Hi guys
I just finished a small job (probably the last one for some time) and I didn't get great closure. The misclosure was 16 seconds which resulted in 7mm.
The traverse was around the outside of a house and through it. One of the legs was shortish (17 metres/51 feet) and two of the legs inside the house were very short (both 6 metres/18 feet). These two short legs were unavoidable.
The total traverse length was 109 metres.
The traverse accuracy was 1 in 15,208.
I would have expected to have got about 1 in 30,000
I use Leica rotating prism carriers with optical plumits. I rotate them through 90 degrees when I set over a nail to check they are centred. I check the setup when take it down. My total station is a 3 second Trimble S7.
I'm confident my fieldwork was good. This has never happened to me before!! I normally get really good accuracies like 1 in 50,000 or more.
I've checked my prism constants - no errors there.
I'm wondering if it is a massive problem, if two of the legs are very short this sort of thing is bound to happen right?
What do you guys think?
The survey was of an empty property that is going to be knocked down. The architect wanted to know the outline of the external walls to establish the size of the site. I imagine an accuracy of 25mm would be okay.
Will my misclosure of 7mm mean I can still fulfill this accuracy.
Space was tight inside the house, hence the short legs - putting some ties in was also out.
Cheers, Andrew
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If you can add some cross-ties for redundancy then run a Least Squares adjustment you will get better information about whether your survey is sufficiently accurate.
thanks for your reply Dave - space was tight inside the house, no chance of doing any cross ties
For such a short traverse incorporating a few very short legs, offhand I would say that is outstanding closure. You don't mention how many total legs the traverse has, which plays into how much error is allowed.
It's been a while since I have calculated a traverse error budget, and my copy of Adjustment Computations is at the office, where I ham forbidden from going. However, 16 seconds is a pretty small number for a traditional traverse with short legs. 7mm is 0.02 feet, which a lot of surveyors around here would not even bother adjusting. Personally, I would probably accept the closure as well within reason, adjust it, and move on. But as Dave suggested, if you can get a cross-tie or two in (unlikely if you are going through a house) that would help you analyze it.
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Just for reference, the old-ish FGCS Geodetic Control Standards?ÿ for third-order surveys give you an allowable angular closure of 10 (or 12) multiplied by the square root of the number of angles in the traverse. If you have the bare minimum of three angles in the traverse, that's either 17 or 21 seconds allowable misclosure.
Hi Rover83
There were 5 legs in the traverse. So that means that 25 secs are allowed. I had 16 secs.
Cheers, Andrew
I??d adjust it and call it good, the error is small but because the traverse is small, it makes the closure not so good.
For those metrically challenged 7mm = 0.025'.
Your simple loop closure calc doesn't tell the whole story. The effect of centering errors on those short legs are the thing. I suspect that a least squares report on your data would pass with flying colors.
Relax.
And think about getting an LS adjustment package of some sort.
Agrre with above....small traverse, no problem.
If you laid out 100 feet and were off 0.01, your error is 1/10,000, but it's still only 1/8 inch.
Plenty accurate for the task.
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AJF
Remember the mission. If this is for an architect and the building is to be torn down, that's one thing. The framing of the building is not going to be within 1/8". If your traverse closes without any significant blunders, you're good. Now were you laying out a particle accelerator, that might be a different story.?ÿ
Just because I'm paranoid, doesn't mean they aren't out to get me.
I would not adjust it like a normal project, I would carefully look at my outside traverse and call it good, then adjust the inside traverse to best fit the outside, easy, peasy.
For a little more work adjust the whole thing but make sure all points have allowance for setup, sighting and angular errors. The two inside the house traverse points having a much higher error budget.
Paul in PA
@fobos8 don’t take this the wrong way, but is this your first day surveying? 7mm closure with several instrument occupations can mean less than 2mm per occupation. Sounds like you’re on a construction project, what are the requirements of the project? I highly doubt the accuracy needed is less than a cm. You’re on a construction job, you’re battling mm? Sounds like one of my first party chiefs, checking and rechecking cross ties to find mm! That’s crazy.
Thank you for posting this problem. The advice you gain from here is valuable and probably applicable in the grand scheme of things.?ÿ And it made me think for a minute, which took my mind off of other less complicated issues.?ÿ
You need to consider what and how the answer that you give your client is going to be framed and ultimately used.?ÿ In my sand box, these areas will be rounded to the nearest square foot or ten square feet and used to calculate a percentage of the total property.?ÿ This is for determining density or existing conditions that are exempt from some form of regulation.?ÿ I cant imagine that in my wildest estimation for a building that is going to torn down, that the error in your traverse, unless it can be considered a blunder, which it isn't, is going to matter.?ÿ ?ÿ
Many thanks guys for all your replies. Your right Daniel the advice is valuable and very much appreciated.
stay safe and lets flatten the curve, Andrew
keep in mind you can minimize the angular error between shorter traverse stations by averaging more angle observations