it in guys, it's an anchor alarm!
Sometimes you don't have room to swing so you set it really tight so that if there is a wind
change it goes off.
Sometimes there is loads of room to swing 360 so you set it to road length plus a couple of fudge factors based on your experience.
Sometimes you have shorelines on so you don't expect to move at all; in this case you set it to 10m and live with an occasional false alarm.
You are all right and you are all wrong in some aspects.
allows you to move the centre of the alarm circle to centre it on the anchor. Mucking about with an app, plotter, GPS, intrument as you drop your anchor is not practical unless it's so quiet that you don't need an anchor alarm anyway. So do it after the fact, you will know your heading and your scope
so it's easy to move the mark.
Watchmate doesn't have a heading sensor built in - great unit though I use one all the time.
You need a heading sensor, not necessarily a fluxgate
, if the unit is to automatically compensate for GPS position as below about 3 knots COG means nothing.
Usually you'll notice that you are dragging about the same time as the alarm anyway. Motion of the boat, the noise
of the chain rumbling across the bottom, the bang as a shoreline parts
in a gust.. whatever.
Dont worry about it too much. Just play with what works for you. SLEEP WITH AN EYE, AND AN EAR, OPEN