So I had a fellow worker explain a leveling technique I never heard of.?ÿ Say you want to run a level circuit and close back on your starting point.?ÿ Instead of running the circuit one way and then running it back, you have two turns on each setup and only run the circuit one way.?ÿ
You would say set one turn with a screw driver, and then set a second turn next to it on say a hatchet.?ÿ Then you read both rods from the setup and move ahead. ?ÿIt was explained to me that this was the same as running the circuit down and back.?ÿ I've never heard of this before and in my mind its not the same running a closed loop but I'm always open for new ideas.?ÿ Opinions?
So you're running 2 separate forward runs and theoretically they would provide a check for each other instead of closing back. Might be crazy enough to work, but keeping 2 books sounds like more work than it may be worth.
Elephino, I havent used a level in years.?ÿ
I have done it that way before for long routes.?ÿ Special care needs to be taken to try and keep the numbers for the close stations to be different.?ÿ Best if they are on a different foot part of the rod or you may make the same mistake on each reading.?ÿ For instance, reading 4.98 and 4.92 on one pair of backsights when they are really 3.98 and 3.92.?ÿ You won't pick up the error in your closure at the end but it will be there...
Yeah, I've done that more than once in my rookie days. That big red number 4 saying 'read me'...
It's a great technique if you don't want to have to trek the route twice (nasty woods, swampy, brutal cold), and accomplishes the same thing. Two independent measurements for each station/setup. The instrument doesn't have to travel a physical loop to obtain the same data.
I used do this with traverses all the time (when I was actually in the field more often), and it goes especially fast with a 3-person crew. Get set up, turn your sets, break setup for instrument and BS/FS, turn another round of sets, then move up and repeat. You then have redundant measurements at each station. Run LSA to get final coordinates.
If it is absolutely required by the project I will compute a closure from the observations, but with redundant observations at each station (out-and-back) least squares is the way to go.
I have seen this technique before and it fails if the stations are not broken in height significantly, as said above.?ÿ Basically if the wrong foot is read once, it will be read that way again, more than likely.?ÿ We read electronically now, so, we aren't really subject to that.?ÿ That concern may have gone the way of the plumb bob.
It will also fail if the rodman lowers the middle section of the Philly rod while walking, and forgets to raise it again...
If only 15 foot high rods were not so unwieldy, we could solve that one
With this method, there is only one observation to the staring BM??ÿ?ÿ
If I'm only going to run one way, I run 3-wire, using high and low to check the direct reading.
Usually on starting and closing measurements, I would break setup and hit it a second time. Also trying to drop at least 0.5-1.0 to read a different part of the rod. I liked three wire too. Haven't done long level runs in years and going digital it takes out some of the human factor.
Always worked good for me. Just have your first benchmark be the second and then next to last shot. I usually start and end on a dummy point. You might also want to consider running a "chain". You run one or two turns then close back. Drive ahead then run one or two more and close out and repeat.?ÿ
You could always run one set in feet and the other in meters if you could figure a way to use two rods, or one with front-feet and back-meters.
If you have one rod, it??s a math check only. Which is pretty reliable with carefully balanced sights.?ÿ
In precise levels the technique is a two pattern rod : double simultaneous reciprocal.?ÿ
Running out and back has great benefit in accuracy. And setting nails or stakes for every turn and hitting them out and back is a very good practice. Averaging each segment.?ÿ
I vote for two setups of the level between each point and the next, making sure you read at least a half-foot different between setups.?ÿ That's essentially the same thing as doing a closed loop except you are doing the pieces in a different order.?ÿ
It's actually slightly better because you are using the same turning point(s) so can compare the two differences right away to look for blunders, and don't have to trust the turning points to be stable and undisturbed for as long a time as re-using them on the way back in a loop.
Three-wire and more setups are also options to get more averaging, but the above gets you a basic blunder check and simple average.
"setting nails or stakes for every turn and hitting them out and back is a very good practice."
I think using the same turning points out and back is a questionable practice, especially on short runs. And of course it's impossible to do so when using a turtle or turning pin.
I suspect the real reason is it makes it quicker to find busts and fix them without redoing the entire loop.
What makes it questionable?
With properly balanced sight length, and given that the HI will be different, but it may be a question of time.
Comparing elevation difference 1-2, outbound, 2-1 return run, isolates errors. Makes a 1 cm reading error easy to identify. On a long run, setting TBMs and running thru them out/back is std practice. Stakes/nails sort of like having more TBMs.
Dont see the compromise. Ive never used electronic leveling and data collection. Mostly used Jena and N3, 1/2 cm two pattern rods.
When using 2 setups per turn, imbalance had best be 0. Small +/- Imbalance needs to be randomized. And accumulated imbalance monitored.
also a return run at different time of day, or an hour later is important.?ÿ
For those interested in a different approach related to the thread??s subject see:
https://geodesy.noaa.gov/PUBS_LIB/NAVD.pdf ?ÿ Article 11 ??Survey Instrumentation and Procedures,? by Harold Beard. It includes a description of ??Double simultaneous leveling? used during leveling for NAVD88. While not mentioned in the article but alluded to by Larry Scott, there are uncorrectable refraction biases due the one-way run. The FGCS leveling specifications https://geodesy.noaa.gov/FGCS/tech_pub/Fgcsvert.v41.specs.pdf allow the method but with conditions.
For those who have never seen one, I attach a photo of the rod used.
Using a level that is reading above or below 90 zenith angle will accumulate error on unbalanced legs. Using the same turn points coming back with same unbalanced legs will compensate for the error generated on the run out. You could be 0.40' off on your leveling but still close flat to your turn points and initial BM. Running balanced legs with different turn points on the way back would reveal the error accumulated on the run out. Granted this situation is typically only encountered on longer runs with large vertical changes in the run and with a level that wasn't properly pegged to begin with. As always there are different benefits and risks associated with whatever techniques are employed.