GPS am I thick or what?

[Mike2822425]

Last Nov. I purchased a Wooden Folkboat, and have done lots of "refurbs" and now have started looking at "electronics" so at this point there is no engine, and nothing to cause compass deviation! BUT the existing SUUNTO Compass tells me that my garden fence is 15/W of North (1/2 mile from the Beach at 000ft. the GPS tells me my boat is in position *** *** looks good.


So I decided to check out my new "Toy" so I walked around and it followed me in all directions, great BUT.

Today was RNLI Fraserburgh Open Day so I did my bit, then went back home and deceided to check the compass direction of my (shore based boat)SUUNTO. about 15 W of N Mag. against the Sportrack GPS.

On board(next the garden fence) my MagellanSportrack GPS (entry model) when switched on seems to have gone into"monkey manure"feature, (ape **it) it tells me I am pointing West this is obviously CRAP because living in a fishing community we tend to know where N is!

It seems to have no ability to up grade direction unless you are moving (compass that is) I walked off and it came in strong telling me I was walking at ** in *** direction.

So I decieded to push the system and walked around the garden ,it followed me every where but the moment I stopped (Port Side On) to my project,it froze the compass pointed W with a bit off, climbing the ladder to my garden based boat, I sat, turned the GPS through 360 and found mycompass direction could not be updated but I was still pointing West/ ish.

Answers please but dont tell me I am sailing in the garden Jimi, things will improve!!

From. Mike (Its a beautifull evening here in the Broch)

It does pose a question, if this system can pinpoint my position why cant it tell me if I am looking N or S ????



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[Tantalus]

Simple answer..some of the more advanced models can...yours can't..matter of getting what you pay for. Yours only tells the direction you're moving in relative to the sattelites.

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[Mariner]

No you are not thick, and you will be glad to know your gps is working fine. A quick and simple answer to your question is this.
The gps works out your direction of travel by comparing the differences between where you were and where you are now. From this it can work out your direction of travel. The gps software once it knows your direction of travel can then convert this information into compass bearings. Obviously if you are not moving it can not do this and this is why you get strange and unpredictable results.
This is a simple explanation if you need more information please pm me.

Martin


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[Solitaire]

Just to confirm Mariners post - if your not moving the GPS can't asses where you are heading. Check the lat long readings, stay still and so will they!

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[hlb]

Yep. There all right. The GPS does not know where your going untill your going some where. So your heading will be all over the place till you set off. This is not a problem Particularly in your garden!!

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Haydn

 

[BrendanS]

This is for interest only....maybe not applicable to your model. There is a battery saver mode on some GPS. Turn on battery saver, and batteries will last longer.

However it doesn't check with the satellites so often, so bearing will stay the same for long distances, even if you change direction. Fine if doing long voyages in one direction, but not handy if navigating in close quarters.


Some of the more sensitive GPS receivers can detect a change of position and hence show bearing, measured in a few feet, and would probably be able to give a bearing as you walked a few feet around the garden

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[hlb]

Mine although old will give an accurate position to, Well accurate? I cant say if it's a foot out but maybe less. But I know whether they have put the pontoon in the right place on Fowey. Most years they have put the D hook through the wrong bit of chain!!!

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Haydn

 

[BrendanS]

Gps is brilliant at telling you how close you are to a position. Not the question asked though.Does GPS double up as compass?

depends on your GPS is my answer. Some are accurate, as long as you don't set them for battery saving or some other innaccuracy inducing setting.

For most, you have to be moving at least a little bit. They gain their bearing from the fact that you are moving, and measuring the delta (changed position) from the satellite transmissions.

There are not many (if any, leisure) GPS or plotters which have an inbuilt compass function which operates separately. Move and it can work out which way your going. Stand still, and it can position you accurately, but can't provide a bearing;

If anyone doesn't get it, it's time you took some training. David L would be happy to take you on.

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[Mike2822425]

Many thanks to every one who replied, at least I have not bought a "duff" piece off kit. I will keep on navigating in my "garden project" with confidence!

Mike.

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[Mike2822425]

I cant sleep so its 03.00 here in Scotland and its light so I took my Sportack out again, and yes, you are all right on your observations of its merits. Carefully walking along the FB/s deck , minding the paint cans etc. for all of its 25ft. the compass trips in and I know the direction of my garden fence. So if this unit is WASS and ENOSS enabled (for 2004) just how accurate is it?

Mike the night time garden navigator.



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[hlb]

Depends what your measuring against. How sharp your pencil is and whether the chart is accurate enough. Linked to a chrt plotter and auto pilot, mine accurate enough to sail down any river in thick fog.

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Haydn

 

[BrendanS]

Have a look at this site. Brilliant, and you'll understand how it works in detail when it's finished. Puts across complex thoeries in a very easy to understand manner. http://www.gpsinformation.net/exe/waas.html

EGNOS is the euro version of Waas, and is not due to go live until 2004 <A target="_blank" HREF=http://www.pocketgps.co.uk/egnos.asp>http://www.pocketgps.co.uk/egnos.asp</A>

Presently, if you're getting a good signal, then should be accurate to about 15metres.When Egnos is enabled, you should get accuracy to 3metres or less





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[Birdseye]

to answer your question - yes!

the gps works like an electronic three point fix. ok it uses more than three points and it fixes by time rather than by bearing, but the principle is the same. and does a three point fix tell you the way you are facing?

but do 2 fixes and you can easily work out the direction you have moved in. gps is the same - it compares 2 position fixes some time apart to tell you the direction you have moved in. when you stop moving, it simply tells you the last direction you were moving in. it is incapable of telling you the direction you are facing in since it does not know the orientation of the set - you will get the same readout whether you are holding the gps at right angles to your body or in parallel(?)

according to my new, waas enabled garmin yesterday, the best accuracy was 7 ft with the waas test signal working. that, however, is the predicted accuracy from the configuration of satellites it could see, and therefore it varied from minute to minute. it is, for most purposes, useless since the accuracy of the charts is not that high.

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[Aeolus_IV]

As others have said - unless it has built in fluxgate compass you'll need to be moving to get a heading - but this heading will be true (dependent on datum chosen, ie OSG33, WGS84 etc..), and not magnetic - so it and your normal compass will always be a few degrees out (depending on your location).

Jeff.

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[gjgm]

GPS is amazing, but as a review of accidents has shown, use it as a tool, not a god.-check with the visual reality outside your boat! Accuracy is improving with dgps and waas, but just because its 99pct accurate to within 10m, it doesnt mean that its only 10m wrong for the last 1pct. 10m maybe ample out at sea, but your eyes maybe more accurate entering a harbour mouth for example.

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[peterb]

GPS compass

Just to add one tiny point: just appearing on the market (though still very expensive) is a GPS set which can tell you which way you are pointing. Effectively it uses two GPS sets, one at the bow and the other at the stern. Comparison of the two positions gives the heading of the boat, as a true bearing.

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[BrendanS]

<<not magnetic - so it and your normal compass will always be a few degrees out (depending on your location)>>

Not on all models. I've a Garmin handheld plotter which can show a magnetic course as well as true. It calculates the magnetic variation from the chart in use automatically

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