I would be interested to know different ways that people use to accurately level to benchmarks which are set vertically in structure faces, like walls, buildings, bridge piers, etc. You know...the ones the geocachers report as "Suitable for satellite observations".
We use a knife blade stuck into the mark, and a 0.5 m invar strip, which is a rigid metal piece about 2 mm or so thick.
I would think there would be some kind of tool to do this very accurately, but I have not been able to find one.
Here is an idea I have, a metal bracket that would rest against the wall, when the bubble is centered the rod seat would be at the same elevation as the datum point on the disk. Maybe a screw driven leg with the rubber foot to adjust? Unfortunately I am not skilled at "making" things, but I don't think it would be terribly difficult for someone with the right tools in his workshop.
I use the same rod I am running levels with. Introducing a different piece of equipment introduces one more thing for an error in my opinion.
There is usually a footing below the point. After the initial shot, take a shot on the footing and do a measure down check.
We use a 6' rule (that's been compared with our Philly rod) held against the side of the structure and a relatively short turn getting out of, and back into, the mark.
SS
This is for second order class I levels, doing a "measure down" isn't going to help at all. We use 3 m invar rods for the leveling, and a 0.5 m invar strip from the same manufacturer is used for this purpose. One reason we don't use the 3 m rod is that it is HEAVY, and has struts on it, so holding it up in the air against the mark is more inaccurate than holding lightweight invar strip, which only weighs a few ounces or so. But, we have done that as well. I am looking for sub-millimeter accuracy, or at least 1 mm.
Just twisting the knife up and down LOOKS like it could affect the readings by one or several millimeters. We hold the knife blade as level as possible by estimation, I am thinking of setting a small level vial on it. But, usually there is not an extra set of hands there to try and get it as stable as possible. The level will reject any readings that have a high standard deviation (i.e. from movement).
It just seems that there are a lot of these marks around, and they are usually very stable, so we don't want to NOT use them.
We made some plates up very similar to the sketch, some 30 years ago. Still in use.
You can use an offcut of T section aluminium, about 3 inches deep. Cut one side of the top face back to about 1/2" inch wide (so you end up with a lop-sided T) and chamfer the underside back at 45 degrees. (This gives a knife edge to fit into the bench mark horizontal cut.)
Drill and tap a hole near the base of the T for a 1/4" bolt - aluminium taps easily, so you don't need any great skills (BUT remember when you use a tap to advance half a turn and then retract by the same amount to clear out the swarf - that way you don't break the tap).
The bolt then enables you to level it up (assuming the bench mark slot was cut horizontally in the first place!!)
Thanks, Chris. That sounds like a good design. If you could, I would appreciate a picture of your "device".
The NGS Instrumentation and Methodologies Branch in Corbin, Va has designed a device for this purpose. I searched my computers for photos or plans but it was too long ago (pre-2008). Contact them and I'm sure they will send you digital copies and maybe even a commercial source. The I&M Branch was once the I & Equipment Branch.
HTH,
DMM
If I've set it, I will always adjust the HI of the instrument to read the mark, especially with painted targets for inside work.
The Cass County BM is in the north wall of the Post Office and is at a height to setup with a HI to read the mark.
FWIW,
It is my understanding that the crosshairs in modern digital levels do not provide an accurate rod intercept reading as they are intended to assist pointing. Therefore, the use you describe cannot be assured to be accurate.
This method using a conventional optical level is fine.
Cheers,
DMM
This method relies on the rod having an accurate zero at the shoe. For most work where the rod is placed on the backsight and foresight points, an accurate zero is not required. Be sure to check your rod before using this method.
If that's a problem for you, then you could drive a stake nearby and use the level or a TS to match the height of the stake to the bench mark. Then you have something to rest the rod on.
Mike: I figured there must be such an item, but it is not shown in the manual NOS NGS 3, which is where I would have thought it might be. I will ask the guys at Corbin.
well, one of the more brilliant minds i have ever known was Charles Glover of NGS and he gave me many ideas for instrument problems in the years he worked there in Corbin. This was a question several people posed to him from time to time at Seminars and places he gave lectures.
