Hello everybody,
I need some help because I do not have much experience in high precision leveling.
The leveling will be carry out by a Topcon DL-101C, 2 invar rods with bipods and standard base plates. They demanded us for a high precision?ÿ leveling ( 2.5 mm??L) for monitoring some buidings maybe involved by a landslide.?ÿ We have only one control BM (elev.=0 m.) outside the area involved by the landslide.?ÿ
What do you think about adopting a closed loop and a "double way run" (starting from a BM and turning back on it after a number of turning points) together? I have attached an image to illustrate more the possible plan:
1) from BM1 up to point 2 and turning back to BM1 (Magenta).
2) Closed loop from point 2 (green)
3) from 6 to 9 and turning back to 6 (red)?ÿ
4) from 14 to 16 and turning back to 14 (light blue)
In the image red points are fixed into the pavement or walls, blue points would be the positions of the base plates (frog). We need more base plates positions because the area is quite steep and the specs do not want more than 40 m. between the rod and the turning point. Orange area are buildings and black area are unaccessible/occlusive.?ÿ?ÿ
?dl=0
Thanks a lot.
Bests
P.S....Why can't I attach files?
Your picture does come up if I click on the little box.?ÿ Maybe it's the way Dropbox works that the picture won't embed?
?ÿ
I think it's because you're new?ÿ Can't post images or links right away
https://www.ngs.noaa.gov/PUBS_LIB/Geodeticleveling_nos_3.pdf
This is a good read I would worry as much about procedure as closure.?ÿ
As mvanhank222 alluded to above, PRODEDURE is actually more important than "closure" alone.
IF you use recognized [documented] procedures specifically designed for your stated [precision] goals, then your closure SHOULD be within your tolerance goal. If it isn't, then you have an observation/calculation/etc. problem that needs to be addressed. Sometimes you will just have to do it again.
In any case, you WILL have to use the same PROCEDURES?ÿeach and every time you re-run your line to verify what (if any) movement there may be in the study area.
Just by 2-bits
Loyal?ÿ?ÿ
We do a lot of high accuracy leveling. The fact that you are going to use invar is very important, not only for the much smaller coefficient of expansion, but also the fact that invar rods are one piece. Another critical thing is to do a C factor check every day before starting. Our level (Trimble) has several different test routines built in, we use the Kukkamaki. And keep the sight lengths balanced.?ÿ
If everything is done accurately and carefully, it probably matters little what sequence your do the level runs in.?ÿ
?ÿ
I would add that's it's important to have as many bench mark outside the slide area as possible. Although you only have one that's available as control, I would set whatever is needed to do later checks. We usually drive screw together rods till refusal for this task.
As stated above procedure is important, closure with digital levels has become routine. 2.5mm is very tight ?
One thing about accurate leveling procedures, they've been developed for accurate level runs, usually along roads/railroads, I can say from experience that they can become almost impossible in broken/steep country.
"We have only one control BM (elev.=0 m.) outside the area involved by the landslide."
Considering what you are doing at least 2 or 3 verifiable benchmarks should be utilized. And take pictures of them, aka documentation.?ÿ
As others have said it is important to have at least two other points near the bench mark so you can check on its stability.
Our procedures, used for over 30 years, are to:
1) Place suitable change points (stud drilled in concrete, paving, peg with stud in top to act as change point). Tape out AND mark each instrument position, keeping a running check on distances and adjust spacing as necessary so that total foresights equals total backsights. Keep individual sights down to about 30 metres.
2) Set these so sightline is always at least 50cm. above ground level.
3) Using two invar staves label one A and the other B.
4) For change point observations always read staff A first, followed immediately by staff B. Intermediate readings are taken either all before or all after the change point readings. This way you nearly eliminate any settlement which might occur at the instrument as such settlement will effect fore and back sights alternately. It takes a little getting used to - better if staff men wear something to distinguish between them!
5) If running closed loops then the SAME staff MUST start and finish - same applies to links between different runs. Each individual ground point must always be occupied by the same staff, just in case there is some minor variation between the staves.
5) When staff man is not being read the stave is kept off the ground - resting on boot. If in doubt, wipe staff base before reading.
As LOYAL said above, always use exactly the same procedure each time - if possible that applies to time of day and break times as well.