Choosing a GPS

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My other option for Christmas (see "Night Sight" thread :
is a GPS unit. I would want it for use in the field, and as an in-car aid.
I have a Compaq iPAQ, so could use one tat works with that.
Any thoughts/ FAQ?
--
Andy Mabbett Reply to [my first name] [at] pigsonthewing.org.uk
USA imprisons children without trial, at Guantanamo Bay:
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Andy Mabbett Reply to [my first name] [at] pigsonthewing.org.uk USA imprisons children without trial, at Guantanamo Bay:
Why dont you keep this shit to yourself ? !
size=2>&gt;)<BR>&gt; <BR>&gt; is a GPS unit. I would want it for use in the field, and as an in-car<BR>&gt; aid.<BR>&gt; <BR>&gt; I have a Compaq iPAQ, so could use one tat works with that.<BR>&gt; <BR>&gt; Any thoughts/ FAQ?<BR>&gt; <BR>&gt; <BR>&gt; -- <BR>&gt; Andy Mabbett&nbsp;&nbsp;&nbsp; Reply to [my first name] [at] pigsonthewing.org.uk<BR>&gt; &nbsp;&nbsp;&nbsp; USA imprisons children without trial, at Guantanamo Bay:<BR>&gt; &lt;</FONT><A href="http://news.bbc.co.uk/1/hi/world/south_asia/2970279.stm "><FONT face=Arial size=2>http://news.bbc.co.uk/1/hi/world/south_asia/2970279.stm </FONT></A><FONT face=Arial size=2>&gt;<BR>&gt; &lt;</FONT><A href="http://web.amnesty.org/library/Index/ENGAMR510582003?open&amp ;of=ENG-USA"><FONT face=Arial size=2>http://web.amnesty.org/library/Index/ENGAMR510582003?open&amp ;of=ENG-USA</FONT></A><FONT face=Arial size=2>&gt;</FONT> </BODY></HTML>
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On Mon, 20 Oct 2003 22:34:59 GMT, yup wrote:

What's the matter? Does the truth hurt?
--
Phil Cook looking north over the park to the "Westminster Gasworks"

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So you don't give a fuck then ?
Lucky it's not you this time eh ?
--
geoff

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NO, I never pointed a rocket launcher at innocent people just cos I don't like burgers and chips (oh and democracy) !!!
writes

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Pot kettle black - does it really take that much attached junk to say one line?
--
79.84% of all statistics are made up on the spot.
The other 42% are made up later on.
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On Mon, 20 Oct 2003 23:22:22 +0100, Andy Mabbett
Probably more useful.

Try the newsgroup sci.geo.satellite-nav There's a good FAQ and an excellent website (reffed from the FAQ) that one of the regular posters maintains.
Most GPS are now pretty good, but some are still better than others - especially for mapping, which is now a near-standard feature. A surprising number have large inaccuracies in their conversion to GB OS grid refs though, which makes them damn near useless.
-- Die Gotterspammerung - Junkmail of the Gods
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Andy Dingley wrote:

Inaccuracies, yes. Large, no.

No.
They were very useful even when SA was on, the conversion inaccuracies are way smaller.
Paul -- The October Project 2003 http://www.wildwales.fsnet.co.uk/october/october.html http://www.wilderness-wales.co.uk http://www.photosig.com/go/users/userphotos?id 8749
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On Tue, 21 Oct 2003 00:42:48 +0100, "Paul Saunders"

1/2km is a bit too big, IMHO.
Can't remember the model, this is a friend's (a Garmin, I think) bought just a few years ago. It was enough to put us on the wrong side of a woodland when out walking.
-- Die Gotterspammerung - Junkmail of the Gods
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Andy Dingley wrote:

Sounds like he was using the wrong datum.
Paul -- The October Project 2003 http://www.wildwales.fsnet.co.uk/october/october.html http://www.wilderness-wales.co.uk http://www.photosig.com/go/users/userphotos?id 8749
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That was not due to an error in the GPS conversions to OS grid. The datum transformations are accurate to a few metres (<10).
Most likely it was due to either poor satellite geometry / obstruction or user error in having the wrong datum set.
The most common error with setting up a Garmin for use with the OS is to select British Grid position format but not change the datum to the required Ord Srvy GB from the default WGS-84. That will lead to an extra 100m of error which is consistent. People also commonly miss read the grid ref reported by Garmins until they realise that it is a 10 figure ref.
--

Dominic Sexton
http://www.dscs.demon.co.uk /
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[snip]

Except, of course, if you have purchased Memory-Map software and OS maps to go with your new Garmin.
Then you need to have the datum in your Garmin set to WGS84.

