Yes, IF the instrument is out of adjustment and IF the legs are not balanced and IF you use the same leg distances for each measurement, then there is a problem. That suggests three ways to make it not a problem.
Turning point choice is irrelevant to this issue. What is important if you want to catch an out-of-adjustment level is the sight distances, whether those are on the same or different turning points for the second setup. Carefully balance the sights or randomize them.
The benefit of using the same TPs is localizing a blunder or serious error to pair of TPs, and that could outweigh the hassle of randomizing the sight distances.
3 pattern, 1/2 cm rods. Love em.
There??s no shortage of documentation on procedures.
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.
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.
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.
+1. It's usually large vertical changes that are difficult to properly spirit level because of shooting low on the rod on the uphill leg then shooting high on the rod downhill rod which introduces atmospheric changes dependent on weather. The key, short sights, a carefully calibrated level and doing the whole run in stable weather conditions.
Seems legit to me.?ÿ a little extra care in reading and recording saves A LOT of walking (time).?ÿ?ÿ?ÿ We did traverses like that in Alaska "diamond traverse" is what the BLM guys called it.?ÿ?ÿ Saved a lot of effort.?ÿ 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.?ÿ
Thanks for all the answers, I've learned a couple of things out of this thread.
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...
David, although this works, (single setup, double data), when you work with field hands, that may not have slept a lot the night before, there is merit to the idea of running all the way down, and all the way back. I worked on a crew in Chicago, that required a completely linear level run.?ÿ A city crew had tried the method above, and had a large blunder. (In the 3 feet range), on a many million dollar project. They told us, "we can wash in a tenth of error real easy, but a couple of feet, is going to cost in insurance, reputation, and the persons paying, are not going to hire us, to find and fix it".
Redundancy is the word, and a value system, that never fails, is indispensable.
Anyway, you can do both. (Single setup, double run), combined with a closed loop.
Nate