The signal error varies depending upon the type of receiver you have and whether you're using vertical measurements, but is about 16 feet on average, according to government sources. All GPS systems use the same satellites so it doesn't matter if it is a plotter, handheld, or smartphone the signal errors will be the same on every device.

Signal errors: The errors inherent in the GPS system itself in generating a position.

Note that when the boat swings 180 degrees, offset error doubles ( Figure 1), so you must include twice the offset in your calculation of the security zone. Offset error: The difference between the anchor's resting place and the GPS antenna at the instant you enabled the alarm. To prevent false alarms, the security zone must then take into account the following factors: Remember, the GPS tracks the antenna, not the anchor, and the electronic center for all alarm calculations will be at the set point, the position the antenna was in when the system was activated. To prevent false alarms, that circle must be large enough so the normal movement of the boat, and the inaccuracies in setting up the circle, don't cause the anchor alarm to go off. Anchor watch systems work by creating a security zone around the boat, which is the electronic circle the boat is allowed to wander around in without tripping an alarm. The challenge lies in understanding the limitations of the different anchor watch systems and choosing which to use.Īlmost all anchor watch systems rely on the global positioning system (GPS), a revolutionary aid to navigation that is nonetheless prone to errors that must be compensated for when used to monitor the boat's position. Today, with just a few keystrokes, you can have an electronic box stand the middle watch, allowing a few hours of shut-eye.

Thankfully the days of wooden boats and iron men are behind us. You escape below only to have your pencil tear a hole in the soaking-wet chart while triangulating the boat's position. It's a pitch-black, nasty night with rain blowing sideways, and your concern about your anchor dragging sends you topside again to shoot bearings through lightning on distant landmarks.

Anchor watch app