I'd seen one of these before on Robinson but hadn't known what it was, but after some reseraching about trig points, I found that this is called a SURFACE BLOCK.
http://trigpointing.uk/info/types.php and
http://trigpointing.uk/trig/7422 I followed the link on the second site to find out what a "third order" site means. It seems that they used the triangulation pillars to do the main survey, found some gaps and set up a second order set to fill the gaps. And then...
