Trouble with 2 divots (punches) on a monument

Pick one, and call it good.

:gammon:

It's a bench mark, isn't it? The "divots" don't matter, shoot the high spot.

> It's a bench mark, isn't it? The "divots" don't matter, shoot the high spot.

I would agree without qualification if I were only concerned with the vertical - but I'm also hoping to tie into NAD83(2011) positions given the mark's NGS-published coordinates determined through recent GPS observations.

To do this we've been performing long static observations (10+ hours), and have set the pole point in the divot to ensure the setup doesn't slip mid-session.

I guess there's no way of knowing how previous observations were made (or why someone bothered punching a second divot in the mark).

My thought is the center divot would be the original if they even set a divot, I also believe there would be no reason to place a divot in a benchmark. I also can't think of any reason for NGS to set an off center divot.

Height modernization would be a good reason to place a divot on a benchmark.

> Height modernization would be a good reason to place a divot on a benchmark.

Precisely. This mark (and the question) pertain to an ongoing height modernization study.

I'm now wondering if this second (off-center) divot marks a Radius Point (thus the 'RP'). I can't spot the roadway curve it might define, but I've seen stranger things happen.

Or perhaps it's a reference point for the roadway alignment.

What you going to do with the .02 '

Could be the disk comes pre-punched. Does it monument elevation only? Does it mark a horizontal boundary or reference a horizontal datum? Someone may have used it for traverse some time ago and found that it may have moved?

While it may not be the case here, there are a lot of USCGS/NGS disks out there that have a hole drilled through, off center, that supposedly prevents air from being trapped under the disk when it was set. or so I was told.

We have had a lot of problems with dam alignment disks that have punch marks. They would set the disks first, then set alignment pins on line between them. Sometimes the alignment pins would either be set accidentally too far off line to be punched or the monoliths would move and the pins would no longer fall on line. In that case they would go back and put a new punch on the disk. Sometimes they would grind off the cross hairs in the middle, other times just put a paint mark in the punch, which over time would wear out. A typical navigation dam would have 4 or 6 disks at least. Some dams had no punch marks at all, others had several with punches.

Here is one from last week where it is clear which one to use.

Others are not so obvious. Since we are trying to get better than 3 mm accuracy overall, these are critical. We had to go back to one problem project and survey all the punches and crosses and try to figure out which were used and when. Usually they are only 3 to 5 mm apart, so it can be tought to figure out after the fact. Now we are much more careful about checking to see if there is more than one mark on a disk.

Now that I think about it I believe the hole was to let excess water in the concrete escape rather than air. I was told the reason a long time ago, and my memory is not real clear.

> What you going to do with the .02 '

In my post-processing adjustments, I think I'll have to let the mark float horizontally.

Oddly enough, I think the centered divot is 7mm (not a bad guess with the 0.02') east of the off-centered divot. My multiple hours of 1-second real-time observations produce a cluster of points that are well-centered around the published coordinates. Moving 7mm east of the published coordinates removes me from the cluster. This leads me to wonder if previous GPS (and horizontal) observations might have occupied this off-center divot as well.

Vertically, I think I have probably lost a millimeter or more from the round top of the monument to the bottom of the off-center divot. If I can get a good pole point adapter for a digital level rod, I could approximate the vertical offset between the two. Alternatively (and perhaps less accurately), I could set up a round prism on a fixed height tripod over each point and make repeated observations from a total station to approximate the same vertical offset.

> Could be the disk comes pre-punched.
Could be, it looks like the off-center divot and the 'RP' stmamping were added more recently.

> Does it monument elevation only?
I imagine it did, initially. Now it monuments horizontal position as well.

> Does it mark a horizontal boundary or reference a horizontal datum?
I didn't find any ROW markers nearby, so I'm not sure that it marks state ROW. I'm wondering if RP stands for reference point. I'll contact the state DOT tomorrow to see if they have more information on the monument - perhaps it has some highway stationing tied to it.

It is tied to NAD83(2011) Epoch 2010.00. GPS observations were made in 2012 to tie it into the datum. I have no way of knowing which divot was used for these GPS observations.

> Someone may have used it for traverse some time ago and found that it may have moved?
I hadn't considered this but it's certainly a possibility. The NGS datasheet lists the 'RP' stamping for the mark, but doesn't indicate if 'RP' was stamped when the monument was set, or if it was stamped at a later date.

Since GPS observations were only made in 2012, I'm not sure what someone traversing through the monument would have used as initial coordinates to establish that the monument had shifted enough to need another divot.

Wow! I've never run into anything like that.

Granted, my experience is sorely limited compared to yours and the rest of the posters on this forum, but it gives me peace of mind to know that these problems exist and how they've been handled by experienced practitioners before I'm confronted with these problems in the field.

I take it you don't need to make any modifications to those disks - you work with the punch marks already set in the disk (whether or not they're well-centered on the disk.

On a very tangentially related note, if you're monitoring movement at the 3mm level, how do you setup your total station for this kind of work? Do you setup over a known point (and assume a well-adjusted plummet and a solid HI measurement), or can you resection yourself in through observations to 'stable' monuments in bedrock and away from the dam structure? I apologize for going off-topic, but I've always been curious about this.

Thank you!

> Now that I think about it I believe the hole was to let excess water in the concrete escape rather than air. I was told the reason a long time ago, and my memory is not real clear.

I've always heard it called an air release hole, so I think your first explanation was correct. Air would get trapped under the disk, which prevented the concrete from sealing tight to the underside, or so it was thought. As you and others have noted before, the practice was abandoned not too many years after it began.

When two hole were found on a USGS Horizonal mon., I figured that after an adjustment it was easier to adjust the center point hole than to adjust the numbers. I have found many of those, but only on USGS Horizonal Control Monuments.

1- second huh? I like it but doesn't the NGS just use 30-second when processing? I recently asked if a new CORS could record at 0.2- second... I have not yet received a response.

As far as the second divot, do you know the velocity of the area? Did some wise guy set a second divot because it is where things pointed at some time when they performed some work?

> 1- second huh? I like it but doesn't the NGS just use 30-second when processing? I recently asked if a new CORS could record at 0.2- second... I have not yet received a response.
>
> As far as the second divot, do you know the velocity of the area? Did some wise guy set a second divot because it is where things pointed at some time when they performed some work?

I believe NGS does use 30-second epochs, but I'm checking testing the accuracy of the real-time network at multiple observation lengths (30-second, 1-minute, 3-minute real-time observations, etc.)

At the moment, I don't know the velocity of the area. I'd love to pick the brain of the individual responsible for the second divot, but I'm not holding my breath for someone to come forward.)

We setup on numerous points. My goal is to shoot each point from at least two different stations. We typically setup on all of the control disks, which on navigation locks are usually set on the gate monoliths, which go to bedrock, as opposed to a regular monolith which are usually built on wood pilings and are not as stable. We also setup on other stations as needed to fill out the network, for example at the end of each long wall. We take great care when setting up the instrument over stations, making sure that the centering error is less than 1 mm.

There are many objects on the walls like railings, buildings, lamp posts, etc that block lines of sight, so it is not possible to get all of the shots that we want. Here is a network we did last week:

After processing the data and adjusting it in a least squares program, the semi-major axis of the error ellipses are in the 1 to 3 mm range, with just one point being 4 mm.

we have always called it the breather hole. Not to be used as the real "dimple" looks like someone incorrectly used a dimple machine on the monument. Many of the HARN points were placed on 1st order benchmarks and sometimes it's not clear just where the point is. This one looks like a DOT bench mark, probably not a real stable point.