Streetmap funny

From Rhaworth

There is a facility, coor_g.php, mainly used by Wikipedia, which takes an OS grid reference, converts it to latitude and longitude and links to a map sources page, thus: TL 473 132. Streetmap.co.uk offers a facility to display the current location in a number of coordinate systems thus.

There are noticeable differences between coor_g's idea of gridref → lat/log conversion and streetmap's as can be seen by clicking on the links in the table below. In the worst case (top left of the table = north west) there is a difference of 39 metres in longitude. (The latitude figures show reasonable agreement throughout.) That example is all at sea - the worst case for actual GB land is Soay, St Kilda where the difference is 24 metres.

Who is right coor_g or streetmap?

0	100 km	400 km	700 km	← eastings
0,1300	100,1300	400,1300	700,1300	1300 km north
0,800	100,800	400,800	700,800	800 km north
0,400	100,400	400,400	700,400	400 km north
0,0	100,0	400,0	700,0	0 north