GB Grid vs Lat/Long problem

[+One Eyed Hound]

Hi All,

 

I've searched and can't find what I need but I'm sure this is a basic question...

 

I've used my Garmin Etrex for years but always on GB grid reference units.

 

So, now i need to learn to use lat/long with decimal minutes so i can solve mystery cache clues etc.

 

problem is, when I enter lat/long coords to the etrex from a cache page I always end up 92 or 93 metres due East of the cache.

 

Example GCXP3X: I enter N 55 36.129 W 002 46.780, I can't find the cache. I then change the units on the gps to grid (just to check) and it gives me a grid ref of 50881 34557. But the grid ref given for the cache is 50973 34553. I go to this grid ref and find the cache 92m E of the lat/long coords.

 

It's not an error on this cache as I see this with every example I've checked (6 so far)

 

Anyone know what's wrong? Is it my time settings somehow?

 

Thanks

[Iwuzere]

from http://www.geocaching.com/about/glossary.aspx :

 

"WGS84

 

The most current geodetic datum used for GPS is the World Geodetic System of 1984 (WGS84). The significance of WGS84 comes about because GPS receivers rely on WGS84.

 

Geocaching uses the WGS84 datum by default. We also use the format HDDD MM.MM, which is a standard for GPS receivers (like the eTrex). "

 

There should be a Datum setting somewhere on your GPSr that will allow you so select WGS84... try that!

I presume (without straining my brain too much thinking about it) that the datum setting applies to the lat/long.s, but doesn't affect the British grid refs.

Edited July 31, 2007 by Iwuzere

[+One Eyed Hound]

Thanks for taking the time to look up the glossary and offer your help - much appreciated!

 

I've changed datum to WGS84 but It's still not right to my mind.

 

on cache GCXP3X, coordinates N55 36.129 W002 46.780 are also given as NT50973 34553

 

My GPS converts these coordinates to NT 50876 35021.

 

I'm confused. How come this is so easy for millions of other geocachers:-(

 

Am I missing something really obvious?

 

1IH

[+Thrak]

Make sure you are entering the coordinates as DD MM.mmm You need to set the unit for decimal minutes.

[+One Eyed Hound]

I thought I'd post to close this one off.

 

I found the answer on the UK forum (geocacheuk.com)

 

There are 2 datums in use in the UK :

 

WGS84 for hddd mm.mm

 

Ord srvy GB for British Grid

 

On my etrex I have to change both the format and the datum or the results are ca100m out.

 

Excellent - problem solved!

 

No stopping me now

 

Thanks for your help

 

Happy caching

[Iwuzere]

Thanks for clearing up the confusion. Pity it doesn't store the datum separately for each format!