--- In ordnancemaps@..., "Chris Higley"
<Chris.Higley@v...> wrote:
> --- 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

I have just joined the group, having also just joined the Charles
Close Society. With reference to the vertical datum change from
Liverpool to Newlyn, you can probably do no better than to consult
the book "The Second Geodetic Levelling England and Wales 1912-21"
by Sir Charles Close and others. There is a scanned copy available
at http://www.deformedweb.co.uk/trigs/data/2GLMain.htm and Plate III
is a map of part of the British Isles showing the difference in
level between the two data. Differences shown on the map vary
between +0.25 ft around Hadrian's Wall to about -1 ft around London.

Incidentally, whilst I was searching for this information, I came
across "The United Utilities Water plc (Ullswater) (Drought) Order
2003." Now, I appreciate that no-one in their right mind would
normally be in the remotest sense interested in such a document, but
it does make reference to a 'mean height not exceeding 145.12 metres
(476.1 feet) above ordnance datum (Liverpool)' when defining the
company's rights to construct temporary works on the River Eamont.
Can anyone shed any light on this apparent legal throw-back?

Nick Eden