why standards for precise leveling specify that stiff leg tripods should be used. Is there something serious with this spec or is it something that is just left over from days of old style equipment when extension leg tripods left a little to be desired.
I find it hard to believe that a modern extension leg tripod that will support and hold steady a robot can't hold a level steady.
This inquiring mind wants to know.
Stiff Leg Does Not Neccessarily Mean Single Piece Leg
Certain fibre legged tripods would be unacceptable.
I have noticed that when my beer belly comes in contact with certain top priced tripod legs that the leg will bow moving the instrument bubble. This has very little affect on a double compensation total station but does have an affect on a precise level.
Paul in PA
Stiff Leg Does Not Neccessarily Mean Single Piece Leg
A possibly more accessible link: wheat belly blog
Many Thanks, Half Bubble
"unattractive stomach bulges preventative cardiologist William Davis calls "wheat bellies." According to Davis, that excess fat has nothing to do with gluttony, sloth, or too much butter"
I was worried and thought I had to discontinue the gluttony, sloth and too much butter. I feel better already.
Paul in PA
Many Thanks, Half Bubble
I'm keeping my girlish figure on 2 ribeyes a day fried in butter.
We used to live on the fat of the land. Since WWII the agribusinesses and the life insurance companies invested in each other and figured out what to sell us to maximize their profits. Throw in the social control aspect of fat shaming -- of making everyone feel guilty for something that is not their gluttony or laziness, and the subsequent market for fat free or diet foods.
There is more to it: Possibly some bacteria similar to what is causing my autoimmune stuff is causing America to get fat regardless of diet or exercise.
http://www.aeonmagazine.com/being-human/david-berreby-obesity-era/
Apologies for the hijack, original poster.
OK - can we get back on track ...
with my original question - please?!
My guess would be to eliminate the chance of a mechanical failure (slippage) of the tripod.
I would also think the refraction correction at the level would be more consistently applied at every set up.
Also in my experience some folks would push the limits of the tripod to get a little more height; and therefore create a wobbly set up.
Of course these are just my educated guesses and opinions.
Maybe ask you NGS state advisor?
*your
The FGCS Standards and Specifications for Geodetic Control Networks does not mandate the use of stiff leg tripods for geodetic leveling. They are however highly recommended by NGS. The rational behind this and any number of other recommendations for geodetic field procedures is to eliminate or at least reduce as many sources of error as possible. Even the best slip leg tripods are subject to mechanical or human failure. When going to all the effort and expense of performing 1st- or 2nd-Order leveling this is a very small expense and has proven it's worth over many thousands of miles of level runs.
:good:
like less moving parts means less movement of the level. you eliminate at least 3moving points from the equation.
OK - I hate to tell "old guy" stories, but...
IMHO (and experience), single piece legs are subject to less thermal expansion and contraction than multiple (collapsible) piece legs, in the direct sun, anyway.
I used "stiff legged" tripods for probably the first seven or eight years of my career as an instrument operator. This included 12 pounds of brass known as a wye level mounted on a stiff legged hickory tripod (I still own it).
Depending on what job we were on, we use to leave the gun set up for extended periods. When we broke a set up, we always checked as a last shot. I never noticed any set up degradation until we bought a little Nikon auto-level with an aluminum collapsible tripod. The optics were great, but you couldn't leave that thing set up for more than 15 minutes without the H.I. going to hell.
My crusty old $0.02 anyway.
OK - I hate to tell "old guy" stories, but...
> .....but you couldn't leave that thing set up for more than 15 minutes without the H.I. going to hell.
A big part of precision levels is timing; if you don't complete your turn, backsight foresight readings, in a certain amount of time, you need to start over....
timed setups..
..fer sure. I was mainly talking about either topo or blue topping; when we would occupy a constant HI for a period of time.
On level loops we usually had two rodmen (one for the BS, one for the FS) to keep a 'good clip' rolling. 😉
My guess is that back in the day, NGS advertised to buy tripods for leveling. When the bids were analyzed, the fixed legged tripod was cheaper, and they decided that it would be more precise. They got good results, and if it ain't broke don't fix it. I if you are setting up to do precision leveling, the cost of a new fixed legged tripod is a lot less than the cost of having to re level a segment . When you have to re reu a segment, you will know that it was not caused by a leg slipping.
