Download - Leveling Methods
-
8/12/2019 Leveling Methods
1/33
4/15/03
CH. 3 LEVELING
-
8/12/2019 Leveling Methods
2/33
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.
-
8/12/2019 Leveling Methods
3/33
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
-
8/12/2019 Leveling Methods
4/33
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)
-
8/12/2019 Leveling Methods
5/33
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
-
8/12/2019 Leveling Methods
6/33
NGVD 29 and NAVD 88
-
8/12/2019 Leveling Methods
7/33
4
-
8/12/2019 Leveling Methods
8/33
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.
-
8/12/2019 Leveling Methods
9/33
3.2 Differential Leveling Procedure
-
8/12/2019 Leveling Methods
10/33
How to Read a Level Rod
-
8/12/2019 Leveling Methods
11/33
How to Hold
A Level Rod
-
8/12/2019 Leveling Methods
12/33
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
-
8/12/2019 Leveling Methods
13/33
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.)
-
8/12/2019 Leveling Methods
14/33
-
8/12/2019 Leveling Methods
15/33
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.
-
8/12/2019 Leveling Methods
16/33
Electronic Digital Levels
-
8/12/2019 Leveling Methods
17/33
Tilting Level (Can be used for precision
work, or use automatic levels)
-
8/12/2019 Leveling Methods
18/33
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.
-
8/12/2019 Leveling Methods
19/33
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.
-
8/12/2019 Leveling Methods
20/33
3.5 The Telescope
High-powered telescope (20x to 45x power) with a spirit bubble tube attached.
-
8/12/2019 Leveling Methods
21/33
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
-
8/12/2019 Leveling Methods
22/33
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!
-
8/12/2019 Leveling Methods
23/33
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.
-
8/12/2019 Leveling Methods
24/33
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.
-
8/12/2019 Leveling Methods
25/33
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.
-
8/12/2019 Leveling Methods
26/33
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
-
8/12/2019 Leveling Methods
27/33
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
-
8/12/2019 Leveling Methods
28/33
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
-
8/12/2019 Leveling Methods
29/33
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)
-
8/12/2019 Leveling Methods
30/33
-
8/12/2019 Leveling Methods
31/33
-
8/12/2019 Leveling Methods
32/33
-
8/12/2019 Leveling Methods
33/33