Why not turn your angles from both directions, BS to FS, FS to BS and adjust the angles to 360°, the full circle. Should do this on all angles of any traverse. Turn as many sets as you think necessary.
I would take the shot .Surveyors love to make there job more complicated than it really is. We use to be the experts at measurements now [sarcasm]Its so easy a engineer can do it. [/sarcasm]
> If you take the error in a 10' backsight and force it into a 1000' foresight, you create 10 times the error. This is also the reason to keep the number of points in your traverse to a minimum. If you have a .04' error in each point, the less points the less error.B-)
Aloha, Radar; not knowing why I always try to find way the number of point at minimum. I guess I didn't want to setup to many times. Now I know the reason.
thanks
Here is a trick question
> If you traverse around a mile (20,000') with 40 points and miss the closing distance by 1', is this good enough?
>
> 1:20,000 closure.
>
> :snarky:
> Radar
Aloha, Radar: I actually have no idea! To what benchmark am I suppose to refer to, to conclude this is good enough? I remember reading that there few orders of surveying. So I guess it depends on the requirement... That said my gut is telling me that it is a lot to be one foot away from where I suppose to be...
That brings up a question...what kind of accuracy should I uphold as a standard and strive to achieve? Thanks!
> In our hills, we try to keep our hubs at a minimum of 200'. Sometimes it doesn't work. If it's close to 200 or less, the rule is to turn the angles to a plumb bob string. I know, it's archaic, but it works because it's WAY skinnier than the 0.1' wide range pole. Robotics really doesn't care as long as your instrument and pole are in good adjustment.
>
Kris, thanks! Knowing 200' minimun that I need to strive for is a very guideline for me as a novice.
> However, sometimes, you just gotta break all the rules. We were running a 9 mile loop one time for O&G. At the end of the second mile, I had a crazy turn with a 300' backsight to a 127' foresight. Shouldn't be an issue right? Well, the next foresight was 2800' down a pipeline from a 127' backsight. So, we turned the 127' foresight down with 8 sets, so we really could see what the angle was. Then, we moved ahead, and I sent the rodman to the next foresight 2800' down the pipeline. I stacked the plumb bob string in a lath and stayed there to watch it while the instrument man then turned 8 more sets from the short backsight to the long foresight. From there, every sight was 2500 feet or so and fairly balanced, except for a 5900' shot down the state line from a 3200' backsight. 🙂
>
Wow! That is impressive to be brave to do that confidently! Earlier you said "robotics really doesn't care" I take it if you use robotic don't worry about the plum bob method. Or when it comes something like what you've done with 127' backsight you'd switch from robotic to manual mode and turn your angle to a plum bob string?
> So a nine mile loop, with a real possibility for a problem only 2 miles in, turned out well and we closed the loop within a half a foot, which for what we were doing, and where we had come from, was WAY more than acceptable.
>
What is your rule of thumb i.e. what you consider as very good closure that is achievable with a 3" gun?
> My point is, if you're gonna break the rules, make up new rules to compensate for the rules you just broke. 🙂 Check check and recheck. When you think you've got it, check it one more time. 🙂
Got it!:-)
> Make traverse lines as long as possible for the job you're doing. It doesn't matter whether a backsight is shorter than a foresight or vice verse. That is to say, if you have a setup where one of the targets is longer than the other, it doesn't matter which one you sight first. it will be the same angular configuration either way.
>
Aloha, Tom: I a bit confused here. I obviously not understanding it clearly. I thought from several of the comments earlier that the error from the short backsight will be increased with long foresight. If I ignore the procedure of keeping the balance wouldn't it impact my traverse closure?
> When setting out a point at a particular angle and distance, it is good to have a backsight that is longer than the point you are setting.
Okay, this is clear!
Thanks!!
> Ideally you will want balanced sight distances. In real world application, that is not always possible, so you have to address those times. Kris described a great procedure in his post.
