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CMgt2340 Surveying
Chapter 5 Open-faced
transit, Vernier readingsand Theodolites
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Angular Measurement Terms
Optical Plummet Theodolite
Plumb Bob Transit
Theodolite Precision instrument
Transit Repeating instrument
Vernier Shorter scale on top of alonger scale used to measure angles
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Uses of Transits and Theodolites
Accurate measurement or layout of
horizontal and vertical angles
Low order differential leveling
Prolonging straight lines
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Major Parts of a Transit
Alidade
Horizontal Circle AssemblyLeveling Head Assembly
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Transits Versus Theodolites
Transits Repeating instruments, 4 screw, plumb bob and string, vial level
on telescope, metal hor./vert. circles with verniers inscribed, and
rotation of instrument occurs on a brass spindle rather than ball
bearings
Theodolites Precision instruments, 3 leveling screws, optical plummet, fish-
eye level on leveling head, Shorter telescope, smallergraduations on verniers if not electronic
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Major Types of Verniers
Direct or single
Double or double direct
Folded Vernier
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Reading Double Vernier Transits
Clockwise (Angle Right) Reading- Find the zero mark on the vernier.
Find the set of numbers on the bottom scale that increase going
right to left.
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Reading Double Vernier Transits
Clockwise (Angle Right) Reading- Determine the last known mark going right to left on the bottom
scale (57 degrees and just past 30 minutes).
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Reading Double Vernier Transits Clockwise (Angle Right) Reading- Moving left on the vernier from zero, find the vernier line that
vertically aligns with a scale mark. These two marks should beframed by two vernier marks right inside two scale marks.
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Reading Double Vernier Transits
Clockwise (Angle Right) Reading- Add these minutes to the original reading
(57d 30m + 7m = 57 d 37m).
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Reading Double Vernier Transits
Counter-Clockwise (Angle Left) Reading-\ Find the zero mark on the vernier.
Find the set of numbers on the bottom scale that increase going left to
right.
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Reading Double Vernier Transits
Counter-Clockwise (Angle Left) Reading- Determine the last known mark going left to right on the bottom
scale (302 degrees).
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Reading Double Vernier Transits
Counter-Clockwise (Angle Left) Reading- Moving right on the vernier from zero, find the vernier line that
vertically aligns with a scale mark. These two marks should be
framed by two vernier marks right inside two scale marks.
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Reading Double Vernier Transits
Counter-Clockwise (Angle Left) Reading- Add these minutes to the original reading (302 degrees 23
minutes).
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Typical Mistakes in Reading
Verniers Not using a magnifying glass.
Reading the wrong direction from zero.
Failing to read directly on the line.
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Setting up a Transit
1.Remove transit from its case. Hold it by its standards, neverby its telescope.
2. Place on tripod. Screw leveling head on tripod snuggly while
holding instrument with one hand.
3. Place instrument roughly over the desired point and adjust
the legs of the tripod so that 1) the instrument is at a convenient
height, and 2) the tripod plate is nearly level.
4. Tighten the wing nuts on the tripod legs when in position.
5. Secure a plumb bob to the plumb bob chain and hook slightly
over desired point.
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Setting up a Transit
6. Slowly secure the tripod legs into the ground without jarring
the instrument.
7. Loosen two adjacent leveling screws slightly to release
tension so that the transit can be shifted laterally until it isprecisely over the point. Then retighten the same two screws.
8. Center the leveling tubes or plate levels directly over two
opposite leveling screws by rotating the alidade.
9. Begin adjusting the level bubble by turning the two (opposite)
leveling screws that are directly beneath the level vial. Propertension is important at this stage. Leave the screws firmly
secure but not bound.
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Setting up a Transit
10. Duplicate this procedure using the two remaining leveling
screws and the opposite level vial.
11. Repeat the leveling steps a second or third time to eliminate
any minor adjustments to the level bubbles. 12. Once the instrument is accurately leveled, the screws may
be loosened slightly and the leveling head shifted to position the
plumb bob point directly over the desired point of setup. Care
must be taken if this is done. It can be shifted right or left but
not turned on the leveling head. Any leveling screw must beretightened back to its original point.
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Measuring Angles in the Field
1. After setting the instrument over the first point, set
the scales to zero.
A. Loosen both the upper and lower motion clamps.
B. Hold the alidade stationary, and revolve the circle by
pushing on the circle underside with the fingertips.
C. When zero is close to the index point of the vernier,
tighten the upper clamp.
D. With a magnifying glass, slowly turn the upper tangent
screw until the zeros are precisely in line.
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Measuring Angles in the Field
2. Sight the initial point (backsight).
A. With the upper clamp tightened and the lower clamp
loose, turn and point the telescope towards the initial point.
B. Once relatively close to the desired position, tighten thelower clamp.
C. While observing the point through the telescope, slowly
turn the lower tangent screw until precisely in line with the
initial point.
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Measuring Angles in the Field
3. Turn the horizontal angle.
A. Loosen the upper clamp and turn the telescope clockwise
or counter-clockwise toward the final point (foresight).
B. When point is close to the vertical cross hair, tighten theupper clamp.
C. Slowly turn the upper tangent screw until the vertical
cross hair is precisely in position.
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Measuring Angles in the Field
4. Read the angle.
A. Observe the vernier and determine the angle using a
magnifying glass.
B. Record the value in the field book in the appropriatecolumn.
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Measuring Angles in the Field
5. Repeat the angle.
A. After the initial angle has been recorded, plunge or
invert the telescope.
B. Loosen the lower motion and sight the initial point ororiginal backsight point.
C. Tighten the lower clamp.
D. Repeat steps 5 and 6 except that the telescope is now
inverted and the initial horizontal angle setting is that of the
initial angle.
E. Record the doubled angle in the appropriate column.
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Recording Angles in the Field Book
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References Cited
Examples and step by step tutorials
were copied directly from the following:
Mid-America Vocational CurriculumConsortium, Inc., Basic Surveying
Technology, Stillwater, OK: Oklahoma
State Department of VocationalTechnical Education, 1987