Hi everyone. I am currently having a little bit of an argument with one of our more experienced field crew members. He's been surveying for 15+ years and in my opinion has some bad habits that he refuses to unlearn. One of these is his firm belief that resections should and I quote "only be done when there are no other options" We are currently working a very very busy construction site with more equipment and machines blocking our line of site than I can describe. This just so happens to be our busiest part of the project, we are both crew chiefs however I am the lead in this project. He is very stubborn and more than likely upset that a 30 year old is trying to tell him how to do his job. I normally would not tell anyone how to do their job as long as they were not screwing anything up however his method of setting on the same two points repeatedly and setting points off this setup if something is in his line of site is starting to just get silly and I believe he is introducing more error into what he is laying out than needed. Behind the project is a large ridge that we have control on (traversed control.) For the past two months I set two backsites on whichever points will give me the best angle and then run my resection and lay out whatever it is I'm laying out. 90% of my layout work is foundations and anchor bolts. I do this for many reasons and will list them. I would just like anyone's input on if any of my theories are flawed because I'm trying to really figure out why he's so against them.
1. With resections you can set up right next to what you're laying out
2. You are establishing control while at the same time setting up for your layout, saving time.
3. With you being so close to what you're laying out the error decreases. When I stake out my backsite which is 900' away after an hour and see an error of .04' I know that that error is much less on what I'm laying out because of how close I am.
4. Easier to re-level the transit when it's sitting 20' away from you.
5. Resections help to average out error in setups.
6. We get a lot of different tasks throughout the day usually I lay out a foundation with 40 anchor bolts in about an hour and then get a call to do the same thing 150' away from where I'm at. This allows me to pick up by transit, move to where I need to be, turn the glass on my backsites if need be and run a new resection.
With all of these advantages I just don't understand his thinking. I do (and not bragging here) but at least double the work he does throughout the day because of this method and it's actually starting to make me mad because part of me thinks he's doing it on purpose so he doesn't have to work as much. Half his day is figuring out control dilemmas where I never have that problem. The way this site is setup is we have a fixed GPS base and are running a local coordinate system. The GPS is calibrated to our traverse so everything jives well. Whenever I stake out one of my control points of something I've laid out from a resection it always hits tight. I guess I'm just looking for others opinions on this issue and to hopefully gain a little insight on why he may be so against them.
Thanks
Resections can be a great solution or a disaster. I use them any time i have good control and can get an angle between about 85 and 275. I prefer to have three stations available, but may settle for 2 with a check from anything i set / tie at the resected station. This assumes i am calculating and adjusting the free point by hand or with proven field software.
Unless you are doing things with unrealistic error budgets there is nothing wrong with resecting with any modern equipment.
My 2 bobs worth.
Not entirely sure of your operations, so keep that in mind.
900 feet (270 metres) is for me way to far to expect elevation control. 2 way, almost simultaneous observations from both ends still stretching it, but depends on conditions.
Clear cold air okay, not warm hazy summer air.
I do like 3 points as it throws in another dimension that has to be accounted for.
However I do a lot of 2 point setups but with checks to something from a previous setup. Something I'd not call 'control' but if I agree then assume a level of certainty to my observations.
Personally I do frequent backsight checks. I'd suggest 1 hour is too long.
Resections are for me a normal way of life and I'd never refute a well done one with the normal checks.
As you've no doubt discovered they can be extremely accurate and time saving.
Maybe demonstrate to this bloke from some real examples.
i agree with the elevation.. the way I personally do my layout is if elevation is critical (top of concrete and anchor bolt elevation at this stage in the project) we pull off a bench with our digital level. If time is of the essence I will settle for benching in with my mini prism, I've found this to be a pretty reliable and quick way to set good elevations.
I resect all the time.
On my construction sites or lots where we are building new houses, I put control points all around on curbs etc at different distances and angles for future use.
As long as you make sure your angles are strong and it solves right on the money.... I have no objections.
Old timers don't like it, but when I run control and then resect later on and 'stake' a third point elsewhere I'm right on.
I'll throw in also, I'm probably past an 'old timer' and way past 15 years 'at it'.
Not sure that's the problem? I've seen young ones who 'know it all' and won't be convinced on other ways.
Maybe need to discover the chink in the armour of his mindset.
From my early days of "surveying in nappies" , I soon learnt the benefits of resection, and it was just another tool in our arsenal of choices.
I normally wouldn't tell him how to setup but everyday the site gets smaller and the time it takes him to complete a task gets larger.
I just don't think he's ever had to rely on it in the past so he's not used to doing them. He just has a way of making a really convincing argument out of no factual evidence. Everything I've seen and done and tested points directly in the opposite direction of what he's saying though.
I would say over 95% of my setups are resection although I am partial to retro targets over tripods and glass
elkboarder, post: 421679, member: 12485 wrote: ...his firm belief that resections should and I quote "only be done when there are no other options"
you've either neglected to mention the reasons/maths behind his beliefs, or he doesn't have any. You should ask him directly for them. If he can't give any then he's got no (or a bad) basis for his belief and it can probably be ignored.
