Jump to content

Petition: Bring back OS Grid Refs!


Followers 4

Recommended Posts

lol rofl OSGB slows a GPS down ! lol

OK misquote me, you are the one who is laughed at here, What I said was.

 

... to make them more accurate would just slow your GPSr down

 

If you wish to quote somebody’s words on here then please read what they have put.

 

The fact that all but the most expensive GPS systems, (we are not talking Garmin GPSMap60CSX here, we are talking professional GPS systems used in surveying) actually just do a rough conversion of the WGS84 projection, to the OSGB36 projection. Remember WGS84 is based on a Geodesic survey of the curved earth, and OSGB36 is a flat mapping system. Go and wrap a sheet of paper around a ball, and see how it creases.

 

As the earth is not a perfect sphere you would have to include an infinite number of points to reference in the calculation model, that they DO NOT, they take a few, and on equipment of greater value the number is greater. The more points you include the slower a calculation would be, due to increased lookups and slight changes in formula to accommodate the change in shape of the earth.

Link to comment

We don't imput the British Grid Coords into the GPS, but they invaluable for getting the gist for where a cache may be on a map (the day I'm able to plot lat / long on a map I'll eat my hat! :) ). You said yourself they are designed for flat paper and that's when we use them.

 

When out caching in the countryside we tend to navigate by map rather than just follow the arrow, and only consult the GPSr once we get within a few hundred yards or so. As we already own loads of paper maps, were not going to waste a rain forest printing new ones showing the cache location when we can have a quick look at the OS grid coords and draw a dot on one of our old maps. It's purely about cost :blink:.

 

 

Ditto.

Link to comment

Now this is serious nonsense, as the British grid doesn't apply (apart from maybe as a mathematical theoretical possibility) in Finland.

Or does it...? The Angry Corrie's OSGB Conspiracy Theory

Heh, funny.

 

Interesting it's all about OV, someone's been there, there's not much to it: http://www.geograph.org.uk/article/In-Search-of-OV0000

 

(To any wondering, the true origin of the British grid is N49° W002°, to minimise distortion. The grid, however, has been shifted some 400km west, and 100km south, which happens to fit most of the UK snugly into the 4 'big' squares (N,S,T,H)

Link to comment

Now this is serious nonsense, as the British grid doesn't apply (apart from maybe as a mathematical theoretical possibility) in Finland.

Or does it...? The Angry Corrie's OSGB Conspiracy Theory

 

Great stuff! Loved it.

 

By a strange coincidence the NE Tip of the monstrous square of Really Big Squares looks to fall just short of Finland ... kinda Kemi/Oulu area at the top of the gulf of Bothnia. So for once it seems that, purely by chance, I wasn't talking rubbish.

Link to comment

Can anyone explain why the 'British Grid' on the 'Other Conversions'Link does not correspond to OS Grid. What British Grid is it supposed to be?

For example, Cache GC106TT is at N 52° 56.747 W 000° 48.738, or British Grid 'SK 79903 39372' but if I use the other conversions link it tells me it is at British Grid 339373 479904. What system is that from?? :)

Link to comment

Can anyone explain why the 'British Grid' on the 'Other Conversions'Link does not correspond to OS Grid. What British Grid is it supposed to be?

For example, Cache GC106TT is at N 52° 56.747 W 000° 48.738, or British Grid 'SK 79903 39372' but if I use the other conversions link it tells me it is at British Grid 339373 479904. What system is that from?? B)

That is the british grid, but in the technical format. (Incidentally the eastings and northings are the wrong way around, too)

 

This gives a bit of background to the system, if you're interested.

http://www.ordnancesurvey.co.uk/oswebsite/...lp.html#gridref

Link to comment

Can anyone explain why the 'British Grid' on the 'Other Conversions'Link does not correspond to OS Grid. What British Grid is it supposed to be?

For example, Cache GC106TT is at N 52° 56.747 W 000° 48.738, or British Grid 'SK 79903 39372' but if I use the other conversions link it tells me it is at British Grid 339373 479904. What system is that from?? B)

That is the british grid, but in the technical format. (Incidentally the eastings and northings are the wrong way around, too)

 

This gives a bit of background to the system, if you're interested.

http://www.ordnancesurvey.co.uk/oswebsite/...lp.html#gridref

:lol:

Ahhh. Now I get it. It was not noticing the fact that they were swapped around that made it look so wrong. Thanks for pointing it out.

Link to comment

Remember WGS84 is based on a Geodesic survey of the curved earth, and OSGB36 is a flat mapping system.

Not quite true. Not true at all, in fact. In fact OSGB36 is a geodetic datum which is applied to the compound curved surface of the Airy 1830 spheroid. The OSGB36 datum is most emphatically not a flat mapping system.

 

Go and wrap a sheet of paper around a ball, and see how it creases.

The Transverse Mercator Projection, which is the mathematical operation equivalent to wrapping paper around a ball, is not the cause or the source of the error shown on Garmins and on the GC.com cachepages. That Projection conversion is performed quite accurately, even on the humblest of machines. The duff conversion on GC.com and the Garmins comes further back in the conversion process. It's caused by using those duff dX;dY;dZ datum shift parameters which are a very rough fit to the whole of the UK, but are very poor locally in most areas. The low-powered Garmins have an excuse for such a crude and crappy conversion. The high-powered GC.com servers do not.

 

Fortunately the most humble GPSr gives you the option of using your own bespoke parameters which are a much better fit for your own caching area. If you scroll through the list of available datums (or as a last resort, read the manual!) you will see "User". Here you can input your own dX; dY; dZ and then your GPSr will show you the correct OS grid co-ords, to within a metre or so. It doesn't use any more computing capacity that any other datum shift conversion and doesn't slow the machine down or drain the batteries.

 

To calculate your own XYZ datum shift parameters, simply convert a local WGS84 position to 3D Cartesians using one of the many freebie online or offline progs, then convert from OSGB36 and subtract one from t'other. It really is as simple as that.

 

Cheers, The Forester

Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
Followers 4
×
×
  • Create New...