Hi,
I'm fairly new to the surveying world, and i'm no full-fledged surveyor.
But I do some surveying in tunnels for my company.
So my question is, how are you guys setting your fixed points ?
I realize there are a lot of ways to achieve this. So I've come to believe that using 'Set of Angles' is the best way.
Also, what is the difference from using the different methods of sets ie: A'A''B'B'', A'B'A''B'', A'B'C'D etc.
What kind of instrument?
Do you mean Face A, Face B?
N
The mean of sets of angles is definitely the more precise method than a single angle. A "set" being a pair of angles, one measured with the scope in "direct", or normal position (aka Face Right) and one being with the telescope in the reverse position (aka Face Left) . It is always a good idea to collect at least one set when running control.
As far as the order the various sights are taken in it doesn't make any difference in theory. But you would typically do BD,BR, FR,FD if you had only one target rod. The trouble with that is that there is a significant passage of time between making your backsights and your foresights, during which time something might happen to the setup - if nothing more than a change in environmental conditions.
If you have separate backsight and foresight tripods or rods you would likely want to do BD,FD,FR,BR. That way not much time passes between your BS's and FS's that form each angle.
The sequence BD,FD,BR,FR was the common way to do it with mechanical (ie/non-electronic) instruments. It was easier to do the arithmetic needed to find the mean angle. So some people might still be doing that because that's how they, like their grandfathers, have always done it.?ÿ?ÿ
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Actually it depends on what kind of tunnels are you doing? For pipejacking tunnels, we have controls on top of each shaft and the TBM uses gyroscope for directions. We will still check it once a week especially before any 'tight' curves.?ÿ
For larger tunnels, we either place the fixed points on the top of the tunnel and use a vertical plummet to set up over it. It is hard to place any marks on the group or even those brackets on the side of the tunnels as the carts and machinery are always driving and hitting into them.?ÿ
Yes we use Set of Angles for our controls, because we measure to a few foresights and set of angles allow us to do that with extra sets in them. The different method AABB ABAB ABBA doesn't really affect the accuracies of the angle sets. It only helps if say during the measurements you know that there will be a cart coming in and you try to time your station to measure when the cart has past a particular prism. This will save time instead of waiting for a timeout because it couldn't find the prism.?ÿ
Pardon me for saying this. If you are new to the surveying world, but you are doing tunnel surveying, you must have a very good survey manager if not its going to be hell. Not sure about your tunnels but here standards and accuracies are really tight for tunnel alignment and it is already hell for experienced people here as there are million of reasons for the TBM to drift. But nothing beats the feeling after 20km of tunneling and the TBM broke through within 20mm tolerance of your design.?ÿ
