Maybe try trigonometric leveling or GPS readings, RTK, or static to transfer elevations? Try them both and compare differences. Whatever error you get from them will not be worse than doing the level run across unstable footings.
Remember, you can drown in two inches of water..........................................or mud.
@chris-mills?ÿ Yep, a load spreading jig for the level legs is a big help in unstable soils.?ÿ Tread lightly around the instrument during observations and don't touch the legs ever once setup.?ÿ More serious efforts involve how to handle the turning points;?ÿ either a long turning point pin driven to refusal or if not possible a 3/4"plywood plate two feet in diameter with a pin in the middle, and the rodman spreads his feet far from the plate and stays still until the backsight observation is done.?ÿ Turtles don't work, they'll subside because of their weight.
Key is levelling doesn't care if torsion rotates the level or lateral movement of the level mislocates it by a few hundredths, it can be releveled w/ sub/mm accuracy; only subsidence or upheaval during the observation sequence between observations screws things up, so work quickly to avoid such problems.?ÿ An alarming trend I note in other posts is the assumption that traversing vs. levelling require the same non-vertical location stability for the instrument & turning point, not so.?ÿ You're just trying to elevate benchmarks, not locate them.
Maybe somebody can chime in here and explain it better than I have tried to.?ÿ BTW I'll assert that traditional precise levelling even today in small arenas (+- a few miles) beats GPS and 3D trig Total station surveys by an order of magnitude.
Also ask yourself, if someone else is going to check your work, will they go through the same process? get the same errors? get worse results?
?ÿ
Maybe try trigonometric leveling or GPS readings, RTK, or static to transfer elevations? Try them both and compare differences. Whatever error you get from them will not be worse than doing the level run across unstable footings.
Trig levels require the same short legs (300' max?) to reach Class 2 specs & of course GPS et. al. is only good to an inch or two in short distances less than a few miles because they observe geometric heights. not orthometric and require geoid corrections to be "correct" and there's a lot of slop in the vertical because the Sats are overhead with none underground to precisely fix elevation.
The key is to meet your client's specs and if an inch or two is good enough then GPS is fine, but if they're looking for millimeter accuracy like at observatories, dams, military drydocks, whip out your level and?ÿ do the job.
@mike-marks?ÿ Where did you come up with "Trig levels require the same short legs (300' max?) to reach Class 2 specs" ????
JOHN NOLTON
@mike-marks?ÿ Where did you come up with "Trig levels require the same short legs (300' max?) to reach Class 2 specs" ????
JOHN NOLTON
Reply hazy, as it was 21 years ago.?ÿ I've looked up the standards of a nearby State DOT and they state 500' max for 3rd order trig levelling.?ÿ Trig not allowed for 2nd order levelling.
Here's a nugget from TxDOT:?ÿ "The establishment of vertical control using the total station or theodolite is not recommended. The slightest variance of vertical angle on the instrument is amplified over the long distances normally associated with trigonometric leveling."?ÿ My recollection is we were trying to tighten up trig levelling by limiting sight distances and got results that (usually) met 2nd order accuracy specs when validation levelling between higher order benchmarks, but at the time there were no procedural specs and of course "usually" is not good enough.
Glory be, CalTrans published 2nd order trig procedure specs a few years later, probably based on our experimentation:
Notice they wrote "site" when they meant "sight"????.
?ÿ
I have used Trig levels with great success and had my own procedures(1963)well before others did (NGS etc.)
6 years ago (2014) NGS came out with procedures (of which I made comments to them about but my comments
fell on deaf ears) on Trig leveling for "crossing rivers, valleys, highways, or other barriers".
This (NGS) will call it chapter (4.5) for the NOAA Manual NOS NGS 3.
This might be of interest to the posters here and can be found on the Internet. Please take a look and let me know what you
think.
?ÿ
JOHN NOLTON