Hello,
NSPS CST Level III Sample Examination (October 2007)
Field Equipment & Instruments (Page 5 of 15)
5. A Total Station is less accurate than measurements made with a steel tape when:
1. the measured distance is more than 1000 feet. 2. the measured distance is less than 200 feet. 3. the measured distance is less than 100 feet. 4. never
The answer is 3. Why, and how less accurate?
Regards,
Mark
> Hello,
>
> NSPS CST Level III Sample Examination (October 2007)
>
> Field Equipment & Instruments (Page 5 of 15)
>
> 5. A Total Station is less accurate than measurements made with a steel tape when:
> 1. the measured distance is more than 1000 feet. 2. the measured distance is less than 200 feet. 3. the measured distance is less than 100 feet. 4. never
>
> The answer is 3. Why, and how less accurate?
>
> Regards,
>
> Mark
Great question. This is true and has to do with the accuracy of the Totalstation. If you read the datasheet on the Totalstations, you will see something along the lines of 2mm + 2ppm. Taking that number and applying it to a 100', 200' or 1000' line, the amount of error is spread out over the length. Short distances aren't too good vs longer distances. A steel tape, if done correctly will be way more accurate at 100'. There is never a never. Someone will come along and do the math for you.
In a hundred feet
2 mm = .0066'
2ppm= .0002'
Not seeing it . Maybe in the past but todays equipment is as good as any steel tape at any distance . Question is bogus or out of date.
You also have to take into account centering and leveling errors at instrument and rod.
However, the instrument constant and scale can be accounted for if verified at a CBL.
Just using the EDM error and ppm calculation, however Scott is correct more goes into such a calculation in reality.
EDM at 1,000.00
1,000.00’ ± 3mm + 3ppm
1,000.00’ ± 0.0098’ + 3(1,000.000’)/1,000,000.00
1,000.00’ ± 0.0098’ + 0.003’
1,000.00’ ± 0.0128’
0.0128/1,000.00
1/78,125
EDM at 100.00’
100.00’ ± 3mm + 3ppm
100.00’ ± 0.0098’+ 3(100.00)/1,000,000.00
100.00’ ± 0.0101’
0.0101/100.00
1/9,900
The true answer depends on the calibration of everything, the instrument specs, and the care given to centering of instrument and target, versus the care with calibration, temperature, and sag/tension correction on the tape.
But 100 ft is a nice rule of thumb.
Well I do not agree. The answer is certainly not 3.
The question should be: what does the precision statement of the instrument manufacture mean when it states a precision of plus or minus 2 millimeters. Most people do not realize the statement is a specification and does not mean that the instrument is making an error at all.
While many assume that the precision specification means the instrument will always make that error it does not. The specification is the statistical limits of the instrument's performance over many measurements of the same two points ( a statistical sample ) and that more often than not the instrument will be making the measurement of 100 feet or what ever the distance is that is being measured.
Think of the normal distribution in a statistical analysis and the plus and minus 2 millimeters representing the occurrences outside the confidence limits.
The only measurement constant is the atmospheric correction.
Knowing the methods used by those who employ steel tapes I would doubt that a greater precision could be achieved compared to a total station that is properly calibrated.
Unless those using the steel tape have a properly certified tape, are using proper tension, sag, temperature corrections and are bucking the tape level on spots that are verified with a properly calibrated level instrument your are not getting more accurate. I really doubt any do all that is necessary when using a steel tape.
I would rely more with measurements made with a properly calibrated total station instrument. So unless this is an essay question then it is misleading because there is not one answer and the question is unable to be answered given the information provided.
Typical ambiguous CST question. My favorite is "how far of a distance can a 1 watt radio reach." I still cannot find a good source for that answer. That may be the question that decides whether you passed or failed.
Thankfully I passed!
Just curious. What does the manual for the tape say about accuracy?
When staking buildings we use a tape every time. Having surveyed 6 plus years before touching an EDM I am pretty decent with a highway or canyon chain. Our instruments are fairly modern and they cannot achieve the results we get on small figures.
They cant build it better than a couple hundredths I will leave my .01' on the job any day . Were not building a watch . 😀
Good one! Just got off the phone with my friend, the ham radio operator. He said one to five miles depending on a truck load of variables such as manufacturer and type of carrier wave being used.
Probably nothing. Would however give you the modulus of elasticity of the tape. The accuracy of the tape is soley dependent on the users.
My response was number four. That was before I read these replies to my questions.
Thanks for your response!
Thanks for your response!
How true!
I'm all for chaining distances two hundred feet or less, but it doesn't seem to be economical for a two man crew to do so. Besides, my data collector doesn't allow me to input chained distances. And you know how the office loves their raw data!
Thanks for your response!
Thanks for your response!
