Rediscovering lost trigs

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Rediscovering lost trigs

Postby FrankR » Sat Jun 09, 2018 7:20 am

Hi, I would have asked this in my last post but it seemed a bit off topic so.....
When rediscovering a trig that has not yet been found, how deep should I expect to be digging for say, a surface block. I know it all depends on how many years growth its got over the top, but I presume if it's a block, then surely it's not likely to be more than about a foot down is it?
Am I going to have to buy a turbocharged shovel? I'm pondering whether my wifes little plastic potting trowel is quite up to the job?
Anyone any hints for finding the lost ones?
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Re: Rediscovering lost trigs

Postby agentmancuso » Sat Jun 09, 2018 11:22 am

There are three types of block referred to by the OS: buried block, block, surface block.

The term 'surface block' only appears with the adoption of a small handful of OSGB36 stations as Passive Stations following the development of GPS-type technology in the late '80s. Unfortunately, whilst OS records for OSGB36 are very thorough, records for the Passive Stations Database are riddled with mistakes. So stations listed in the PSD as one type often turn out to be something different e.g. bolts and rivets listed as surface blocks or vice versa. But in principle, I think surface blocks are just blocks reused as Passives.

The distinction between blocks and buried blocks is often misleading, as you've already said. I have a theory that the difference in terminology refers to the state OS surveyors found these stations in when they were later used and listed in the OSGB36 database, rather than a conceptual difference in existence at the time the stations were constructed, but that's just my speculation. Some blocks have been found at least a metre down, whether they are labelled buried or otherwise.

As Passive Stations often have accompanying sketches, you could sum up by saying it's quite likely that you'll find a surface block, not all that likely you'll find a block, and pretty unlikely you'll find a buried block, unless someone else has in the recent past.

As for tools, I used a wee trowel and an old kitchen knife for years, but now have a 10" screwdriver and a folding metal spade in the car.
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