Surveying I.
Lecture 2.
Outline
Levelling
Structure of levels
Adjustment of levels
Error sources
Procedure of levelling
Levelling
MSL - Mean Sea LevelLevel line - a line with constant height (a curved line)Horizontal line - a tangential to the level line at the instrument
MSL
lineLevelh
linehorizontal
curvatureofeffect
ncollimatioofLine
Levelling
Over short distances the horizontal line and level line coincide.For a distance of 100m the effect of the curvature is less than 1 mm.
The levelling device (called level) must be set up so, that the line of sight is perpendicular to the gravity vector (plumb line). -> the line of sight is horizontal.
sightofline
Horizontal
heightin
Difference
staffGraduated
staffGraduated Level
Levelling
Levelling
Levelling - Bookkeeping
Rise and fall method:
Levelling - Bookkeeping
Height of Collimation method:
Elements of Surveyor’s level
How to set the line of sight to be exactly horizontal?
More general: how to set anything to be exactly horizontal?
The bubble tube
The bubble tube
The radius determines the sensitivity of the bubble tube:
R2R1
R greater thanR1 2
The bubble tube
The determination of sensitivity:
R1
L
l1
R1
L
l2
radians
L
ll 12
8.206264" radians
The Surveyor’s telescope
Object lens
Eyepiece
Object
Virtual image
Note that the virtual image is magnified and inverted!
The Surveyor’s telescope
The diaphragm (cross-hairs)To provide visible horizontal and vertical reference lines in the telescope.
Line of collimation
With adjustment screws the diaphragm can be moved in the telescope to adjust the line of collimation.
The Surveyor’s telescope
Parallax
When focusing the telescope, the real image formed by the objective lens is made to coincide with the diaphragm.
What is the parallax?
When viewing two distant objects approximately along a straight line, and the eye is moved to one side, then the more distant object moves relative to the other in the same direction.
This can lead to observation errors (wrong reading, wrong targeting).
If the real image formed by the objective lens does not coincide with the diaphragm a parallax is observed -> the reading depend on the position of the eye!
diaphragm image
The Surveyor’s telescope
Focusing the telescope
External focusing
Internal focusing
Focusing lens
Variable length
Fixed length
The Surveyor’s level
Tilting level
Levelling head
Tilting screw
Diaphragm
Bubble tube
Tilting axis
Clamping screw - to fix the telescope in one vertical plane
Tangent screw (slow motion screw) - to finely rotate the telescope along a vertical axis
Circular bubble
The Surveyor’s level
Tilting level
How can we view the bubble tube?
• Using a mirror (older instrument)• Prismatic coincidence reader (modern instruments)
Bubble tube
Prism
Bubble tube is tilted Bubble tube is horizontal (leveled)
The Surveyor’s level
Setting up the level
Primary axis
Secon
dary
axis
1. Fix the level on a tripod
2. Center the circular bubble by adjusting the foot screws.(to approximately level the instrument)
3. Sight the levelling staff, and eliminate the parallax.
4. Adjust the sensitive bubble tube by the tilting screw.
The Surveyor’s level
Automatic levelWe must adjust the bubble tube before every reading when using the tilting level -> takes a lot of time, may cause blunders
An automatic level contains an optical device, which compensates the tilting of the telescope - called compensator.
The Surveyor’s level
Operation of the compensator
Advantage: faster observations, elimination of a possible reason of blundersDisadvantage: vibrations (wind, traffic, etc.) have a bad impact on the operation of the compensator
The levelling staff
Adjusting the level
The two-peg test
d1 d2
a1b1
A BP
1d 2d
Collimation error - the line of collimation is not horizontal, when the level is levelled
The effect of collimation error cancels, if d1=d2.
Thus the height difference is:
11 baH AB
Adjusting the level
The two-peg test
321 ddd
3d
323212 dbdddaH AB
d +d1 2
a2b2
A B d3 Q
2122 ddbaH AB
11 baH AB From the previous configuration:
21
1122
dd
baba
Systematic errors in levelling
MSL
lineLevelh
linehorizontal
curvatureofeffect
ncollimatioofLine
The effect of curvature
Solution: the instrument should be set up exactly in the middle between two points.
Systematic errors in levelling
The refraction
The air has different optical properties everywhere. Air pressure, humidity etc. Have an impact on the refractivity. Thus the light does not propagate along a straight line, but along a curve:
Systematic errors in levelling
r
d
13,0:
22
2
22
2
r
Rkgintroducin
r
R
R
d
R
R
r
d
EarththeofRadiusRr
d
r
r
r
Solution: the instrument should be set up exactly in the middle between two points, thus the effect of curvature is the same for the backsight and foresight.
Systematic errors in levelling
The effect of collimation error
d1 d2
a1b1
A BP
Solution: the instrument should be set up exactly in the middle between two points and the collimation error must be constant, thus the effect is eliminated
Systematic errors in levelling
Tilting of the staff
l
lDepends on the:• tilting angle• reading (the higher the reading is, the bigger the error is)
Solution: staffs should be equipped with circular bubbles
Systematic errors in levelling
Settlement of the tripod
a1b1
A B
h
hbaH AB 11
a2b2
A B
hbaH AB 22
Solution: the reading should be taken in both order, and the mean value of the height differences should be computed
Orders:• backsight - foresight• foresight - backsight
Systematic errors in levelling
Settlement of the staff
Solution: all lines should be run twice in the opposite directions.
Procedure of levelling
1. The instrument must be set up in the middle between two staffs.
2. The bubble tube must be levelled before each reading.
3. You must not use the parallax screw between the backsight and foresight readings
4. The bubble tube must not be affected by strong heat.
5. Readings must be taken 30-50 cm above the ground.
6. Staff should be set up vertically.
7. A change plate should be used to place the staff on the ground.
8. Levelling must be done in two opposite directions.
Procedure of levelling
9. All the observations should be made with a constant speed.
10. Observations should be made only in suitable weather: cloudy sky, constant temperature, early morning, or late afternoon.
11. Staff should be calibrated.
12. If there are three hairs in the diaphragm, one should use all of them to take a reading.
Thanks for the Attention!
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