To calculate your position, GPS chips compare the receiving time of packets they receive from visible GPS satellites. (In)accuracy is caused by a combination of GPS satellite trajectory variation, GPS clock inaccuracy, atmospheric variations that distorts the path of the signal and changes of tracked satellites as the satellites cross the horizon.
Your device might also get signals that rebounded on reflexives surfaces such as seawater, cliffs, buildings, metallic masts ... When the signal reach your device after a reflection, it makes a longer trip than a direct signal. Your GPS chip does not know it got an indirect signal and its calculation will thus place you further away from the satellite than you are.
Each time the GPS calculates a position, it estimates how (in)accurate the position is. You should read the GPS accuracy as "The GPS estimates I am about 10m from the displayed position".
Reading the accuracy as "the GPS estimates I am within 10m" is quite common, yet wrong. Standard GPS chips provide no such estimation.
The first positions are always very inaccurate as the GPS has insufficient data. The accuracy might then become quite good (20m/ 60ft) or even very good (5m / 15 ft) when the device has a clear view of the sky. The accuracy varies over time, over a full night it is very unlikely for the accuracy to remain at 5m /15ft.
A good alarm app will show you the current accuracy estimation. It is critical to set an appropriate margin.
Don't expect an accuracy better than 5m -15ft.
Will I get false alarms?
It depends on the margin you did set. If your GPS (in)accuracy is or becomes larger than the margin, you will probably get false alarms.
Consider the exemple : You have an alarm radius of 100' with no margin and an estimated accuracy of 40'. You are in the circle at the green position, yet the GPS might position you 40' away in the red position. In this situation you are likely to get false alarms.
In pink, the false alarm zone. When you are there the GPS might place you out of the circle.
False alarm probability is about 50% when you are close to the alarm limit, it decreases as you get closer to the center.
Will I get false negatives?
Yes, you will have false negatives. When you physically cross the red line (your real position in green) and until you get further than the grey zone; the GPS might still see you in the circle (red calculated position).
In that case, your alert will be delayed until accuracy improves or you move further out.
In gray, the false negative zone. When you are there the GPS might place you in the circle.
False negative probability is about 50% when you are close to the alarm limit, it decreases as you get further away.
The good margin
Ideally, the margin is larger than the worse (in)accuracy you expect for the duration of the anchorage. You'll thus get less false alarms.
However, the depth must be sufficient in all the false negative zone that surrounds your alarm zone. Otherwise, you'll have to reduce the margin to reduce the limit the false negative zone. It's your call to define the acceptable trade off between false alarms and timely alerts.
Accuracy and custom shapes
The circle shape minimizes the false alarm zone and the false negative zone. It makes the circle the best shape for your alarm.
If your alarm handles other shapes, you might draw a shape that will often trigger false alarms, because it is entirely in the false alarm zone.
As a rule of thumb, remember that useable custom shapes have an alarm radius of at least 5 times the expected accuracy.
Here, the radius is only 2.5 times the accuracy which is insufficient
This shapes looks nice in app, while in reality most of your perimeter is in the false alarm zone.
The one on the right might not alert you at all when you get in the forbidden arc : the forbidden arc is within the false negative zone.
Better accuracy means less false alarms, less necessary margin and earlier alerts. Your alarm will work better if you can achieve an average accuracy better than 10m / 30ft.
When displayed accuracy is not good enough, try move your device to another place in the boat. Avoid metallic parts, covers and mast...).
Never expect a good accuracy in town. In a building, the roof blocks direct GPS signal. Near buildings, the walls blocks direct GPS signal and might also reflect GPS signal. This makes your position much less accurate than what you can get in an anchorage.
On some devices, displayed accuracy might be inconsistent with your observations:
Switch from normal to satellite view : Maps are very inaccurate in some places, and might show you ashore while at the same position the sat view shows you above the sea.
Make a full shut down and restart your device after waiting a few seconds: this will reset the GPS chip calculations.