Confusion re WGS72 & WGS84

[+JandJD01]

Hi Guys!

 

I'm far from a techie so please bear with me ..

 

I'm still getting used to my new 'toy' - Magellan eXplorist 710 but I think I'm doing ok between playing with it and finding out stuff via t'interweb etc.

 

However ....

 

I'm a little disappointed with the primary/secondary coordinate system/type and map datum.

 

When I installed the OS maps it said to set the primary as OSGB which meant there was no choice over the map datum .. fine.

 

I read somewhere that if on Lon/Lat the map datum should be set to WGS84 so I set the secondary to that.

 

However ... when saving current location as a waypoint, for a new cache or whatever, the GPS shows the grid reference .... I want the coordinates to be the first reference I see so I decided to set the primary to Lon/Lat instead.

 

Am I making sense so far?? :-/

 

But ... in the dropdown box for primary map datum there is no WGS84 only WGS72. I set it for that and left the secondary on WGS84 ... I'm finding that the coords are slightly different .. only very slightly but I'm thinking the WGS84 is going to be the most accurate so am I forever going to have to refer to the secondary coords??

 

Can I somehow get WGS84 in the primary dropdown menu?

 

And does this make any difference when searching for a cache?

 

Eeeeek .... my brain is now bleeding - I need to go and lie down.

 

Your thoughts would be greatly appreciated ....

[+The Blorenges]

I've got the 710. In Settings-Navigation select Primary Coords System = Lat/Lon. Primary Coord Type = Degree Minute. Primary Coord Accuracy = 1 meter. Primary Map Datum = WGS84. Set the secondary as OSBG and datum OGB_M.

This is how mine is setup and coords on the OS map seem fine.

 

By the way, if you're not aware I've written a detailed review of the 710. You'll find it on my resource website.

 

Chris

Edited June 1, 2013 by The Blorenges

[+JandJD01]

I've got the 710. In Settings-Navigation select Primary Coords System = Lat/Lon. Primary Coord Type = Degree Minute. Primary Coord Accuracy = 1 meter. Primary Map Datum = WGS84. Set the secondary as OSBG and datum OGB_M.

This is how mine is setup and coords on the OS map seem fine.

 

By the way, if you're not aware I've written a detailed review of the 710. You'll find it on my resource website.

 

Chris

 

Oh My goodness Chris ... I feel so stupid ... I can't tell you how many times I've scrolled the map datum menu looking for WGS84 to only be able to find 72 .... after your reply I thought 'ok - one last go' - and there it flippin' is - right at the top ... not in the alphabetical list at all ...

**shakes head in disbelief**

I have read your review - more than once and find it really useful.

 

Thanks so much for your reply, I really appreciate your trouble.

[+JandJD01]

I've got the 710. In Settings-Navigation select Primary Coords System = Lat/Lon. Primary Coord Type = Degree Minute. Primary Coord Accuracy = 1 meter. Primary Map Datum = WGS84. Set the secondary as OSBG and datum OGB_M.

This is how mine is setup and coords on the OS map seem fine.

 

By the way, if you're not aware I've written a detailed review of the 710. You'll find it on my resource website.

 

Chris

 

.. I just read your signature out to my husband as he's from near Bridgend and he said .. 'The REAL one'

[jri]

What Chris said for the menu settings! (I haven't got a Magellan), but here's the explanation for why there's a difference in the coordinates:

 

A geodetic datum is a mathematical description of the shape of the world. Because the world isn't perfectly spherical, it's hard to describe its shape accurately. OSGB36 is a datum that gives a relatively good match to the shape of the part of the world around Great Britain, but is somewhat ropey everywhere else. The WGS84 datum isn't such a good fit for GB (or anywhere else), but overall gives a reasonable match for the whole planet.

 

The National Grid and Ordnance Survey grid references are based on the OSGB36 datum, so if you want to give an accurate grid ref, you need to set your datum to OSGB.

 

Geocaching (and most other GPS applications) use the WGS84 datum, so if you want to share lat/lon coordinates with other geocachers, you need to be using WGS84.

 

Where the confusion comes in is that you can use the OSGB datum with latitude/longitude coordinates as well as grid references. However, these coordinates are based on a different shape planet from WGS84. What this means is that the same numerical latitude and longitude can refer to two different points in the two datums. The difference varies across the UK, but can be over a hundred metres. In practice, accuracy isn't really a problem. The issue is that OSGB and WGS84 are two different systems, and to share their coordinates correctly, you need to know which system the people you share with are using.

 

Further reading: the first few sections of https://en.wikipedia.org/wiki/Ordnance_Survey_National_Grid and the Coordinates & Map Datums section of http://www.follow-the-arrow.co.uk/resources/gps.html

 

Hope that helps!

[+JandJD01]

What Chris said for the menu settings! (I haven't got a Magellan), but here's the explanation for why there's a difference in the coordinates:

 

A geodetic datum is a mathematical description of the shape of the world. Because the world isn't perfectly spherical, it's hard to describe its shape accurately. OSGB36 is a datum that gives a relatively good match to the shape of the part of the world around Great Britain, but is somewhat ropey everywhere else. The WGS84 datum isn't such a good fit for GB (or anywhere else), but overall gives a reasonable match for the whole planet.

 

The National Grid and Ordnance Survey grid references are based on the OSGB36 datum, so if you want to give an accurate grid ref, you need to set your datum to OSGB.

 

Geocaching (and most other GPS applications) use the WGS84 datum, so if you want to share lat/lon coordinates with other geocachers, you need to be using WGS84.

 

Where the confusion comes in is that you can use the OSGB datum with latitude/longitude coordinates as well as grid references. However, these coordinates are based on a different shape planet from WGS84. What this means is that the same numerical latitude and longitude can refer to two different points in the two datums. The difference varies across the UK, but can be over a hundred metres. In practice, accuracy isn't really a problem. The issue is that OSGB and WGS84 are two different systems, and to share their coordinates correctly, you need to know which system the people you share with are using.

 

Further reading: the first few sections of https://en.wikipedia.org/wiki/Ordnance_Survey_National_Grid and the Coordinates & Map Datums section of http://www.follow-the-arrow.co.uk/resources/gps.html

 

Hope that helps!

 

Thanks so much for your reply. It's much appreciated ... Even if my brain IS now bleeding a little bit more lol!