Land Surveyor Forum

Replies

• Land Surveyor

  J. Anthony Cavell October 17, 2017 at 9:30pm

  Dear Mr. Win,

  I will try to give an explanation, but first, here are a few links to previous comments.

  Link 1
  Link 2
  Link 3

  Link 4
  Link 5

  • Explain how differences in heights are reduced by adapting the raw measurements to refined data.

  Perhaps the best short explanation I have is to consider people who speak different languages but who think it is the same one, e.g., Britain and U.S. natives. Some of the sentences almost make sense but some of the words used need to be correlated.

  Let's take a simple example of elevations in which the measured elevations seem slanted. They are higher on the east  relative to the ones on the west. If our project controls are supposed to rule the project and adjustment of our observations by tilting your observations until they agree with the control. that was an oversimplification, of course, but not too far off.

  An adjustment (or localization/calibration) develops the necessary functions to bring your observations into conformity with the control. Adjustment can be quite simple or ridiculeslycomplex, but their purposes are to do what I described.

  Often one or more observations or control points will prove to be outliers and removed from the process.

  • Explain reductions relating to sea level, close, and mean.

  MSL:  Sea Level is an average value of the level of the sea at a tide guage over an 18.6 year period, the time it takes for all the influences on the tides to cycle through.

  At one time, (in large part because running levels for any great distance was a daunting task) the CSG&GS took the MSL at several guages around North America averaged their MSLs and developed the MSL29 vertical datum. It bacame obvious that these were not actually MSL, so the datum was renamed NAVD29. Since leveling was a local affair it served until the 1980s.

  We now have a better datum, a hybrid datum called NGVD88. Its zero is far more consistent but NOT MSL because MSL only exists at a tide guage. So much for MSL. Except at tite guages, we don't know where MSL is.

  Close:  I presume you mean the verb close = to shut. In surveying parlence, to close means for an observation to agree with a previous one. A level loop closes if the ending value is the same or very nearly the same as the original value.

  Mean:  Again, I'm not sure of your request. Mean = simple average of some numbers.

  I hope you find this helpful.

  JAC
  • Student Surveyor

    Mat Win > J. Anthony Cavell October 24, 2017 at 4:36pm

    im guessing the question is written way that everyone interrupts it differently.

    The explanantion is meant to include or show how differences in heights are reduced by adapting the raw measurements to refined data. Their explanation must cover reductions relating to sea level, close and mean. Explanations may include these points:
    •
    Curvature and vertical refraction zenith angles measured for horizontal and vertical reduction of long lines require curvature and refraction corrections.
    •
    The manufacturer’s specifications for most total stations indicate that the earth's curvature and atmospheric refraction are internally computed and the corrected horizontal and vertical distances displayed. This correction can be eliminated (or balanced) by taking the mean of two reciprocal measurements.
    •
    The reciprocal procedure means that a distance and a zenith angle are measured at point A to point B and then from point B back to point A.
    •
    At longer distances -- over 1000 meters (3280.8 feet) -- it is better to use this averaging solution than the on board correction factors provided within these total stations, even when they are in perfect adjustment.

    i maybe readying into the question way too much who knows

    thanks for your help tho

    Cheers

    Mat
  • 

    Kim Noblitt > J. Anthony Cavell October 18, 2017 at 4:28pm

    http://opencommons.uconn.edu/thmeyer_articles/2

    What Does Height Really Mean

    Meyer, Roman, Zilowski     2006
    • Land Surveyor

      J. Anthony Cavell > Kim Noblitt October 18, 2017 at 8:49pm

      Yes! Tom's, Dan's and Dave's essay is a great source.

      Thanks, Mr. Noblitt.

      JAC
• Land Surveyor

  Luann Glenn October 17, 2017 at 9:07pm

  I need a more clear question. What's your survey project and are you dealing with littoral data???