1. Define Levelling
    • The method of determining height differences using a horizontal line of sight.
    • Levelling is used most commonly to connect tidal datums and tide poles to and from benchmarks ashore.
  2. 5 Levelling Terms
    • Basksight
    • Foresight
    • Change Point
    • Rise & Fall
    • Reduced Level
  3. Backsight
    The first staff reading taken from any position or 'set-up' is called a backlight. It is taken to a station of a known height.
  4. Foresight
    Is the staff reading taken to the next change point in a level traverse.
  5. Change Point
    Th staff position to which both a foresight and backsight are taken.
  6. Rise & Fall
    Is the defference in height between 2 staff readings; a 'rise' is a positive difference or rise in height & a 'fall' is a negative difference or fall in the height.
  7. Reduced Level
    The height of a point calculated above or below the reference datum at the start of a level traverse.
  8. Systematic Error
    Errors which follow certain physical or mathematical rules and therefore we can correct.

    • To avoid one may:
    • - Calibrate instruments carefully before field work starts.
    • - Design & use suitable measurement routines & procedures
    • - Correct measurement results afterwards
  9. Gross Errors
    Can be of any size or nature and are due to human mistakes, malfunctioning equipment or wrong measurement methods.
  10. Random Errors
    Errors which behave randomly and affect the measurements in a non-systematic way.
  11. Curvature & Refraction Errors
    • Curvature of the Earth's Surface causes and object to appear lower than it really is.
    • Refraction of light rays through the Earth's Atmosphere causes an object to seem higher than it really is.
  12. 2 Peg Test
    • The level should be checked on first issue then daily before use.
    • The 2 peg test enables the surveyor to find a vertical calumniation error in the level.
    • 3rd order work should be less than 2mm over 80m
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