How to Adjust a Side Shot
We're running a traverse down a road in order to locate property corners, fences, wood utility poles, and the edge of pavement. The traverse points are nicely spaced at 600' to 700'. Unfortunately, we overlooked a property corner and had to set another point (side shot) in order to locate the corner. The side shot from a side shot scenario is definitely a no no. We don't want to incorporate the side shot into our traverse because the side shot is less than 200' away from the traverse point it was shot from. Are there any viable alternatives we should consider?
The side shot from a side shot scenario is definitely a no no.
Well, how tight do you really need things?
I can think of several solutions.
I have run tall stakes around 40 acres, with 15 setups. 0.08' and 5" angular error. Over and over.
Gimme a call if you want. I'm tired. My num is in my profile. What kind of equipment are you operating?
The best way to adjust a traverses, is to have so little error, that it does not matter.
Working tolerance is a good rule. Double angles to SS. And to the foresight, .
Ain't nobody perfect. Do it and rock on with your work
We don't want to incorporate the side shot into our traverse because the side shot is less than 200' away from the traverse point it was shot from.
I don't understand why this is an issue. As long as the sideshot measurement errors are properly characterized, you can incorporate the data into your adjustment without contaminating the main traverse.
What kind of equipment are you operating?
Topcon ES 105 5" total station. I would rather have a 3" instrument. Specifically, a Leica 3" instrument. The office has a 5" or better tolerance on doubled angles. Our BS/FS tribrachs are all old robotic system tribrachs. Geodimeter, I think.
The office has a 5" or better tolerance on doubled angles.
That may be a good rule for most angle measurements, but side shots tend to be short distances where, in the error budget, centering error far dominates over angles, making that spec unimportant.
At 40 ft, for instance, 5 seconds is 0.001 ft and I defy anyone to consistently center/plumb that well with the usual equipment, even if you can sight the same spot on the target to that accuracy.
At 200 ft as mentioned it would be 0.005 ft and consistent only with very very careful work.
I think you misunderstood my issue. The side shot to the property corner isn't the issue; it's that the side shot is from a side shot. My party chief doesn't want too many traverse points. That's why he designated this point as a side shot. I think it should be part of the traverse. An alternative method would be to put a point on line between two traverse points, then locate the property corner from that point. That's a lot of work.
I would use resection. Once the initial traverse is made and adjusted, fill-in points can be collected with great accuracy using resection. I believe this usually has a better accuracy potential than a spur traverse point because the occupation point does not have any setup error. This has been my experience anyway.
You are absolutely correct! I always keep this kind of thing in mind when shooting points under 200', especially when sighting a prism pole that's 0.12' wide. What's interesting too is that a distance shot from under 100' is not as accurate as chaining it. I don't know why that's true.
Resection is one of the potential alternatives I was thinking about before posting this topic. I assume you typically use 2-point resection. In my case there will be an adequate number of traverse points to resect to. We will be running a closed loop traverse along our project corridor, first to the east and then back to the west. The project corridor is a paved road with a 60' right-of-way.
It appears to me that Jim does have a handle on what you are doing. No worries.
Putting a points on line between 2 control points of a traverse was a valid tactic when we where using transits with 1' and theodolites with 20" least counts. Angles turned with such instruments just weren't super precise so you wanted to minimize the angles and rely on distances, which could be less of a problem. Also, the math - pounded out on a calculator - was easier. Nowadays with 3" least count instruments very common, automated multiple set turning, and computerized data reduction there isn't much, if anything, to be gained.
Setting a strategic control point off your main traverse, from which to tie some snookered boundary mark, is not a mortal sin. You do want the distance from your main traverse control point to the strategic point to be equal to or greater than the distance from the strategic point to the boundary mark (ie/ you want the backsight at the strategic point to be equal to or greater than the foresight to the boundary mark. And you want to tie the strategic point (stubbed out control) off your main traverse redundantly. That can be accomplished by:
- tying that strategic point from two different main control points, or
- by tying it off two different backsights if the strategic point can only be seen from on main control point. And if neither of those options are possible ....
- tie the strategic point, then make the strategic point you backsight and then retie your former backsight (aka "closing the horizon").
Then you tie the boundary mark from your strategic control in the usual fashion.
The fact is that if your instrument and accessories are in decent adjustment the split between these redundant ties is going to be trivial. What you are primarily doing is trapping blunders, which can be any magnitude. It is the blunders that get you in trouble.
I'm not sure whether you have access to StarNET or not, but one of its most underrated and underutilized features is the PreAnalysis function. If you have rough coordinates of the points you need to tie in, you can load those into a project, set all of your standard errors and instrument specs, and add however many theoretical observations you are considering making. Fix whatever control points you might have, run the PreAnalysis function and it will give you a very good estimate of how good final positions will be. Then you can add, remove, or switch around observations until you reach tolerance, or get the most bang for your buck (efficiency).
This might be a good way to demonstrate to coworkers exactly how one could ensure they meet tolerances on a project such as this, even when stubbing off a main traverse. Just a thought.
tie the strategic point, then make the strategic point you backsight and then retie your former backsight (aka "closing the horizon").
We did that yesterday.