> :good: :good:
> Reduce as many potential sources of error as possible. I like it.
> Like hands-off instruments. Wireless DC; less on-board; less touching of the instrument that has been set up and leveled (plus it means I can sit in the truck on my fat okole and run the robot via the camera) :>
Aloha, thanks to Dave and everyone. This is a good educational post for beginners like me!
Gschrock, I didn't realize you all use the work "okole" in mainland! 😉 Unless you are somewhere in Hawaii??
Back in the 1980s we did a lot of local 2nd order geodetic leveling that was Bluebooked in metro New Orleans. I handled the office stuff & Al Hansen (NGS retired) handled the field stuff. We could not find stiff-leg tripods to purchase, so Al got a couple slip-leg tripods and re-fitted them with oak dowels to make them stiff-leg tripods. They still do just dandy.
A similar question is why balance backsight and foresight distances when performing precise leveling? My comments apply to both stiff-legged tripods and balancing distances.
First, several questions:
1. What are the consequences of violating the criteria?
2. What is the relationship between "cause/effect" and "correlation?"
From 1987 to 1996 I was responsible for monitoring elevations of 120+ points in and around a middle-school building (on several levels) that was built on a geologically unstable site. During "active" periods we leveled the entire network every several months. Other times the re-observations were about twice a year. There were about 12-15 interconnected loops in the network. Normal residuals rarely exceeded 0.3 mm and if, for any observing session, a loop misclosure exceeded 1 mm, we went looking for the reason.
We used a parallel-plate micrometer level, calibrated invar rods, a stiff-legged tripod, and an umbrella for instrument setups outside the building. In 1994 we switched to use of a precise digital level and calibrated invar scale rods. The quality of results remained consistent. My agreement with the structural engineer repsonsibile to the local school district for the integrity/safety of the building was that our survey would detect and document vertical movement of any monumented point more than 2 mm - total vertical movement at various points on the site have exceeded 0.5 feet since 1987.
We used a stiff-legged tripod but, due to physical contstraints, we could not balance all backsight/foresight distances. We did however, perform a C factor check at the begining and end of each observing session and a correction for the "inclination of the line of sight" was applied to all backsight/foresight observations.
Question - Was there a correlation between the number of times we went back to re-observe a particular point and use of a tripod with extendable legs? No, we used a stiff-legged tripod. Was there a correlation between the re-observations and failure to balance backsight/foresight distances? Repeat observations were not very frequent but, because of applying the inclination corrrection on each sight, the imbalance of backsight/foresight distances were never identified as being the cause for a discrepancy. Here too the answer is 'no.'
I go into so much detail because I wish to avoid logical fallacies. Using a stiff-legged tripod will not guarantee acceptable precise leveling results. Failing to balance backsight/foresight distances does not mean the results will be defective.
Mandatory continuing education is an example for which we are collectively guilty. I am all for continuing education for surveyors but, in my opinion, it is a logical fallacy to assume that requiring MANDATORY continuing education fulfills our obligation of competent professional service to society. Undoubtedly there is a high correlation between active participation in continuing education and competent professional services but, as a seminar instructor, I have signed attendance certificates for persons who sat through the required number of hours but who were mentally "disconnected" from the learning experience. Yes, I have been guilty of participating in that "fraud." Collectively, I believe we, the surveying profession, can and must do better.
> Mandatory continuing education is an example for which we are collectively guilty. I am all for continuing education for surveyors but, in my opinion, it is a logical fallacy to assume that requiring MANDATORY continuing education fulfills our obligation of competent professional service to society. Undoubtedly there is a high correlation between active participation in continuing education and competent professional services but, as a seminar instructor, I have signed attendance certificates for persons who sat through the required number of hours but who were mentally "disconnected" from the learning experience. Yes, I have been guilty of participating in that "fraud." Collectively, I believe we, the surveying profession, can and must do better.
amen
