Installed by HMS survey ships mainly around the coast of Scotland, with 1 recorded to date on the Welsh coast. Their known number is gradually increasing as more are found. As at Sep 2022 there are 33 listed in trigpointing.uk
There are 4 OSGB36 trig stations that take the form of a cannon buried upright in the ground, they mark the terminations of the Hounslow Heath and Salisbury Plain baselines. 3 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.
- Hounslow Heath Baseline North-west http://trigpointing.uk/trig/7816
- Hounslow Heath Baseline South-east http://trigpointing.uk/trig/7804
- Salisbury Plain Baseline North http://trigpointing.uk/trig/8928
- Salisbury Plain Baseline South http://trigpointing.uk/trig/7570
A simple scored mark on a pre-existing structure, such as a drain. There are 28 listed on T:UK of which 13 are record Destroyed. Most are located in England, with 3 in Scotland and 1 in Wales. 2 are included in the OS's Passive Network 
Approx 1" diameter pipe set upright and mortared into rock, protruding sub 1". There are 21 listed on T:UK, all in Scotland.
- Lewis and Harris (1)
- Islay (16)
- Gigha (3)
- Kintyre Peninsula (1)
Pre 2022 there was only 1 possible finding recorded on T:UK, subsequently confirmed, Meall Ard on Islay . During 2022 several others on Islay were found.
The TrigpointingUK database contains a few artifacts which are not actual survey marks, but which are interesting nonetheless. And some genuine OS trigpoints do not fit into any of the standard categories. Some interesting examples include TP7314, TP7637