Good to know when using Anchor Alarms
GPS Reliability
Accurate monitoring depends on the quality of the device’s GPS signal. Even when the boat is stationary, small variations in satellite accuracy can cause the displayed position to shift slightly on the map. This behavior is normal and does not indicate that the anchor has moved.
- Place the phone where it has a clear view of the sky — near a window or outside the cabin.
- Keep the device powered and connected; background restrictions or battery savers can pause GPS updates.
- Wait for the GPS accuracy to stabilize before setting the anchor position.
Tip: Some devices reduce location accuracy when the screen is off. You can prevent this by excluding the app from battery optimization or keeping the display dimmed while monitoring.
GPS Accuracy
GPS accuracy varies with signal conditions and device quality. On most modern phones, accuracy is typically
within 3–5 meters (10–15 feet) in open areas, but it can drop to
20–50 meters (65–165 feet) when the sky view is obstructed by cabins, masts, or terrain. Even when stationary, the reported position may drift slightly on the map — this is normal and does not mean the anchor has moved. For best results, wait for the GPS accuracy to stabilize below
10 meters before setting the anchor position.
Anchor Position
It is impossible to determine the exact position of the anchor. The accuracy of the anchor position depends on when and where it is set. If the position is recorded before the anchor is fully set, or if the anchor drags slightly after setting, the stored reference point will be offset. This can cause the displayed swing circle to appear shifted even when the anchor is holding correctly.
When standing over the anchor to mark its position, the phone’s GPS may still have a few meters of drift, and the anchor itself can move slightly as the chain tightens.
The bearing to the anchor should be used only as a rough indication. Most phone compasses are not that accurate, which can make the displayed direction unreliable. For accurate heading information, use the boat’s magnetic or electronic compass instead.
Conclusion
The size of the security circle should be determined by the
anchor chain length,
water depth plus freeboard, and the
distance between the GPS antenna and the bow. An additional
allowance should be added to account for the normal stretch and catenary of the anchor chain, and GPS and position inaccuracies — typically
5 to 15% of the total chain length, with a
minimum of 30 feet (≈10 meters). If the circle is too small, normal boat movement can trigger false alarms; if it’s too large, real dragging might go unnoticed. The allowance should be increased if the GPS signal is inaccurate.
