--- In ordnancemaps@..., "Andrew Rowbottom"
<Andrew.Rowbottom@g...> wrote:
> Does anyone have any information on the Datum by the OS before 1936?
> Ideally the one in use in the early 1930's.

Since no one else is rushing to reply, let me try offering a few
pointers to the sort of information that I think you may be looking
for.

All(?) 1" and smaller scale maps of England and Wales issued between
the late-nineteenth century and the advent of the current Transverse
Mercator Projection and National Grid are based on the Cassini
Projection and the Delamere origin. Ed Fielden's very useful
converter between Cassini and National Grid co-ordinates is available
for download from http://www.fielden-productions.co.uk/mapcol/cconv.
This also handles the Bonne Projection used in Scotland and the Irish
grid.

You may also be interested in Yo Hodson's definitive book on the
Popular Edition of the one-inch map, 'Popular Maps', published by the
Charles Close Society in 1999, ISBN 1-870598-15-6. Appendix 7 gives
a complete list of the geographical and Cassini co-ordinates for each
corner of each Popular Edition map.

The 'Old Series' 1" maps issued earlier in the nineteenth century
(and all 6" and larger scale plans published right up until
replacement by sheets based on the National Grid) were based on a
wide variety of different 'County' origins.

The vertical datum used by the Ordnance Survey is based on Mean Sea
Level at Newlyn. I think that you can assume that this is what is
meant on all maps that refer to 'Ordnance Survey Datum'. Earlier
maps may make specific reference to heights being based on the old
Liverpool datum. The difference is, I believe, less than 1 foot.

Note that Ordnance Survey maps are based on the Airy spheroid. This
is not the default spheroid now used by GPS systems. An online
converter between GPS geographic co-ordinates and National Grid is
available at http://www.gps.gov.uk

Hope that helps,

Chris Higley