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CH. 3 LEVELING
4/15/03
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Read Kavanagh Ch. 3

• 3.1 Know these definitions (not verbatum)
• 3.2-3.3 Understand the divergence between a
  horizontal line and a level line, and the
  proportionality of error due to curvature and
  refraction with distance of the shot.
• 3.4 Skim read, except read 3.4.2 Level Tube.
  Understand the relationship between the optical
  quality and precision of the level, and the
  radius of curvature of the level tube.
• 3.5 Skip.
• 3.6 Know what a compensator does, and
  conceptually how it works.
• 3.7-3.8 Skim read. Become generally familiar
  with what a digital level is, and what a bar
  code is, and how they work.
• 3.9-3.10 Skim read.
• 3.11 Know what these terms mean.
• 3.12 Understand differential leveling procedure
• 3.13 Skim read for Field Exercise. Know how to
  hold a rod, and rocking (waving) the rod.
  Know how to read the rocking rod. Understand
  field notes for leveling
• 3.14 Skim read.
• 3.15 Skim read. Understand Table A.11
• 3.16 Differentiate between plan, profile, and
  cross-section views. Understand Fig. 3.22.
  Understand profile and cross-section field note
  formats.
• 3.17-3.20 Skip.
• 3.21 Understand the concepts of allowable error
  and adjusting a level loop.
• 3.22-3.24 Read for Field Exercise.

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3.1 Definitions

• Leveling a procedure used to determine
  elevations of points or differences in elevation
  between points
• Elevation vertical distance above or below a
  reference datum.
• Datums
• Mean sea level a universally employed reference
  datum.
• National Geodetic Vertical Datum (NGVD) of 1929.
• North American Vertical Datum (NAVD 88).
• MOST AREAS USE MEAN SEA LEVEL AS THEIR DATUM,
  either NGVD 29 or NAVD 88

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VERTICAL DATUMS

• MEAN SEA LEVEL DATUM OF 1929
• NATIONAL GEODETIC VERTICAL DATUM OF 1929
• (As of July 2, 1973)
• NORTH AMERICAN VERTICAL DATUM OF 1988
• (As of June 24, 1993)

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COMPARISON OF VERTICAL DATUM ELEMENTS

• 
  NGVD 29
  NAVD 88
• DATUM DEFINITION 26 TIDE GAUGES
  FATHERS POINT/RIMOUSKI
• 
  IN THE U.S. CANADA
  QUEBEC, CANADA
• BENCH MARKS 100,000
  450,000
• LEVELING (Km)
  102,724
  1,001,500
• GEOID FITTING Distorted to Fit
  MSL Gauges Best Continental
  Model

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NGVD 29 and NAVD 88
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• Benchmark (BM) a reference mark whose
  elevation is known relative to a given datum.
• Backsight a point which is to be used to
  determine the elevation and/or angular
  orientation of the surveying instrument
• Foresight a point to which an instrument
  sighting is made for measuring or establishing
  its elevation and/or its horizontal position
• Turning Point a temporary point whose
  elevation is determined during the process of
  leveling used to establish the Height of
  Instrument
• Height of Instrument in leveling, the height
  of the line of sight of the leveling instrument
  above the adopted datum in horizontal angle
  measurement, the height of the center of the
  telescope (horizontal axis) above the ground or
  station mark.

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3.2 Differential Leveling Procedure
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How to Read a Level Rod
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How to HoldA Level Rod
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Notes on How To Perform Differential Leveling

• Level the instrument by centering the bullseye
  level
• Focus two things 1) cross-hairs 2) object to
  avoid parallax error
• Rodperson starts at backsight (pt. of known
  elev.), rocks rod or uses level rod bubble
• Field notes (see example). Note that sums of BS
  and FS should equal.
• Rodperson choose turning points for
  reproducibility
• Avoid collimation error by making backsights and
  foresights the same length

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3.3 Common Methods of Leveling

• There are 2 common methods of leveling
• Direct Differential Leveling (Spirit Leveling)
  usual method of determining elevation
  differences. Uses a spirit level and a rod, or a
  digital level and rod. The instrument does not
  tilt you set it up so the line of sight is in
  the horizontal plane.
• Trigonometric leveling horizontal and vertical
  distances are measured to compute elevation
  differences. Good for inaccessible points e.g.
  mountain tops, offshore construction, etc.
  (Nowadays when large distances are involved, GPS
  is commonly used instead of trigonometric
  leveling.)

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3.4 Instruments Commonly Used for Leveling

• Dumpy Level in common use up to the last few
  decades. Some contractors still use them.
  Called dumpy because optical system allowed
  them to be shorter than previous levels (for the
  same magnifying power).
• Main components telescope, leveling tube,
  leveling head.

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• Automatic or Self-Leveling Levels modern types
  most commonly used nowadays. Automatic levels
  have bullseye level to get instrument
  approximately level. The instrument then sets
  itself level. It has a swinging prism or mirror
  compensator which maintains a horizontal line of
  sight by allowing only the horizontal rays coming
  into the instrument to pass through the optical
  center of the instrument. Good instruments to
  use because they can maintain level even if the
  instrument is jiggled around a little.

