Thanks for your response!
lol, the answers where
a) 1
b) 2
c) 3
d) 4
e) 5
Very ambiguous questions when the all answers could be correct.
The question comes from a textbook. I can't remember which one, might have seen it in more than one. If I recall it was 200' but maybe modern edm's are 100'. Anyway, it's stated that edm's can't properly resolve atmospheric stuff at less than a certain distance. Look it up in your textbook under total station or edm precision/accuracy. That's the best I can do from memory, I'll leave the research to you.
One aspect of technical exams is your knowledge base. There absolutely are several correct answers to the basic question. When you think about it in terms of current technologies applied to survey problems the field gets real narrow real quick.
Like it or not, exams are written by people. Regardless of effort our subjective (and occasionally downright evil) side comes through in the questions. I just took a State specific exam that frustrated me for months. When my license arrived I got over it...
Questions like that turn multiple choice into multiple guess.
http://science.howstuffworks.com/question431.htm
The accuracy specifications for EDM or the EDM portion of a total station has improved over the years. In the 1970s ±0.02 ft plus 5 ppm was common. Now ±2 mm plus 2ppm is common. As others have pointed out, accurate taping needs proper corrections for systematic errors, especially temperature correction.
> The answer is 3. Why, and how less accurate?
Why, because 100 feet is the length of the typical steel tape. If you have to measure more a tape length you lose accuracy. How much? That is difficult, if not impossible, to quantify.
As Norman said, 100' is one tape length. While it's true that a modern total station can measure to 2mm+/-2ppm, or around 0.0068', there are a ton of other errors associated with a total station measurement, including:
- Centering error at the gun
- Centering error at the target
- Possible atmospheric error
- Possible prism constant error
- Parallax
- Sighting error
- HI / HT error
- Instrument errors (vertical indexing, EDM constant)
Even assuming that your prism constant, temperature, and pressure are perfect, the cumulative error from all potential sources is most likely going to add up to much more than what can be measured with one pull of a good tape.