't just use the American GPS satellites — it simultaneously tracks GLONASS (Russia), Galileo (Europe) and BeiDou (China). More satellites in view means a faster, tighter fix.
• A clear view of the sky. On a boat at anchor, out in the open with nothing but water and air around you, your phone has close to ideal conditions — far better than the urban canyons GPS is usually criticised for.
• Sensor fusion. Phones blend raw satellite data with their accelerometer and other sensors to smooth the result.

In practice, a phone on deck with an open sky is typically accurate to within about 3–5 metres, and often tighter once it has had a few minutes to settle and lock onto a full set of satellites. That is the same order of accuracy as a dedicated marine chartplotter, which uses the same underlying satellite systems.

What Actually Causes GPS Drift at Anchor

“Drift” is the term for the small, random wandering of your reported position even when the boat is physically stationary. It is not your boat moving — it is the position estimate jittering within its margin of error. The usual culprits:

• Atmospheric delay. Satellite signals bend and slow as they pass through the ionosphere and troposphere, shifting the calculated position slightly.
• Multipath. Signals bounce off the water, a nearby cliff, a steel hull or a marina building before reaching the antenna, arriving a fraction late and nudging the fix.
• Obstructed sky. A phone left below deck, under a sprayhood, or wedged against the hull sees fewer satellites and drifts more than one with an open view.
• Satellite geometry. When the visible satellites are clustered rather than spread across the sky, the fix is geometrically weaker and wanders more.

The important takeaway: drift is normal, it is usually only a few metres, and a well-set alarm radius absorbs it completely. Drift becomes a problem only when your radius is set so tight that ordinary jitter pushes you across the line.

GPS Drift vs. Real Dragging: How to Tell Them Apart

This is the heart of the matter, and the difference is bigger than it sounds:

• GPS drift is small, random, and bounded. The position wobbles a few metres this way and that, then comes back. It has no consistent direction and no trend — over an hour it averages out to roughly the same spot.
• Dragging is larger, directional, and progressive. A dragging anchor moves the boat steadily downwind or down-current in one direction, and the distance keeps growing. It doesn't come back.

A good anchor alarm is built around this distinction. Rather than shrieking the instant a single noisy reading pokes past the line, it watches the trend — a sustained move in a consistent direction beyond your safe radius — which is the unmistakable fingerprint of real dragging. If you want to recognise it with your own eyes too, our guide on how to tell if your anchor is dragging covers the visual and instrument signs, and why anchors drag explains the underlying causes.

Why False Alarms Happen — And How to Stop Them

Nothing erodes trust in an anchor alarm faster than being woken at 3 a.m. for nothing. But genuine false alarms almost always trace back to setup, not to the technology. The three causes, in order of how common they are:

1. Radius set too tight. By far the number-one cause. If you set a 20-metre radius but you have 40 metres of rode out, your boat's normal swing alone will trip the alarm the moment the wind shifts.
2. Legitimate swinging. When the wind or tide turns, your boat pivots around the anchor to a new position on the far side of its circle. That is healthy behaviour — but a radius that only covers where you happened to be sitting will read it as a breakout.
3. Anchor point set before the boat settled. If you mark the anchor position while still drifting back and setting the hook, the centre of your circle is wrong. Set it once the boat has settled and the rode has come taut.

All three are solved the same way: set the alarm centre after the boat is settled, and choose a radius that covers your whole swinging circle plus a margin for GPS noise.

How to Set Your Alarm Radius for GPS Accuracy

The goal is a radius large enough to ignore drift and normal swinging, but small enough to catch real dragging early. A reliable way to arrive at it:

Alarm radius = Deployed rode length + Boat length + GPS margin

• Deployed rode — the radius of your swing is, at most, the length of rode you let out (the boat can lie anywhere on a circle of that radius around the anchor). If you don't know how much that should be, our anchor scope calculator gives you both the rode and the swing radius, and anchor scope explained covers the 7:1 rule behind it.
• Boat length — your bow swings on the rode, but the phone is somewhere amidships or aft, so add your boat's length to be safe.
• GPS margin — add roughly 5–10 metres to absorb normal drift. This is the buffer that converts “3 a.m. false alarm” into “sound sleep.”

Worked example: You let out 35 m of rode on a 10 m boat. Swing radius ≈ 35 m, plus 10 m of boat, plus an 8 m GPS margin = a sensible alarm radius of around 50 m. Set it tighter than your rode length and you are fighting physics; set it absurdly wide and you lose early warning. The formula keeps you in the safe middle.

One refinement worth knowing: some sailors deliberately let the alarm gather a minute or two of readings after dropping the hook so it can centre itself on the boat's settled average position, rather than the single noisy point of the first fix. A slightly calmer centre means a slightly more honest radius.

Where You Anchor Affects Your Signal

GPS accuracy isn't uniform — your anchorage shapes it. A few situations to be aware of:

• Steep-sided coves and cliffs. Beautiful, but the surrounding rock blocks part of the sky and reflects signals, increasing multipath and drift. Add a little to your GPS margin.
• Crowded marinas and harbours. Metal masts, buildings and other hulls all bounce signals around. Worth a wider margin if you're anchored among them.
• Phone stored below. The single biggest self-inflicted accuracy loss. A phone in a cabin sees a fraction of the sky. Keep it where it can see up — near a hatch, a window, or in the cockpit on charge.

The fix in every case is the same: give the phone the best sky view you reasonably can, and let the GPS margin in your radius do the rest.

Battery, Background Monitoring, and the Screen Off

An anchor alarm is only useful if it runs all night with the screen locked while you sleep — so it must keep tracking in the background. On iOS this requires granting “Always” location access (not “While Using”), which is what lets the app read your position with the app closed and the phone in your bunk.

The trade-off is power. Continuously polling GPS is the heaviest draw on a phone, so two habits matter:

• Keep the phone on charge overnight. A dead phone is a dead alarm — the one failure mode an anchor alarm can't recover from. Treat “plug it in” as part of arming the alarm.
• Use a sensible update interval. Checking position every several seconds rather than continuously catches dragging perfectly well — an anchor that breaks out drags for minutes, not milliseconds — while easing the battery load.

For the wider overnight routine, our night anchoring safety tips pull the whole checklist together.

Phone Alarm vs. Chartplotter: Which GPS Should You Trust?

A common question: if my chartplotter has an anchor-watch function, why use my phone? On raw signal, a fixed plotter has a mild edge — its antenna is mounted high with an unobstructed sky view. But anchor monitoring is rarely lost on raw signal. It is lost on whether the alarm is actually armed correctly and whether you'll hear it.

The chartplotter is at the helm; you are asleep in a cabin two bulkheads away. The phone is on the shelf beside your pillow. It uses the same satellite constellations, it is far quicker to arm correctly every single night, and it goes off where you'll actually wake up. That is why many experienced cruisers run both but rely on the phone as the alarm that genuinely gets used. We compare the wider trade-offs in anchor watch vs anchor alarm app, and review the leading apps in our best anchor alarm apps comparison.

The Bottom Line

Phone GPS is accurate enough to keep watch over your boat — not because it pinpoints you to the centimetre, but because an anchor alarm measures change in position, where shared errors cancel out and real dragging stands clear of the noise. Drift is normal and small; dragging is large, directional and progressive, and a good alarm is built to tell them apart. Set your alarm centre after the boat settles, choose a radius of rode length plus boat length plus a GPS margin, give the phone a clear view of the sky, and keep it on charge. Do that, and a GPS anchor alarm becomes exactly what it should be: a quiet watch-keeper that only ever wakes you when it truly matters.