Cautions when using automatic levels 1) the
compensator is hung by fine wires that easily
break with rough handling 2) the compensator
can occasionally get hung up. Tap the end of the
telescope or turn one of the leveling screws
slightly. The cross hairs should appear to
deflect momentarily before returning to its
original rod reading.
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• Electronic Digital Levels

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• Tilting Level (Can be used for precision work,
  or use automatic levels)

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• Laser Level commonly used by contractors for
  grading, setting forms, etc. Two types 1) fixed
  single laser 2) rotating laser. The rotating
  laser provides a level plane from which
  particular distances can be measured. Good 1000 ft.

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• Transits and theodolites may be used in lieu of a
  level, but give poor results. Total stations
  give comparatively better results, but are not
  generally as accurate for levelling as automatic
  levels, and should generally not be used for
  vertical control of construction projects, or
  where 3rd order or better accuracy is needed.

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3.5 The Telescope

• High-powered telescope (20x to 45x power) with a
  spirit bubble tube attached.

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• Main parts of the telescope
• 1) Positive objective lens forms an image of
  the object sighted. The image would be formed
  ahead of the cross hairs.
• 2) Negative focusing lens diverges the light
  rays to bring them into focus on the cross hairs.
• 3) Reticle glass with the cross hairs on it.
• 4) Eyepiece actually a microscope to enlarge
  the image from the reticle. Focusing the
  eyepiece, e.g. focusing the cross-hairs, changes
  the distance between it and the cross hairs
  (twist the eyepiece to focus).
• 5) Hanging prisms swings on wires to keep
  line of sight level

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3.6 Level Bubble

• The accuracy of any survey instrument is
  generally most affected by the alignment (or
  misalignment) of the level bubble.
• Sensitivity f ( radius of curvature) angle of
  tilt / one division of scale on glass
• But the larger the radius of curvature, the
  more difficult it is to level!

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• Example If it takes 20" of arc to move the
  bubble by 2 mm then the radius of curvature is
• For first order leveling, the instruments have
  2" bubbles (2" of arc to move the bubble 2 mm)
  with "680 ft radius.
• Two types of level bubbles 1) tube and 2)
  bullseye. Sensitivity principle same for both.

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3.7 Sighting Through the Telescope
Inaccurate sightings occur if the cross-hairs and
the scope are not properly focused. This is due
to the problem of parallax.

• Parallax the apparent displacement of the
  position of the point being sighted occurring
  when moving the eye up or down while looking
  through the telescope
• Proper procedure to avoid parallax
• 1) Focus the cross-hairs on the eyepiece. Hold
  a paper about six inches in front of the lens so
  that it appears fuzzy, and twist the eyepiece
  until the cross-hairs come into focus
• 2) Sight the intended rod or object. (Use the
  pointing system on top of the barrel to help
  locate the rod or object). Focus on the rod or
  object.
• 3) Check for parallax by moving the eye up and
  down or sideways while watching the rod. If the
  cross hairs appear to move with respect to the
  image sighted, then either the cross-hairs or
  object are not properly focused.

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3.8 Correction for Inclined Line of Sight
(Collimation Error)

• If instrument is not quite level but distance D
  is same for both BS and FS,
• then the errors cancel.

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3.9 Common Leveling Mistakes (Blunders)

• 1) Misreading rod
• 2) Moving turning point
• 3) Field not mistakes
• 4) Rod not fully extended
• 5) Forgot to level the instrument

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3.10 Common Leveling Errors

• 1) Level rod not vertical
• 2) Settling of level rod on turning point
• 3) Mud, snow or ice buildup on bottom of rod
• 4) Rod damaged
• 5) Incorrect rod length (same as incorrect tape
  length)
• 6) BS FS distances not equal (collimation
  error)
• 7) Bubble not centered / compensator not
  swinging free
• 8) Settling of level legs (tripod)
• 9) Instrument out of adjustment
• 10) Improper focusing of instrument (parallax
  error)
• 11) Heat waves
• 12) Wind or vibration causing instrument
  movement
• 13) Bumping into tripod

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3.11 Corrections Curvature and Refraction

• Curvature error, c the divergence between a
  level line and a horizontal line over a specified
  distance
• c 0.667K2 c in ft, K is dist. In miles

Rays of light are refracted downward under normal
P,T conditions. Thus, line of sight is bent
downward, and curvature effect on error is
reduced. Under normal atmospheric conditions,
refraction error is . 1/7th curvature
error. (cr) 0.574K2 0.0206M2 M in
thousands of feet
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3.12 Level Loop Adjustments

• Application when you close a level loop and find
  your closing elevation for the benchmark to be
  different than your initial value.
• Use judgment. If you suspect that some points
  are weaker than others (either foresights or
  backsights) apportion more error to those weak
  points rather than other, stronger points.
• Examples of weaker shots 1) long distance
  shots 2) heat waves 3) poorly defined turning
  point 4) instrument settling 5) reading high up
  on extended rod (on hill)

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