Deviation card

[Anonymous]

Re: Deviation and HBC

In essence you are using the gps as a pelorus. I'm not sure that it will be as easy as you might imagine. As someone suggested earlier (in another thread?) a good way for a simple compass check is to motor at slack water on a wind-free day and use the gps as a 'compass'. I haven't got to the bottom of how a gps derives the local magnetic variation and the accuracy of that needs to be looked into before using on to swing a magnetic compass.

 

[BrendanS]

Re: Deviation and HBC

[ QUOTE ]
I haven't got to the bottom of how a gps derives the local magnetic variation and the accuracy

[/ QUOTE ]

by use of lookup tables stored in the gps. Accuracy will depend on how good the algorithm used to interpolate the variation at your current position.

 

[Anonymous]

Re: Deviation and HBC

[ QUOTE ]
by use of lookup tables stored in the gps. Accuracy will depend on how good the algorithm used to interpolate the variation at your current position.

[/ QUOTE ]Then they could be horribly inaccurate in some parts of the world, when the variation does something unexpected. Since the military also use magnetic compasses, I would have thought that they would transmit new data regularly, based on actual observations rather than predictions.

 

[BrendanS]

Re: Deviation and HBC

No, this information is not sent by satellite. It's stored in the GPS - maybe sometimes it's updated when you do software updates, but you'd have to query the manufacturer about that.

 

[Robin]

Re: Deviation and HBC

You don't even need a waypoint as a target, just motor on a steady heading and compare COG with compass heading. It only works if there is NO tide effect or leeway, so flat calm at slack water neaps or better still inside a locked water (inland of Arzal Dam for example in S Brittany or in the Rance above the barrier perhaps). If you are concerned about the algorithms (or any other aliens for that matter) then set the GPS to read 'True' and do the comparison by using your own variation figure.

 

[brownsox]

Re: use of hand bearing compass

Many thanks for that tape idea - brilliant!
As newbies we need all the help of this kind that we can get.
Lovely simple idea (for steering at least as much as for compass swinging). Tape will appear on both sides of the boat next time out.
Cheers.

 

[peterb]

Re: use of hand bearing compass

You can add a bit more usefulness by working out the angle between the two pieces of tape when viewed from the helm. Helps you to estimate the angle off the bow of anything you see.

 

[Ships_Cat]

Re: use of hand bearing compass

Alternative to the tape is to just use something already on the pulpit to line up and know how far outboard to stand in the cockpit for determining dead ahead.

I use the motoring/low level port/stb nav lights which in our case are seperate units on port and starboard sides of the pulpit - an attraction for those is that they can generally be seen at night too.

John