OGB to WGS84

[Manxman]

I have a Garmin 188 chart plotter sounder and wish to fish over a couple of wrecks, the co-ords I have been given are OGB Datum.The plotter was set to WGS84 when I created the waypoints, going out to fish I changed the Datum to OGB.
Arriving at the first waypoint I couldn't find the wreck even after a thorough search.
Should I have had the plotter set to OGB before creating the waypoints?
Does anyone know how to convert OGB to WGS84

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[Ovni]

you may try this program: Pc trans (freeware)

Download from

http://www.hydro.nl/pgs/en/pctrans_en.htm

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by Ovni on 04/05/2004 00:06 (server time).</FONT></P>

 

[LadyInBed]

WGS 84 to OSGB 36 is about
offset 0.03 S /183 ft S /61 yds S
offset 0.08 E /300 ft E /100 yds E


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[Birdseye]

The offset varies from area to area. Look at the paper chart and (assuming its not a real oldie) you will likely find instructions on it to tell you how to convert.

Your gps converts the position it works out into a lat and long using the datum to do so. So if you have put waypoints in using osgb, they will have shifted using wgs and vice versa.

<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it.

 

[Aeolus_IV]

I would have expected the waypoints to be kept internally on the GPS's own datum - then converted to/from the display datum (WGS84/OSGB etc) as required. This would then ensure that changing the display datum didn't actually move the waypoints - so, yes - if you know your waypoints in OSGB, set datum to OSGB then enter your waypoints, then when you change to WGS84 the waypoint values should appear to move - but stay in the same place.

I haven't got a GPS handy to check this view - and of course, it may not be true of all GPS's, so I could be talking a load of rubbish.

Regards, Jeff.

<hr width=100% size=1><A target="_blank" HREF=http://users.swing.be/FDB/centurion/index2.html>Centurion 32 Web site</A>

 

[tome]

The GPS receiver will do the conversion for you, but you are correct in assuming that you need to enter them in the reference datum, ie OSGB36. If you then switch to WGS84 you can read off the converted co-ordinates. There will not be a huge difference, but certainly enough to miss a wreck.

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[Birdseye]

You are quite correct - the gps does store the wapoints in terms of some internal datum. Just tried it out on my Garmin using a waypoint for the Manacles. Changing from WGS to OSGB caused the gps to alter the lat / long given for the waypoint.

The lat reduced by 0.032' and the long reduced by 0.055' which suggests an difference of about 120 metres, assuming I've not got my maths wrong - which I probably have.

<hr width=100% size=1>this post is a personal opinion, and you should not base your actions on it. <P ID="edit"><FONT SIZE=-1>Edited by birdseye on 04/05/2004 13:58 (server time).</FONT></P>

 

[DavidP]

Hi
Have a look at this site.

http://www.divedata.co.uk/

Dave

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[Manxman]

Thank you all for your help, I shall try your suggestions this weekend

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