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Leveling technique?  

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GeeOddMike
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For those interested in a different approach related to the thread’s subject see:

https://geodesy.noaa.gov/PUBS_LIB/NAVD.pdf   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.

5A2381C1 F38B 4A3A 9421 95BA3CFAD0CE

EFFC7A40 026F 48F5 99F4 720B8414701E

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(@larry-scott)
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3 pattern, 1/2 cm rods. Love em.

There’s no shortage of documentation on procedures.

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I'm a fan off double simultaneous leveling using 3 meter rods with the 301.555 centimeter offset between the left and right sides of the Invar strip.

I haven't needed them for years, and if anybody has a use for them just come to Pasadena, California and pick 'em up.

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(@chris-mills)
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I'd have posted earlier, but I was out using our pair! To get over the refraction bias we still generally do the run each way when we can. Probably more important is to minimise the instrument settlement (even on hard surfaces, where dust can still move from under the leg tip) by alternating fore and back sights as the first reading. ie. first set up BFFB, second set up FBBF, etc.

I've experimented with doing the same run by both methods and the difference is surprising: the standard method does consistently accumulate the odd 0.1mm and over twenty or so change points you can quite easily end up with a mm. adrift.

our pair gets used regularly, probably around 40 miles a year.

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Bill93
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I can see how refraction can cause a bias because your uphill line of sight is closer to the ground than the downhill one, and the refraction won't be the same.

But how does a two-way run correct for that any better than a double rod one-way run?

At best, the different times would give you two different temperature gradients, but they would tend to be similar.

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Andy J
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Seems legit to me.  a little extra care in reading and recording saves A LOT of walking (time).    We did traverses like that in Alaska "diamond traverse" is what the BLM guys called it.   Saved a lot of effort.  I don't think we ever had to re-run one because you knew it was critical to get the readings right the first time out there. 

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(@rover83)
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I heard a lot about the diamond traverse while living in AK. It is certainly faster than traversing out and back, but from an adjustment standpoint it is worse - takes the same number of observations that the traditional out-and-back method would have, and increases the number of unknown stations by 50%.

I preferred to have two independent, redundant setups on each traverse point. I get that the idea was to run down one "side" of the diamond and then back the other for closure, but is just as easy to use the first setup at each point for the out leg, and the second setup for the return leg. Same amount of time spent, fewer overall stations to compute, a check at every station, and a lot more degrees of freedom for adjustment...

 

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David Livingstone
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Thanks for all the answers, I've learned a couple of things out of this thread.

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