The Charles Close Society is the society for those interested in the products, past and present, of the Ordnance Surveys in the British Isles.
There is now an internet discussion group started by members of the Charles Close Society, but not a part of that organisation, at http://groups.yahoo.com/group/ordnancemaps/. It is intended for anyone with an interest in Ordnance Survey Maps to pose questions, assist in research and to give details of relevant sources - including web sites.
The Ordnance Survey (OSGB, or just OS) are, and have been for over 200 years, the publishers of maps of Great Britain at scales from 1:500 to 1:1 000 000. (The OS site is full of useful information but, sadly, pages on this site take a long time to load because of the heavy use of graphics.)
Ireland is covered by maps produced by the Ordnance Survey of Ireland (OSI) and the Ordnance Survey of Northern Ireland (OSNI).
Up to date mapping at various scales from Ordnance Survey and Bartholomew can be viewed online at http://www.streetmap.co.uk/
Alan Godfrey Maps publish reprints of old Ordnance Survey Maps of towns and cities throughout the British Isles. These maps are mainly those originally published at the 1:2500 scale, which are reduced to about 1:4340 in the reprints, but there are a few maps of cities at even larger scales, and reprints of the One-Inch to One Mile maps of England and Wales are now being produced.
Early editions of the Ordnance Survey Six Inch to the Mile (1:10 560) mapping for most of Great Britain can be viewed on-line at http://www.old-maps.co.uk/. There is a discussion group for users of that site at http://groups.yahoo.com/group/old-maps/
To make a map, you need to know where things are.
In traditional surveying, triangulation is used to determine horizontal positions - more about triangulation pillars ("Trig Points") at TrigpointingUK.
while levelling is used to determine heights above a chosen datum - read more about Flush Brackets (FBs) and Fundamental Bench Marks (FBMs).