Hard to find item

Ebay is probably going to be your best chance. I've got one and it came off Ebay. I'd guess there are a lot of them lying around unused in the UK as we have a lot of concrete trig pillars here and you used to need a pillar plate to occupy them and tie into the Ordnance Survey's National Grid. You also had to pay £75 to obtain the co-ords of each one!

The trig pillars are still useful for a really permanent GPS station, bronze grooves on top and a removable cap with a tube down the centre to a bronze bolt in the foundation, always cast on bedrock.

I think Swisstek in the US did have one advertised for sale on their website but that was a while back.

It doesn't look like it'd be very hard to make one. Am I correct in assuming that centering is accomplished by the 120° distribution of the points, and that leveling is done via the instrument tribrach?

> ...in the UK as we have a lot of concrete trig pillars here and you used to need a pillar plate to occupy them and tie into the Ordnance Survey's National Grid.

Happy birthday to the Trig Pillar

Jim

It does and it doesn't! If you mounted the plate on a perfectly level surface with the 120 degree radials scribed on it from a centre point then levelling with the tribrach would be ok.

On the first photo of the plate you can see a circular bubble stored inside it, this has a stem, ball and spring arrangement which you use for centring, down through the hollow centre of the threaded part which receives the tribrach.

http://picasaweb.google.com/lh/photo/dC13qYB37nbnHJ0UEFGLVw

I use the pillar plate for mounting the GPS base on old trig pillars or for total station set-ups directly on steelwork or concrete with a centre punched station mark, you need the stem and bubble for that.

Thanks - I will try there

Thanks for the info - will let you know if it pans out!
Like I say they are not common any more.

Thanks - and you are right. I will have one made as a last resort.
But off the shelf is always easier.

Thanks for the history lesson!
I would kiss you if you were female 😛
Got a trig plate to share?

Try this:

http://www.sccssurvey.co.uk/trig-pillar-plate.html

> If you mounted the plate on a perfectly level surface with the 120 degree radials scribed on it from a centre point then levelling with the tribrach would be ok.

Not to discount the value of precise centering, but if I've done the calculations right the pillar would have to be out of level by about 5 degrees (roughly 2 cm from one side of the trivet to the other) in order to move the center of the tribrach 1 mm off the intersection of the radials. This assumes that the station mark is coincident with the intersection of the radials, and that the latter are accurately machined. I don't know if either condition normally pertains to pillar installations.

Jim, what you say is true for the trig pillars with their cast bronze grooves. In fact the Ordnance Survey used to 'maintain' the published co-ordinates of each pillar by checking whether the pillar top had moved horizontally relative to the bolt down at the base. There is no 'centre point' at the top, only the radial grooves.

The bubble and stem is useful though when centring over a centre punch mark in steel or a stud in concrete - less effort than trying to scribe out the radials.

Here's a pillar in good condition!

The last time used one was around 1979 or 1980 in Pine Bluff, Arkansas on top of the concrete wall of a rail car dumper.

We called it a trivet and it was made of either cast aluminum or pewter.

Most of the time the setup surface was level and it was the only time I carried a drafting compass into the field for drawing a circle around a painted target instead of using a plumb bob.

If needed one now I would go to the local machine shop and ask them to put something like plumb bob or tripod point feet on a plate to mount under a tribrach.

0.02

May be a stupid question, but why is the bubble on the side?

Okay, I can see how this may be stable in the bronze groves of a trig pillar but what hold it in place on a concrete wall?

The bubble on the side is misleading- that's actually where the centering bubble is stored- it's removed from the side holder and inserted in the middle of the trivet, and lets one level the trivet.- it's only stored in the side-position.

John

You're depending on the weight of the fairly heavy pillar plate (mine is 8 pounds) along with the sharp points cutting slightly into the concrete and the weight of the total station above. You've just got to be careful and not nudge anything, think of the plate as being a very low tripod.

It's also useful for setting a prism over a station in a concrete slab or wall top.