Leveling Methods

8/12/2019 Leveling Methods

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4/15/03

CH. 3 LEVELING


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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 refractionwith distance of the shot.

3.4 Skim read, except read 3.4.2 Level Tube. Understand the relationship betweenthe optical quality and precision of the level, and the radius of curvature of thelevel 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 procedure3.13 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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4


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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 Hold

A 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 avoidparallaxerror

Rodperson starts at backsight (pt. of known elev.),rocks rod or uses level rod bubble

Field notes (see example). Note that sums of BS andFS should equal.

Rodperson: choose turning points for reproducibility

Avoid col l imat ion error by making backsights andforesights the same length


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3.3 Common Methods of Leveling

There are 2 common methods of leveling:

1) Direct Differential Leveling (Spirit Leveling)= usual method ofdetermining 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 theline of sight is in the horizontal plane.

2) Trigonometric leveling= horizontal and vertical distances aremeasured to compute elevation differences. Good for inaccessiblepoints e.g. mountain tops, offshore construction, etc. (Nowadayswhen large distances are involved, GPS is commonly used instead oftrigonometric leveling.)


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Automatic or Self-Leveling Levels= modern types most commonly used nowadays. Automaticlevels have bullseye level to get instrument approximately level. The instrument then setsitself level. It has a swinging prism or mirror compensator which maintains a horizontal lineof 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 leveleven 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 levelplane 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

stationsgive 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 ofthe object sighted. The image would be formedahead 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 toenlarge the image from the reticle. Focusing theeyepiece, e.g. focusing the cross-hairs, changesthe distance between it and the cross hairs (twistthe eyepiece to focus).

5) Hanging prisms = swings on wires to keepline of sight level


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3.6 Level Bubble

The accuracy of any survey instrument is generally most affected by the alignment (ormisalignment) 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 ofcurvature is:

For first order leveling, the instruments have 2" bubbles (2" of arc to movethe 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

Parallax = the apparent displacement of theposition of the point being sightedoccurring when moving the eye up ordown while looking through thetelescope

Proper procedure to avoid parallax:

1) Focus the cross-hairs on theeyepiece. Hold a paper about six inchesin front of the lens so that it appearsfuzzy, and twist the eyepiece until thecross-hairs come into focus

2) Sight the intended rod or object.

(Use the pointing system on top of thebarrel to help locate the rod or object).Focus on the rod or object.

3) Check for parallax by moving theeye up and down or sideways whilewatching the rod. If the cross hairsappear to move with respect to the

image sighted, then either the cross-hairs or object are not properly focused.

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


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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 rod2) 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 free8) 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 distancec = 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.

(c+r) = 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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Leveling Methods

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