Cruising Sailors Forum Archive

Re: Yes heading sensor and fluxgate compass) are interchangeable But ... heading should not

have any effect on range and bearing to waypoint.
I have miniscule operating experience with chart plotters, but I think I know a bit about the math -- enough to be dangerous, anyway.
The GPS does not need a compass to figure bearing. It needs the boat's (GPS) position and the position of the target . . . cursor or waypoint.
It then figures the difference in longitude between the two positions and the difference in latitude between the two positions. These differences form two sides of a right-angled triangle (for our purposes). The range to target is the length of the hypotenuse of the triangle, the bearing is the direction of the hypotenuse. GPS figure COG the same way.
The heading of the boat is irrelevant -- it can be going backwards, sideways, spinning on its axis.
(This is why you need a compass on a boat that has GPS -- the compass is the only instrument that can give you heading.)
From the screen shots Liz posted, it appears that the heading sensor (compass in English) was off kilter. Maybe someone put a radio on top of the fluxgate compass--- whatever. But that should not have had any effect of the internal calculations of the GPS/plotter. That is a mystery.

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