--
Jake

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jake wrote:

No you don't. Garmin units always send and receive data in WGS84 format, no matter what the datum is set to. Setting the OS datum simply affects the coordinates that are displayed on the unit, not how they are sent to the software.
I don't know about Memory Map, but with Ozi Explorer I set the upload/download datum to WGS84, so that the data is transferred correctly, but set the map datum to Ordnance Survey, so that the data is correctly shown on the map.
Setting your Garmin to WGS84 will give you incorrect readings.
Paul -- The October Project 2003 http://www.wildwales.fsnet.co.uk/october/october.html http://www.wilderness-wales.co.uk http://www.photosig.com/go/users/userphotos?id 8749
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Yes. Seems your right.
Just checked. My Garmin Map Datum is set to 'Ord Srvy GB'.
Now when memory-map displays the position where the cursor is sitting, it always appends '(WGS84)'
So a waypoint on a map is SP 75562 54701 (WGS84), but when down-loaded is displayed on the Garmin as SP 75561 54704.
Hmm ....... must try and understand all this one day ;-)
regards.
--
Jake

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jake wrote:

Actually that doesn't sound like WGS84, that sounds like the conversion to OSGB inaccuracy that Andy Dingley mentioned.
I've tried inputting a grid ref in WGS84 on my GPS and on OziExplorer, then changing the datum to OS and they both produce slightly different results. I think Ozi is more accurate because the conversion in a GPS is relatively simple.
I have two highly accurate lists of trig point waypoints in the UK, produced by the OS themselves, one in WGS84 Lat/Long format, the other in OS BNG format. When I enter the WGS84 waypoint (in either my GPS or OziExplorer) then switch to OS BNG, the result is different to the OS BNG coordinates (the OS use a much more complex conversion). The difference is usually only a few metres, sometimes more, but always less than 10 metres.
Paul -- The October Project 2003 http://www.wildwales.fsnet.co.uk/october/october.html http://www.wilderness-wales.co.uk http://www.photosig.com/go/users/userphotos?id 8749
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I think I'll keep quiet ;-)
--
Jake

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Not for the transfer of waypoints, routes and tracks as these use the Garmin Protocol which always uses WGS-84 no matter what datum the user has selected in their GPS.
However if you are using a program which takes the real-time NMEA information from the GPS to display the position on a map you may need to set the GPS datum to WGS-84 in order to have the position displayed correctly.
--

Dominic Sexton
http://www.dscs.demon.co.uk /
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wrote:

Wrong set-up, or very old unit. From the tracklog of my Garmin Vista you can usually see which side of the road I walked.
Jim Hutton
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With that size of error it was your friend's set-up of the GPS that was faulty, not the GPS. Google will reveal lots of folk complaining of similar problems, the problem being diagnosed, the complainer being told how to fix it, and the complainer reporting it fixed.
If it was easy to use a GPS correctly everyone would be doing it :-)
-- Chris Malcolm snipped-for-privacy@infirmatics.ed.ac.uk +44 (0)131 651 3445 DoD #205 IPAB, Informatics, JCMB, King's Buildings, Edinburgh, EH9 3JZ, UK [http://www.dai.ed.ac.uk/homes/cam /]
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wrote:

I've met people on mountains with a Garmin Etrex (the basic yellow one) reporting a position around a km wrong.
Answer - incorrect setup. The instruction manual is at fault, telling people to use a particular datum. Quite possibly the correct setting for the USA, but wrong for UK OS maps.
I changed the setup for them, and the position reported was where we were, reporting to a higher accuracy than any of us could measure from the maps we held.
Nigel. (not a GPS owner).
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