Meridian Treasure Hunt

[Greg]

Last chance to register for the Meridian Treasure Hunt.

So far we have ten cars taking part. There is still room for a few more.

It will be a great day out.

[Mike]

Greg, what type of OS maps are those as those numbers are different parts of the country depending on which OS map you pick?

Also, does anyone know a good place you can buy them please?

[Sue]

The maps are the Explorer series 1:25 000 scale(Orange cover). Large/independant book shops or W H Smiths stock some but can usually order them for you in a couple of days if you ring your closest store.I got mine from the Sevenoaks Book shop - rang Saturday they were in yesterday before 10 am.

[Rick]

...and if you're in London there's always Stanfords near Covent Garden...

[Mike]

I've managed to get one of them from WH Smiths. Another shop had another one, but it didn't have the Meridian Line marked on the map, which Greg says is a requirement.

I shall have to take a trip to Bromley on Saturday and hope for the best !

[Rick]

Try the Waterstones in Bromley. They have (or had, last time I looked) a fairly reasonable selection of maps.

[Sue]

Mike, just had a look at my maps - only 1 has the meridian marked. Glad you mentioned it. However I've spotted that longditude is marked at top and bottom of the maps so I'm off to the shed to find a long straight edge!

[Mike]

OK Thanks Rick.

Sue, I think the meridian line is at an angle to the longitude line, correct me if i'm wrong !

[Ian]

Surely the meridian line is the prototype of longitude lines? Having said that, spherical geometry is not my strongest skill...

[Mike]

Dont' ask me why, but on the maps it is at an angle to the longitude and latitude lines!

[Rick]

The OS map uses the OS grid, which is a projection with some slightly strange properties. Conversion between long/lat and OS grid coordinates is a non-trivial exercise....

[Greg]

It's not absolutely vital the maps have the meridian line marked on them. It is just that all the questions relate closely to the line so it makes finding the clues a lot easier.

[Greg]

Mike, you are quite right. the meridian line is used as a guide for referencing everything to the Earth as a whole. The Ordnance references are those based on a mapping system for the UK. I was going to use the Meridian references but since they are not on the Ornance Maps (I know they are placed as add-ons around the perimeter) it would have made life very difficult!!

It is interesting as to exactly how many mapping systems there!!

Sue, I think we should have a talk about mapping the Earth, the UK and anything else (GPS??) that's been mapped it could be very interesting.

[Greg]

We now have 12 cars although some may drop out on the day.

[Ian]

and so it all becomes clear. The meridian is a longitude line, but does not line up with OS maps due to differences in projection and datum(and to confuse it further, the angle difference will change as you move North/South and East/West depending on where the coordinate datum is).
Clear as mud really. Definitely like the idea of a mapping presentation. I was reading about the mapping of Mars a while back and that got a little confusing too.

[Rick]

Somewhere I've got a note of some of the formulae needed to convert between Universal Transverse Mercator and geographical co-ordinates. I forget whether the OS grid is a UTM grid or some other projection though. I do know it's not based on the WGS84 spheroid that's usually the default setting for GPS. I can get my GPS to display OS grid co-ordinates, but it plays merry havoc with the waypoints I have saved for locations in Germany, Kenya and Australia if I make the mistake of trying to save and then restore them....