-
traverse closure question
I did a fun test for a friend this weekend that included topoing his 3 acres with feature identification. Pretty complex lot with 4 buildings. steep terrain, lots of retaining walls, sidewalks, several building pop outs, a cascading water feature with a bridge over it and about 1/3 of it heavy brush. Anyway, resected on 2 bounday corners and got 1/2″ difference from his record. From there I ended up having lots (hundreds) of side shots and 8 traverse hubs that I average multiple shots, flipping face. Most worked very well. On two of the hubs I was getting 0.073 or so horizontal mis targeting on the backsight/new setup. At the end I closed back on one of my original hubs and got very good vertical, but horizontal was misclosed by 0.75/0.82 NE. It’s a friend. For fun (and info). But in real life, if I were yet a surveyor, that would not be acceptable to me.
Question is: I’ve closed a few traverses by hand before (school) using compass, transit and learned about least squares – I don’t think we hand calced one that way though. I don’t think we addressed side shot adjustment in those. What’s the best way (or is there one) to adjust a traverse and have it make the side shot adjustments as well? I separated my side shot points by code to group each with a particular station setup.
Mahalo,
Dave
-
10 Replies
-
Anyway, resected on 2 bounday corners and got 1/2″ difference from his record.
Resection is not the way to go unless you’re using good control points. Compass Rule adjustment is what I used when using a total station. Side shots are rerun after a traverse is adjusted.
MH -
Anyway, resected on 2 bounday corners and got 1/2″ difference from his record.
Resection is not the way to go unless you’re using good control points. Compass Rule adjustment is what I used when using a total station. Side shots are rerun after a traverse is adjusted.
I agree with Field Dog, Compass Rule first, then SSs after adjustment. With that bad of traverse results, all if your side shots are as screwed up as the traverse, and you dont want to have to adjust all of your SSs from each setup.
-
With a traverse closure that bad I would investigate the source of the mis-closure prior to adjusting anything. One of three things is going on here, you are either mis-sighting your back sight (least likely), you have a bad control point (not likely as you state your check shots looked ok), or you are not using the correct prism constant. Prism constant seems to be most likely. Assuming you are using -30 mm prisms and the prism constant is set to zero you would have accumulated 0.8′ of error through your network of 8 hubs on your closing shot.
-
What’s the best way (or is there one) to adjust a traverse and have it make the side shot adjustments as well?
I assume that most least squares adjustment software does this, but I know that Star*Net does.
-
Good points. I did check my prism constants in the fiels when I saw that 1″ bs error, It was good, but your comment makes me want to go back and double check those two lines. Thanks.
dd -
Great. Besides the needing to know what’s not right, I’d definitely like to make those adjustments in the office than going back out for it. Thanks. I’ll take a look at StarNet.
dd -
There are many unknowns in your original post. We don’t know the configuration of your traverse, we don’t know how these 2 hubs that were 0.073′ out come to be, we don’t know what software or method you are using to do your compass rule adjustment. It seems likely that you have a bad angle at one or more points that have propagated itself through your traverse. If you draw a perpendicular to your “misclosure leg” it should point back at your bad angle.
Compass rule has the advantage of being relatively simple to understand and perform. And therefore easy to teach. Least Squares is much more complex math, and more difficult to teach. But it is vastly more robust. And luckily the computer program does the math so you don’t have to. If you have access to LS I can’t think of why you would want to continue to mess with compass rule. And I can’t think why anybody would attempt to run a surveying business without access to LS. The main reason for doing the LS analysis to trap errors like the one you have, and to know what the precision of your work is – not to try to tickle some imaginary precision out of poor data.
Like Jim I use StarNet and have for over 20 years now.
While it would be a simple matter to devise software that would compass rule adjust your traverse and then recalc the side shots from the adjusted control, most programs which do CR don’t.
-
A misclosure in the vicinity of a foot with modern equipment is not something you can adjust out. You have a busted measurement somewhere.
-
Too little info, post a plot of the traverse.
Like, how far apart were the two points that you used for reference and whether you closed into where you started or at the other end of the property into know locations and how long was the traverse and in what directions you traversed.
I’ve gone back to some of the same hubs thru the last 40+ years and find them differing as much as 0.10 frequently because of where they are they tend to move around due to soil type and travel over them by tractors, livestock and other forces of nature and man.
Had a pencil whipper chief to turn his closing angle with his Berger transit to a point 2.3ft away one time and his angle closure was 0?ø00′ for a traverse around a couple hundred acres. The entire staff went bonkers over that for awhile.
I know that your survey was “for fun” and during the chaining days we would correct a survey with a closure like that. In modern times after the addition of EDM, TS that is something that happens due to some drastic error somewhere or the introduction of error in a little bit everywhere.
good luck
-
As others mentioned, adjustment is for random error not systematic error or blunder. Also, not enough to close back on the hub, you have to set on it again and turn the final angle. Then balance angles to see if angular problem. You can only analyze that which is part of the loop, sideshots simply get calced after removing systematic error (such as prism constant problem) blunders (such as kicked the hub before backsighting) and adjusting random error of the loop. Sideshot coordinates will then be based on the corrected coordinates of the loop points, but there’s no check on whether you got correct angle and distance to individual sideshots unless you already had a coordinate for them to compare. The points you re-sected from are not part of the loop from what you describe, so wouldn’t have anything to do with the adjustment. If there’s a problem with the points you re-sected from the whole thing will just be off by whatever even if you get the loop to close perfectly, but the traverse analysis will not tell you one way or another.
Close the traverse by turning final angle. Then start checking angle and distance on each traverse hub till you find mistake (not error). Small site like that shouldn’t be too difficult. Large site least squares can help find where to start checking.
Log in to reply.