In 1784, Major-General William Roy undertook the measurement of a triangulation base on Hounslow Heath. The work was carried out with glass tubes approximately 18 feet in length 
This was followed, in 1794, by the measurement of a base on Salisbury Plain, this time using steel chains 
The ends of these base lines were marked by cannons.
These cannons are not recorded in OSGB36 but are of historical importance.
The four trig stations in T:UK corresponding to these are:
- Hounslow Heath Baseline North-west
- Hounslow Heath Baseline South-east
- Salisbury Plain Baseline North
- Salisbury Plain Baseline South
Three of the cannons are easily accessible, with the Salisbury Plain Baseline North marked on T:UK as 'inaccessible'. It is co-located with the Beacon Hill trig pillar and is assumed to be buried.
The Hounslow Heath NW cannon has been moved a short distance to accommodate Heathrow Airport boundary fencing.
In 1944, work on the runways of the future Heathrow
Airport resulted in one of the terminals of the Hounslow
Heath base having to be removed. This terminal, a
32-pounder cannon, had been emplaced there in 1791, when
the base was remeasured. Ordnance Survey was pleased to
receive it back and, to give a historical dimension to the drab
and utilitarian new offices, it was mounted with due care on a
plinth in front of the Chessington headquarters. Here it
remained as a kind of talisman until the new headquarters
were built at Southampton. In 1972, the cannon was restored
to its original position at Heathrow