Naive GPS question

[cliffb]

Sorry if I'm a bit naive, but....
The compass heading shown on a GPS, is that true or Magnetic? /forums/images/graemlins/confused.gif
Guess I should know the answer...but I've always been too embarassed to ask!!Thanks for your time.

 

[johna]

It is normally "user selected". Have a look at the Nav setup page.

 

[cliffb]

Ahh... Thanks, Johna... didn't think of that. See, told you I was naive! /forums/images/graemlins/blush.gif

 

[tcm]

careful

in most cases, a gps doesn't "know" magnetic north - it's a number you enter. Best is to set it to "true" - all the magnetic north stuff is because of compasses being wrong.

You can tell which it's showing cos it will have an M or T after the heading in degrees

 

[Talbot]

Dont forget that the GPS course is course over ground, not over the water. Thus it is a combination of ship's course, leeway, and tide.

 

[Alastairdent]

Good point, Talbot.

This will show up dramatically when you hold the GPS and turn round in circles (you, not the boat) - the GPS still shows the same heading, cause it is the direction you are travelling in, not the direction that the GPS is pointed.

 

[Guest]

a) it is NOT compass heading ..... GPS has no idea which way it faces ....
b) it is the direction the gps unit moves
c) you can set it to either auto GPS corrected magnetic or to true ....

Best is to set it to true as only fancy sets will allow deviation to be applied etc.

 

[cliffb]

Thanks all, but ....
The reason this question was in my mind was because when one sets a waypoint the GPS tell you which course to steer. If one sets it to True, then one has to apply, erm... deviation? Or is it Variation??? I always get it mixed up!! /forums/images/graemlins/tongue.gif The amount by which Magnetic is off from True!!!
So perhaps setting to magnetic would be more user friendly? No?
Or have I got the whole concept completely wrong? /forums/images/graemlins/grin.gif

 

[srm]

Its not even the direction the GPS is moving in, but a running average of the direction it calculates it is moving in, which includes system errors. Just leave your set displaying course and speed while tied up alongside, or better still securely aground and see how fast you are going.

GPS is a great toy and can also be a very useful nav aid, but remember it can only give one view of reality.

 

[nealeb]

[ QUOTE ]
Thanks all, but ....
The reason this question was in my mind was because when one sets a waypoint the GPS tell you which course to steer. If one sets it to True, then one has to apply, erm... deviation? Or is it Variation??? I always get it mixed up!! /forums/images/graemlins/tongue.gif The amount by which Magnetic is off from True!!!

[/ QUOTE ]
You should apply both - variation which is a number for your current location (I was going to say "fixed amount" but it does change very, very slowly with time) and deviation which is a correction for your particular steering compass on that particular heading. If you are lucky, deviation will be zero on all headings - or at least smaller than your helmsman's typical steering error!

If you are going far enough that variation/deviation is a significant factor then it may be worth doing a bit of homework on basic nav theory - there are plenty of good books around if you don't want to go as far as an RYA Day Skipper course, for example.

 

[Guest]

Correction of heading / course ....

True is what it says ... TRUE
Magnetic is true corrected for Variation - local error due to mag north not being at N.pole ... actually in Alaska !!
Deviation is the deflection of compass needle due to ship structure / fittings etc. Usually changes with direction boat is laying ...

Little memory saying .... Error East - Compass Least, Error West - Compass best

As to setting GPS to magnetic .... ok if boat has no deviation ....

 

[tcm]

completely wrong

ok, for small trips esp under motor, you could indeed just put in a waypoint and head there. There's a "highway" function that allows you to stay "on track".
Steeringwise like this, it makes no differecne - it'll tell you left or right a bit either magnetic or true.

But this is no good at all for ideal navigation over any distance.

Imagine, frexample, you were going somewhere and draw a straight line to it. But now imagine there is a nice tide or current or wind or all three blowing sort-of towards that place your going - at 45 degrees - so it comes over your shoulder,and at the same speed as you travel.

Hurrah! this means you don't have to take the direct route - you can set off with that wind/tide on the beam and sail along one short side of the triangle whilst the wind and tide will take you the rest of the way for free. This is much quicker, but the gps shows it as "wrong" - until the very last minute before arrival. The slower you travel under your own steam (sail/power) the more dramatic these effects - the faster you travel the less relevant, but never totally irrelevant.

Ignore these effects and jsut use gps you'll always be fighting the tide or wind insteasd of using it. The dayskip stuff covers it but unfortunately it is not always taught as a positive skill - sadly sometimes a necessary evil.

hope this helps

 

[tome]

Re: careful

<in most cases, a gps doesn't "know" magnetic north - it's a number you enter>

Most gps sets have a built-in model which derives local variation based on current position.

As stated previously, gps doesn't know which direction you are heading, only the course you've made over the ground. This is why it jumps about so much when at slow speeds.

We stabilise our gps systems with a fluxgate compass at speeds below 2 knots.

 

[graham]

Simple solution, most GPSs have a display which has distance and bearing to waypoint and COG(course over the ground )and SOG(speed over ground)all appearing on the same display.

Look at the bearing to waypoint and point the boat roughly in that direction.Now adjust your heading until the bearing to waypoint and COG are nearly the same.

You are then travelling in a straight line over the ground towards your next waypoint with no calculations for tide or leeway neccessary.

lazy yes but highly effective .

On a long passage where the tide is across your course and will change during the leg to your next waypoint this method is inefficient as you will be constantly fighting the tide one way or the other .

 

[andy_wilson]

Magnetic

All the GPS's I've had gave no option other than Magnetic.

They have a built in variation model which should last until the Earths mangnetic field reverses, or we die, whichever the sooner.

Easy way to plot on passage is to set a waypoint to the compass rose on the chart and plot bearing and distance directly from GPS display.

 

[Anonymous]

Re: Magnetic

[ QUOTE ]
All the GPS's I've had gave no option other than Magnetic.

[/ QUOTE ]Could you name some of those models?

 

[john_morris_uk]

Re: Magnetic

I've never seen one yet that you couldn't specify either True degrees or Mag as part of the user setup function. So which models were these GPS's?

 

[Anonymous]

Re: Magnetic

You meant to address that to andy_wilson, not me.

 

[graham]

Even with a basic hand held like mine you can set up for Mag or True the "T" next to the bearing indicates true in this case.I prefer it like this as it relates directly to the chart.

You can also set up the speed /distance for nautical miles ,miles or kilometres .In this pic I have accidently left it on Miles after playing in the car with it.

 

[Guest]

Re: Magnetic

ALL GPS that I have encountered .... even back to the old Transit had TRUE and MAG ....... OK Tansit you had to type it in !!
If a GPS does not have true capability - then its sadly lacking .... IMHO