There are four gravity stations in the UK, all around the Cheshire area:
Cat and Fiddle TP7177
Hatton Heath TP7237
North Rode TP7178
The outward appearance of these stations is similar to that of an FBM, except that they have a Flush Bracket (11171-11174) on the side and there is no bolt or height tag on the top surface. There is no underground chamber associated with gravity stations . However, they are classified as FBMs in T:UK. Gravity stations were purpose-built in the 1960s. All are designated as Passive stations.
The following description appears in  "... a Short Calibration Line near Macclesfield. Here a 60 milligal (mgal) difference in gravity between the terminals could be achieved by locating one in Macclefield itself and the other 1200 feet higher up on the Pennine scarp near by. This base was supplemented later by a second between Hatton Heath, near Chester, and Prees in Shropshire, which gave a 55 mgal difference for a much smaller difference in elevation."
Gravity measurements are needed to determine the Geoid Model. The Geoid Model is used to convert ellipsoidal heights (usually on WGS84, ETRS89 or other GPS compliant datum) to so called orthometric heights measured relative to some determination of sea level. The ellipsoid is rarely parallel (never mind coincident!) to the equipotential surface we call sea level.
- ↑ galium_verum, Trigonomy post, https://groups.yahoo.com/neo/groups/trigonomy/conversations/messages/4976
- ↑ Rod Sladen, Trigonomy post, https://groups.yahoo.com/neo/groups/trigonomy/conversations/messages/4981
- ↑ A History of the Ordnance Survey, W.A.Seymour(ed), 1980, p.348
- ↑ Ian Wilson, Trigonomy post, https://groups.yahoo.com/neo/groups/trigonomy/conversations/messages/4977