How to measure height above sea level

  1. When drawing sewer pipes, I often include the height above sea level within one centimeter to get the correct fall, simply cause I've been told to do so. I'm just curious as to how this height is actually measured by the people who actually lay these pipes? Is there some kind of instrument that tells you this height directly and with such good precision?
     
  2. jcsd
  3. GPS style Satalites do it. I think

    they time how long it takes to get from the satalites position to the ground and back to the satalite, feed oit into a program and, whala its there. thats what i suspect.
     
  4. And it's good down to one centimeter?! Don't "normal" GPS handy-things have a precision of like 10-20 meters or something...? Maybe these instruments are perfectly tuned for this...thx!
     
  5. the Military keep all the best ones so they might have one up to 1mm , maybe.

    you could always use a topography program for it
     
  6. FredGarvin

    FredGarvin 5,087
    Science Advisor

    I have had surveys done for my installation. They used GPS. The accuracy I was given was, if all goes well, down to a couple of inches. That being said, that was also doing, what they refered to, as a chain traverse (or something like that) in which they started at their control point and traversed about 20 points in succession back to the original point. In that circumstance I could see the couple of inches being an accumulated error. You do have to ask yourself if the accuracy you are looking for is necessary.
     
  7. Elevations are typically transfered from a known point, called a bench mark, to a project point with 'Level Loops' using equipment that is just for leveling. You run a loop to determine closure, i.e. subtract the change in elevations going one way from the change coming back and divide it by the length of the run.

    There are different levels of rigor for different types of work. The closure depends on both the error and the length of the loop. A typical closure of 1:10,000 is achieved with good equipment and methods.

    GPS is not typically used to transfer elevations if a high degree of accuracy is required.
     
    Last edited: Jul 28, 2006
  8. russ_watters

    Staff: Mentor

    When taking many stationary fixes and averaging them, you can get much more accurate than the typical individual fix accuracy of 3m (with Dgps).
     
  9. Mech_Engineer

    Mech_Engineer 2,299
    Science Advisor
    Gold Member

    IIRC, Differential GPS can be accurate down to 1cm (theoretical). They do use DGPS units with enough accuracy on an airplane to accurately measure pitch, yaw, and roll, so I know they are very accurate in the correct applications.
     
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