>
> Even with modern instruments it is useful to balance angles. Even though you may not notice the reason as much as with older equipment.
>
> When you are setting the instrument on the back sight, you have some amount of angular error in sighting the target. If the sight distances are balanced (barring other sources of error), you carry that same amount of error through to the next station. If you sight farther away to the fore sight, the error increases in a proportional triangle.
>
> With careful procedures, in the real world, you are likely talking hundredths of a foot. However, you can see that the error increases as you move farther away from the source of the bad angle.
>
> One reason I prefer a longer back sight distance is that there is a sweet spot where aligning the scope cross hairs with your target (pole or string) involves less guesstimation than if the target is very close. Of course if the site line is too long, you can go past that sweet spot.
>
> In my opinion, balancing the sight distances perfectly is much less important than choosing the proper target for sighting. Again, as Kris Morgan points out it is often wise to break out the old plumb bob as needed.
Aloha, Jon: Thank you for reaffirming what Kris said. Are using robotic mode or you operate from the instrument? If you are using robotic would you switch to plumb bob method?
Thanks
Back2Basics: Long BS & Short FS Leveling
> Now, if you want to put the robot up for a minute and break out a level rod and auto level, now balancing stuff really begins to be more important. Why? Well, if you can keep your backsights and foresights balanced, then you do not need to worry about curvature of the earth.
>
Aloha, Kris: Now you are talking carrying elevation right? By the way do I need to worry about curvature when working with my 5603?
> For those ULTRA-RARE moments where we need a level loop now, and I tend to step them off ahead of time and look at the relief. So, if I'm gonna get short on a turn, I'll probably begin slowing transitioning the turns shorter and shorter so that it's not a big deal and then as I can, slowly make the turns longer and longer, out to about 250'. That's about as far as I wanna turn with a level.
>
You have to go 9 miles while setting up every 250? Or you normally carry the elevation with your total station for that distance and pull out your level for critical work only? I was told by one of the local surveyor "if your want a good elevation use digital level and not trigonometry level from your total station." What are the governing factors when to use what?
> Some people don't make the distinction between the two. I suspect that they were blanket rules for crews even though the surveyor knew when to cheat and how to cheat, the crew didn't and some of the archaic rules have carried on to this day. At least that's my take on some old procedures with new equipment.
Thanks!
> Here is the real trick question which might help you visualize the mathematics of the problem. If you are measuring angles between existing points, does it matter if you shoot the long line first then the short line, or is just as accurate to shoot the short line first then the long line.
Aloha, Bruce:
Let me take the bait:-D
I don't think it matters because I have the angles already. If it is wrong I will know right away. But when traversing I will only know when I close the traverse or if I find another know point to verify it.
How did I do?:-|
Called closing the horizon. BS, FS, BS flip BS, FS, BS = One set. Can have multiple FS if needed.
jud
OMG!!!
This was one of my biggest peeves.
Early in my career we had a simple angle to turn, two legs.
I set up the gun and for whatever reason backsighted on the "short" leg.
The PC went berserk.
"ALWAYS BS on the Long leg!!"
When I asked what possible difference it would make there flowed a bunch of baloney.
None of which justified the point of view.
I have never heard of a solid reason.
And have met many guys who rabidly insisted that you always sight the Long leg.
Never a good logical reason.
I don't even think it would make a difference with directional instruments, like a T2, in setups with multiple FS.
You are using the mathematical differences in the several directions.
There's a difference in measuring the angles and using the measured angles in later calculations.
i.e. Using a short leg as a baseline for calculating coordinates and "orienting" your traverse.
That is a BAD idea.
God, there's a bunch of old wives tales like this.
P.S. I like the idea mentioned above (RADU) of using a natural or setting a distant target on a short leg to, ugh, "preserve aiming consistency" on a pair of legs sharing a short component.
Again, always had a hard time convincing coworkers of this technique.
"You MUST sight the actual point!"
Once had to sight a brass tack 3ft away. No kidding.
