Where possible map references are given in three different formats:-

(a) National Grid Reference

X:(0)000000m,Y:(0)000000m

Notionally accurate to one meter with an origin 200 Kilometers West and 200 kilometers South of a point near Alleston Wood about two kilometers SE of Pembroke, UK. The latitude of this Pembroke position is 51:39:47N and the longitude 4:53:35W, whilst its Landranger OS Grid Reference is SS000999.

The co-ordinates of the old UK National Observatory at Greenwich are:-

X:538900m,Y:177300m

51:28:40N,0:00:00W

TQ389773

(b) Latitude and Longitude

00:00:00N,00:00:00W

(c) British Ordnance Survey Landranger Grid Reference

The Ordnance Survey is the British state cartographical agency. This grid reference format is an old alphanumeric code for UK Govenment maps.

XX000000

So far as Internet map resources are concerned, all three systems are only trustworthy to plus or minus a hundred meters.

To determine the grid reference(s) of an arbitrary place it is convenient to use interactive maps such as those provided by Multimap at http://www.multimap.co.uk to converge to the precise location at 1/10000 scale ( 1/50000 in the Scilly Isles and outside the UK ).

The National Grid Reference can be fed into ( for example ) http://www.old-maps.co.uk to invoke centered Victorian maps of the given location at approximately 1/25000 scale.

Few of my readers need to concern themselves with arbitrary fixed points such as the location of the Flamstead Observatory and much less with imaginary sea-stations off Finistere. Those are quoted to assist people who wish to calibrate their own GIS programs or the like. It is usually sufficient for travellers to know only the Landranger OS Grid Reference as this can be entered directly into Multimap to locate any feature in the United Kingdom.