Hypothetical Application
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Let us say that Dimensions on a long / rigid / free standing steel stucture - say @ 41 meers long and @ 11 meters wide - have to be checked.
Can one use Total Station? Will there be any limitations?
Presuming that four sharp punch marks are made on this steel structure representing corners of 40 m x 10 m rectangle - these marks were made in - say - a controlled temperature - say 20 degrees - shop. Let us say that these marks were verified for dimensions using Total Station in the same shop under the same controlled temperature and results are found satisfactory. Say within 0.5mm.
This structure is then transported to site - where SAME Total Station is available for reverifying the measurements in a similar manner. The only issue is temperature on this site is not controlled. For that matter the morning and afternoon temperature difference could be as different as 20 degrees C - say.
Let us say we took two measurements - one in the morning and one in he afternoon.
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What dimensions do we expect to see on the Total Station? Should we expect same readings? If different in morning and afternoon - why? To what extent?
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I am asking this with the thought that using Total Station for this application is inappropriate - for the desired confidence level of measurement of 0.5mm+/-. And it is possibly because of the fact that steel which has a very different temperature behavior w.r.t. mother earth and the errors of measurement could be very large.
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My customer is using this to measure long steel structures and I am sure that my tension tape readings give a correct result. I am not sure aout Total Station.
This is the first time I have seen total station. So while allowing some wrong notions of the novice, please comment.
....Thanks for going through the long write up. I could have been mor brief really.
nrk
Tension Tape Readings Need To Be Adjusted
As does everything else.
Assuming the tension tape is invar steel the coefficient of expansion is different from the coefficient for the steel structure.
Total station distance readings are affected by temperature, humidity and atmospheric pressure.
Total station angular precision is most likely insufficient for said purposes. There are industrial class instruments available.
Paul in PA
Tension Tape Readings Need To Be Adjusted
Thanks Paul..for a very immdediate response.
But I think I am contemplating about the repeatability and reproducibility of total station for the given application WHERE SAME POINTS ARE BEING CHECKED FOR DIMENSION EVAUALTION and want to conclude whether the application is constrained.
So for the time being - let us keep the tension tape aside.
Let us talk the variance we could see in the dimensions concluded by the total station for the 40 m x 10 m marks on the structure.
Targeted points (marked points at 40 m x 10 m) do not change. Multiple sets of reading - say - are taken for the same ponts.
What may change....mainly temperature over day time and possibly over seasons.
Under this situation - with a theoritical change of 20 degrees of the steel structure - what could be the variance seen in the dimensions (which was presumedly good in controlled temperature environment)?
If I want to have a confidence level of 2mm+/-, would the Total Station provide this?
rgds
nrk
Tension Tape Readings Need To Be Adjusted
I'm no expert on this. You will get more advice. But:
2 mm in 40M ?
Total Station or Steel tape, you had better be right on top of all sources of error.
Calibration, Temperature, tension, sag. etc., etc.
Calibrated steel tape might be the best. Wouldn't you want the measuring tape and the steel you are measuring to expand the same?
Some Total Stations can do it, but there are a lot more things to introduce errors.
Accuracy specifications for the EDM vary from make and model to make and model, but a typical spec would be 2mm + 2ppm. And that is assuming proper allowance is made for pressure/temperature conditions, and also that other source of error in the measurement system are nullified. There are more precise models of EDM available but one that approached 0.5mm accuracy would be a very specialized - and expensive - piece of equipment. So I do not believe that a total station is a good tool for this job.
For relative accuracies at the 0.5mm level a properly handled and corrected invar tape is probably a better tool than a total station. A steel tape is not likely precise enough for the job.
FWIW- I should think that stresses on such a structure would be affecting the separation of these points at that level of precision. You know, unstressed on a flat shop floor vs. installed in the building, or perhaps layed out on a less than flat surface at the site, etc.
> Hypothetical Application
> ......
> Let us say that Dimensions on a long / rigid / free standing steel stucture - say @ 41 meers long and @ 11 meters wide - have to be checked.
> Can one use Total Station? Will there be any limitations?
> Presuming that four sharp punch marks are made on this steel structure representing corners of 40 m x 10 m rectangle - these marks were made in - say - a controlled temperature - say 20 degrees - shop. Let us say that these marks were verified for
As others have stated below, a Total Station is probably not the best tool for the job. If what you are trying to determine is the precise distance between the marks relative to temperature, then I'd suggest a set of three very precise gauge sticks (one for length, one for width, and one for diagonal). These would be made of carbon fiber or even wood, and would be carefully match drilled in the shop at 20 degrees C (or whatever). You could use the holes to act as guides to make the marks on the steel structure.
Then, in the field, at higher temperature, place one end of each gauge at one mark, then swing an arc around that point, and make a scribe mark at the other end, and measure the difference. Taking into consideration the coefficient of thermal expansion of the gauge (2.1 x 10^-6 in/in/degree F in the case of Fir), you'd know exactly how much the structure has grown, and (possibly more important--you don't say), also whether it's grown proportionally or differently in each direction.
A gauge is really no different than a good steel tape, as long as it's properly tensioned and temperature compensated. Probably easier to use too.
By the way, given a number of assumptions about the internal structure of the structure, you can expect an increase in distance between the 10 meter marks of about .0575", or a little more than 1.45mm with an increase in temperature of 20 degrees C.
Oops...did my math wrong.
> By the way, given a number of assumptions about the internal structure of the structure, you can expect an increase in distance between the 10 meter marks of about .0575", or a little more than 1.45mm with an increase in temperature of 20 degrees C.
I mistakenly used the coefficient of expansion per degree farenheit, not centigrade. Per 20 degrees Centigrade could result in nearly 3 mm increased space between the marks. I leave it to the experts here to form an opinion as to whether that's within the capability of a high end total station.