Picked a "natural" of a quartz grain in the concrete next to the tack.
PC had been surveying for 30 yrs.
In traversing between existing points it makes no difference which line you shoot first. Let me illustrate by saying you can shoot the long line perfectly, right to the second. The short line, because of heat waves rising off the pavement, has an uncertainty of 30" on each pointing. So, no matter which you shoot first, the angle between them has an uncertainty of 30". Long then short, or short then long, the uncertainty will always be 30".
This does not apply to layout work, where you always want a decent backsight.
> > @yswami: To intuitively grasp the implications of short and unbalanced sights in meatspace, you can make three points on paper along a good straightedge (like a scale or drafting triangle) to represent unbalanced instrument stations,
> >
> >
> > ___V________V________________________________V____
> >
> >
> >
> > then use any two points to reestablish the third point precisely and accurately with the straightedge after moving it from its original position.
> >
> >
> > 😀
>
> :'(
Maybe it would help if the swami used pins instead of pencil marks? :whistle:
sighting the actual target and picking a "Natural" farther away can be deceiving. The problem is, if you can only (as example) sight the close target to +/-15", and if your first sighting of the target is 10" off of "true". When you lift the scope and find your "natural" @ 10" off, you will have a consistent set of angles that appear to be within (for instance) 2" but are all about 10" off of true in the same direction. If you sight your real target as precisely as you can, and you show a deviation of +/-15", your mean will be closer to "true" and your least squares weighting can apply a more precise standard deviation to that set of angles. The other way, you will have a certain 10" of misclosure that is less clear where that error could be based on the data.
:good: :good:
so, that 30" error is the same on a short bs/fs as long one? I tend to think differently, myself.
30" at 100' = 0.0145', not to shabby and certainly worth the shot.
30" at 1000' = 0.145' certainly not what I'm looking for in a traverse.
So, if I backsight my 100' and turn 1000' down the road I have introduced a larger unknown than if I back sighted my 1000' line and then turned to my side shot 100' away...
Maybe I'm whacko and someone can clear this up for me.
:good: :good: 🙂
> so, that 30" error is the same on a short bs/fs as long one? I tend to think differently, myself.
>
> 30" at 100' = 0.0145', not to shabby and certainly worth the shot.
>
> 30" at 1000' = 0.145' certainly not what I'm looking for in a traverse.
>
> So, if I backsight my 100' and turn 1000' down the road I have introduced a larger unknown than if I back sighted my 1000' line and then turned to my side shot 100' away...
>
> Maybe I'm whacko and someone can clear this up for me.
You made the 100' shot a side shot. That is good. Nothing to clear up there. But I hadn't heard about it as a sideshot until now. If the 100' shot was a leg of the closed traverse, then the error in the 100' shot would be incorporated in the traverse along with whatever angular error that introduces. That error is there based on it being a foresight in the traverse as well as being a backsight in the traverse.
Not if you are measuring between two points. The instrument has no idea what point you sighted first, it just reports the angle. Staking out points then you are correct, you will multiply your sighting error as you go farther than the back sight. When I traverse I always have my crews wrap angles to the right regardless of the bs distance. Lots of guys would have problems with this procedure until I would have them peg book the angle right (short to long) them have them turn the angle left (long to short) and omg it would be the same +- the accuracy of the instrument. My 2 cents, Jp
Aloha, After reading all these posts...I think I am bit more confuse than I was:'(
One thing that I am reading in many posts are that if I am setting a point then I need to have longer backsight. However, it doesn't matter if I am traversing. Is this correct? So the accuracy of the point set temporarily for traversing is not important? Wouldn't it cause trouble at the end of the traverse?
Another question (I asked earlier in the thread but I notice the way the posts in the thread are structured, reply to post can easily buried somewhere).
Let say you are traversing to setup your control points on a job what kind of error tolerance you would set as a standard? Both in terms of angle and distance.
Thanks!