I guess I'm just looking for others opinions on this issue and to hopefully gain a little insight on why he may be so against them.
I think you've got to press old mate for his reasons, which should be based on principles of measurement and errors. Resections have helped to lay out many a project with tight tolerances.
elkboarder, post: 421689, member: 12485 wrote: I just don't think he's ever had to rely on it in the past so he's not used to doing them. He just has a way of making a really convincing argument out of no factual evidence. Everything I've seen and done and tested points directly in the opposite direction of what he's saying though.
These sort of situations, i.e. arguments about uncertainties inherent in different ways of skinning cats, are easy to deal with if you are using least squares adjustment software such as Star*Net to maintain the project control.
The way that would work is:
- project control is surveyed and adjusted via LSA (depending upon the project requirements, the initial control network should be so well surveyed that the point of diminishing returns has been passed - new measurements do not have any significant effect upon the adjusted coordinates of project control points),
- all supplemental control points are added to original network and adusted as needed by simply adding the measurements by which the new points were positioned to the original network,
- all resection points or free stations are carried as network points and the measurements by which they were positioned are atted to the original network.
- all stakeout measurements from project control points are added to network,
That way, the size of the uncertainties in both the resection points and the points staked out from them can be easily calculated by the LSA software. Obviously, point nomenclature would be critical to avoid duplicating point names on non-identical points, but otherwise, it would be a breeze to document the quality of results by different methods.
Resect from good control. Is fine.
Resect from poor, or unknown quality control...is bad.
N
The most important thing to remember is that resections are only as good as the control they are shot from. As long as you have good, tight control to start with, there shouldn't be any problems as long as you use good procedures. That is, turn multiple angles for your resection and use stationary prisms that are property aimed, especially if using "0" offset prisms. And as always, instrument and tribrachs must be properly adjusted. Of course, all of these things are true when occupying the existing control as well, so it really just comes down to proper procedures.
I have been using resection often, for stakeout control and for settlement monitoring, for the past year. I works very well. I'm able to repeat ties down to the hundreth.
I'm not sure that I would trust every field hand to get it right. It's quite possible to get a lousy solution if the geometry isn't good - even when the control is tight.
I also use resections on a daily basis
But ALWAYS with at least three points and good geometry
jim.cox, post: 421699, member: 93 wrote: I also use resections on a daily basis
But ALWAYS with at least three points and good geometry
I'm much easier to please since I'll settle for two points as long as the angles and distances are measured at least twice on each face of the instrument and the residuals are within expectation. On the other hand, if the setting out is critical and there is some question about the stability of the control points, a resection using three or more points will answer any questions.
If you're going to be processing the setting out measurements in the general adjustment, one of those third or fourth points could as easily be a point set out from another setup.
C Billingsley, post: 421697, member: 1965 wrote: That is, turn multiple angles for your resection and use stationary prisms that are property aimed, especially if using "0" offset prisms.
Be very careful about stick-on targets, since although you can get a signal back from quite an oblique viewing angle the distance may not be to the centre of the target.
All the targets need to be very stable - getting low residuals from two of them may not be enough if they have both moved together. I had one insurance job where the structural consultant insisted there was very little movement on the building, up to the time it fell into the river. He was using resection from points stuck on the building (yes, I haven't mistyped that!) and an adjacent street lighting post which moved along with the main structure.
I traversed in from a few hundred yards away to be sure of not being influenced by the site movement. Yes, it took longer and there was some ground movement going on, but it was easier to identify and the outer two points kept a decent almost undisturbed reference line.
http://www.chroniclelive.co.uk/news/north-east-news/homes-spencer-court-newburn-demolished-5765294
Your degree of error in a resection is only as good as it is when you stay within the distances you shoot for the resection and will increase greatly when you exceed them.
With any construction site, your number of control points must grow with the job because of obstructions being built.
Shoot as many known control points as possible and other known points to be able to calculate a mean of your resection points.
Sometimes it's hard to teach old dogs new tricks. I'm guilty of this to I know. I use resection quite a bit too. Will always make sure I can triangulate between three and four points. The more the better. It's also become standard practice for me to set hard control points on the perimeter of the site to hopefully insure they don't get disturbed. I like storm boxes. Sometimes you have know choice but to go with only 2 control points to turn into tho. In this case I usually run a translation and rotation between two points then trav. To another point to check in.
people seem to be omitting the fact that traversing is only as good as the control you come off too. Why is this only mentioned about resections?
I would argue that resections involving three or more points may start to give you a better solution on-site due to the statistical model involved in a least squares solution. If the instrument observations are of a high enough quality then many-point resections can serve to roughly test the control, better than a backsight distance difference alone. And a robust free station solution may be even better again where defective control or observations are present.
