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OSGB v hddd (degrees) mm.mmm confusion

 
 
Andy
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      08-23-2004, 04:55 PM
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I have owned my Garmin GPS2+ for some years and there is something that
I don't understand about it, which was highlighted on a recent
geocaching trip within the UK.
I thought that introducing my son to geocaching might encourage outside
even more and help him learn about GPS' and maps. At the top of each
"hidden cache"page on www.geocaching.com is a 10 figure grid reference as
well as a grid in hddd (degrees) mm.mmm format. I decided to use the 10
figure grid reference at the top of the geocaching page. Both my son and
I fed this into our Garmin GPS'. We also took along paper maps.
When we arrived at the car park my son put his GPS into GOTO mode, with
the geocache grid ref selected. I had already looked at the map and had
envisaged a move away from the car park in a certain direction, but he
was heading off at 90 degrees to what I had intended. I followed him,
putting my GPS into GOTO mode, with the same grid ref selected.
When we arrived at the location that both of the GPS said we should have
been heading for we were on the wrong side of the hill and about a
third of a mile away according to the map. I took the matter in hand and map
read to the location manually. There was a lot of ground clutter with
no obvious landmark so we headed back to the car park and changed our
GPS' nav setup to: -
hddd (degrees) mm.mmm

then punched in the location given in that format on the geocache page.
This time we used the GOTO function and the arrow took us to where we
were when I had manually map read. This time, with more confidence, we
searched and found the cache.
Our GPS' are set up the same now: -

Posn - hddd (degrees) mm.mmm (It was previously OSGB)
Datum - WGS 84
CDI Scale - +/- 0.25
Units - Statute
Heading - Auto Mag W006

I have read and re-read the manual and other than the last setting,
which I am not sure is relevant to this problem anyway, I am sure that these
settings are correct.
Needless to say, until I find an answer I am going to stick to hddd
(degrees) mm.mmm, but as I use OSGB paper maps I really would prefer to use
grid references.
Any advice would be gladly accepted.

Thanks

Andy Hayes


 
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Andy
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      08-23-2004, 09:47 PM
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In message <(E-Mail Removed)>

>
>>Our GPS' are set up the same now: -
>>
>>Posn - hddd (degrees) mm.mmm (It was previously OSGB)
>>Datum - WGS 84

>
>There lies the problem. For the correct grid references you need to set
>the datum to OrdSvyGB when using the OS grid. Newer Garmins
>automagically change the datum for the OS grid but older ones have to be
>manually changed.


The GPS "was" set to OSGB when we put in the 10 fig grid ref.

When it took us to the wrong side of the hill I used the map to get us
to the correct place, but couldn't find the hidden cache.
We then went back to the car and changed the GPS setting to hddd
(degrees) mm.mmm and put the alternative reference in (that was given in that
format. That took us to the place that we had found with the map, but
with more confidence to finish the search for the cache.
Thanks

Andy

 
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Andy
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      08-23-2004, 10:06 PM
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>
>There lies the problem. For the correct grid references you need to set
>the datum to OrdSvyGB when using the OS grid. Newer Garmins
>automagically change the datum for the OS grid but older ones have to be
>manually changed.
>


That will teach me to assume. I just looked at the GPS and you are
absolutely right!!
It should be set to : -

Posn - British Grid
Datum - OrdSrvyGB
etc

Thanks for your help

Andy

 
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Neil Pugh
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      08-24-2004, 08:59 AM
In message <(E-Mail Removed)>, Dominic Sexton
<{da-sep03}@dscs.demon.co.uk> writes
>OS grid refs rely on the OSGB 36 datum which differs significantly from
>the WGS 84 datum hence the position shift noted.
>
>It seems that the lat/long co-ordinates you had were referenced to WGS
>84 so they matched the datum set in your GPS and you were led to the
>correct location.
>

Useful info on datums etc:

http://www.gps.gov.uk/guidecontents.asp

http://www.bangor.ac.uk/is/iss025/osgbfaq.htm
--
Neil Pugh
 
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Andy
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      08-25-2004, 09:27 PM
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>>

>Useful info on datums etc:
>
>http://www.gps.gov.uk/guidecontents.asp
>
>http://www.bangor.ac.uk/is/iss025/osgbfaq.htm
>--
>Neil Pugh
>.
>


Very interesting sites Neil. Thank you.


 
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