tri station rehab

Yes. On the NGS website. If you cannot find them then call Dave Doyle at the NGS and he will help you sort it out.

You first have to ask yourself if it's worth the effort. With the availability of OPUS, the value of passive horizontal stations is pretty low these days.

Jim,

i'm trying to set up a bit of a hard control network for our use. this tristation is the only one remaining that i know of on the quad sheet which it is located.

before we perform VRS GPS jobs we check into something known at the beginning and at the end of sessions. it's nice to have something close by to reference to instead of driving 15-20 miles and spending a couple of hours of butt time.

also, without sounding too pessimistic, what makes you think you will always enjoy the level of technology that you do today? strange things happen in a world full of whackos. 😉

> i'm trying to set up a bit of a hard control network for our use. this tristation is the only one remaining that i know of on the quad sheet which it is located.

The published position for a triangulation station that hasn't been updated via GPS is unlikely to be as accurate as a well-observed OPUS position, so I don't see the value in replacing an obliterated surface mark if accurate positioning is the only goal. It could be a fun exercise in "doing things the old way," though.

> i'm trying to set up a bit of a hard control network for our use. this tristation is the only one remaining that i know of on the quad sheet which it is located.
>
> before we perform VRS GPS jobs we check into something known at the beginning and at the end of sessions.

I agree with Jim that for the same or less effort, you could probably establish a new monument and tie it to NAD83 via OPUS at much better accuracy than the published position of the restored triangulation station would have. You are concerned with the vertical component as well as horizontal, aren't you?

If I were faced with a similar exercise of monumenting some check points to verify VRS positioning, I'd probably focus on finding some locations that were most convenient to my office (if that's how you intend to use it) and in some protected location on public land.

FWIW,

I would first contact your NGS Geodetic Advisor for definitive advice.

In any event, as the subsurface monument was set first and the surface monument plumbed over it when the monument was established, it is straightforward to do the same yourself.

I'd do the following:

1. Build a plumbing stand over the subsurface monument. Be sure it is stable and out of the way when pouring concrete into the hole.
2. Cover the subsurface monument with at least ten centimeters of soil (as you may need it again in the future).
3. Pour concrete into the existing hole until flush with the surface. If it is no longer a nice cylinder use a sonotube and back fill around the tube.
4. Plumb the new disk over the subsurface monument using the stand.

Unless the subsurface monument was also disturbed, and if done with care, the new disk should be as accurately located as the old disk.

As you did not provide the station PID, I have no idea of its accuracy or compatibility with the current implementation of NAD 83.

HTH,

DMM

Do you use a tribrach or a plumb bob? What accuracy is practical to hold for the relationship of the subsurface and surface marks (i.e. 0.5 mm, 2mm, ... ) ?

Howdy,

Rather than continuing to rely on my memory, I refer you to the following document (based on NGS documents I no longer have):

http://www.olmweb.dot.state.mn.us/manual/SM_Manual.pdf

See: Figure 2-3.0502 in document for sketch of plumb bench and monument with both surface and subsurface disks. Do a search in document for this figure.

(Copied from above document)

The procedures to follow in setting poured concrete monuments for horizontal control marks are summarized below:

a. Station Mark - Dig hole for the station mark 1.5 m to 2 m (5 to 6 ft) deep and a minimum of 35 cm (14 in) diameter at the bottom and 30 cm (12 in) at the top. (Figure 2-3.0502)
b. Underground station mark - Pour a mass of concrete at least 25 cm (10 in) in diameter, and at least 15 cm (6 in) deep, at the bottom of the hole and set a station mark disk, stamped with the same designation as the surface mark, in the concrete and centered in the hole.
c. Set a plumb bench over the hole and mark a plumb point over the underground mark. Then cover the underground mark with paper and 15 cm (6 in) of soil.
d. Enlarge the bottom of the hole about 5 cm (2 in) or more and center a 30 cm (12 in) square tapered form around the top of the hole. Leave the form flush or projecting 5 to 10 cm (2 to 4 in), and centered over the plumb point of the underground mark.
e. Fill the hole with concrete mixture. Shape and smooth the top and set the pre-stamped station disk under the plumb point of the plumb bench so the mark is exactly over the underground mark. Place a piece of iron or steel in the concrete to make the mark magnetic.

The method uses a plumb bob not a tribrach. Assuming careful adjustment of the tribrach, it could be used as well. Note the reference to "exactly over" and not a numeric estimate. With careful attention, millimeter-level accuracy is expected.

Hope this is responsive,

DMM

Plumbing Bench

> See: Figure 2-3.0502 in document for sketch of plumb bench and monument with both surface and subsurface disks. Do a search in document for this figure.

Here's that figure:

Plumbing Bench

thanks to all of you for the great info