His pat answer for Vertical Benchmarks was "Buck in on the Center"
which meant set the leveling instrument at the same height as the mark and turn off to a horizontal bench. Then, as an after thought he would always say "Be sure the foresight (length)is the same (length) as your back sight."
Most folks did not pick up or understand the latter statement as they were still trying to figure out the first statement.
asked him many times why he did not make such a dvise and his comment was always, "Not worth the price of making anything."
If a new piece of equipment needed a definitive reference test of quality, this man devised the test and then built the test.
As can be seen from the photo, sometimes it was necessary for NGS to build things themselves.
BTW, I still think it is incorrect to use a digital level as an optical one in the situation under discussion.
Cheers,
DMM
Mike: you are absolutely correct in that the crosshairs of a digital level are NOT what the level uses as the horizontal axis for observations. The crosshairs on the level I have (Dini) can be adjusted by a routine so that it is CLOSE, but never gonna be close enough for precise levels.
As requested, images of a bench mark plate. This is slightly different to my earlier description as it is from a section of steel T and the working end is longer. However, making it from aluminium means you can do it easily yourself without any tools other than a drill, file, hacksaw and a suitable size tap.
We do use this for precise levelling on monitoring schemes where a bench mark has been specified as the reference datum. A cicular bubble placed on the platform enables the screw to be accurately adjusted to set everything level. When we have to use 3 metre (10 foot) invar staves we always use the second staffman (you do use two staves when you do precise level runs, don't you?) to hold the staff while the first staffman holds the plate in position.
From "Precise Leveling in New York..." 1909ish.
Thanks, Scott. Looks like something that would be easy to make. It would be easy to carry a couple of pieces of 2X4 in the truck and cut them to fit when needed.
Chris: thanks for that, now it is much clearer on what it is.
To answer your question, no, we only use one rod. Not that two wouldn't be better, but the reasons are several:
1) small company, myself and one employee, with a part time guy available.
2) only have one 3 m invar rod. We have two 2 m invar rods, which are great for leveling on dams (which are flat), but not so good anywhere there is elevation change. The current project (about 15 km) is mostly flat, but it does have a 75 foot deep valley to cross about midway
3) related to #2, only the 3 m rod has been calibrated at a laboratory (a requirement for second order class I leveling). This was a major ordeal to get done. The actual calibration was free, but the rod had to be shipped by common carrier to the west coast (California), as it was too long for UPS or FedEX to take it. The cost to ship it there and back was about 1/2 the cost of the rod.
4) most of our work is on dams, so the lines are usually quite short, although as I mentioned in #2, our current project is longer.
This project (and the previous one) is being bluebooked. Here in the US that means that the data is submitted to the NGS for inclusion in the NSRS. I had to convince the NGS (National Geodetic Survey, the agency here in the US that maintains the National Spatial Reference System) that it would not create any issues by using only one rod. After we discussed it, they agreed that it was tradition and productivity that called for two rods, not really anything to do with the actual accuracy requirement. Yes, it is slower, but not terribly so, and as I mentioned above there are other issues.
Another P-I-A requirement for second order class I is that temperature profiles must be measured and recorded at each setup. This is done with a pair of thermistors high and low on the tripod that have a digital readout.
If we start doing a lot more of this, I may try to purchase a new pair of rods WITH calibration. I think the manufacturer's calibration is not sufficient, it has to be done at a lab (SLAC here in the US).
Second order class II does not require calibration nor temperature profiles. But, the clients in these two projects (subsidence monitoring) require second order class I.
We have done a number of other bluebook projects second order class II, much shorter and also much easier to do without the above mentioned requirements.
The reason for using two staves isn't for speed - it's to control any settling errors at the instrument while the staff man walks from one change point to the next.
If the staves are read alternately backsight/foresight and then foresight/backsight on the next set-up (ie. the SAME staff is always read first) then any small settlement of the instrument tends to cancel out over the run, especially since the time elapsed between each set-ups readings is very small.
That is one reason, Chris. But, running forward-back can also help minimize or detect any error due to settling of the instrument. Since we are in an urban area (all concrete sidewalks, etc), I am not too worried about settlement of the instrument during the setup.
