2OutlineLevellingStructure of levelsAdjustment of levelsError sourcesProcedure of levelling
3Levelling MSL - Mean Sea Level Level line - a line with constant height (a curved line)Horizontal line - a tangential to the level line at the instrument
4LevellingOver 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.
8Levelling - Bookkeeping Height of Collimation method:
9Elements 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
10The bubble tubeThe radius determines the sensitivity of the bubble tube:
11The bubble tubeThe determination of sensitivity:
12The Surveyor’s telescope ObjectEyepieceObject lensVirtual imageNote that the virtual image is magnified and inverted!
13The Surveyor’s telescope The diaphragm (cross-hairs)To provide visible horizontal and vertical reference lines in the telescope.Line of collimationWith adjustment screws the diaphragm can be moved in the telescope to adjust the line of collimation.
14The Surveyor’s telescope ParallaxWhen 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!diaphragmimage
16The Surveyor’s level Tilting level Bubble tube Diaphragm Tilting screw Circular bubbleTilting axisLevelling headClamping screw - to fix the telescope in one vertical planeTangent screw (slow motion screw) - to finely rotate the telescope along a vertical axis
17The Surveyor’s level Tilting level How can we view the bubble tube? Using a mirror (older instrument)Prismatic coincidence reader (modern instruments)PrismBubble tubeBubble tube is tiltedBubble tube is horizontal (leveled)
18The Surveyor’s level Setting up the level 1. Fix the level on a tripod 2. Center the circular bubble by adjusting the foot screws.(to approximately level the instrument)Primary axisSecondary axis3. Sight the levelling staff, and eliminate the parallax.4. Adjust the sensitive bubble tube by the tilting screw.
19The Surveyor’s level Automatic level We must adjust the bubble tube before every reading when using the tilting level -> takes a lot of time, may cause blundersAn automatic level contains an optical device, which compensates the tilting of the telescope - called compensator.
20The 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
22Adjusting the level The two-peg test Collimation error - the line of collimation is not horizontal, when the level is levelledThe effect of collimation error cancels, if d1=d2.Thus the height difference is:
23Adjusting the levelThe two-peg testFrom the previous configuration:
24Systematic errors in levelling The effect of curvatureSolution: the instrument should be set up exactly in the middle between two points.
25Systematic errors in levelling The refractionThe 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:
26Systematic errors in levelling 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.
27Systematic errors in levelling The effect of collimation errorSolution: 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
28Systematic errors in levelling Tilting of the staffDepends on the:tilting anglereading (the higher the reading is, the bigger the error is)Solution: staffs should be equipped with circular bubbles
29Systematic errors in levelling Settlement of the tripodSolution: the reading should be taken in both order, and the mean value of the height differences should be computedOrders:backsight - foresightforesight - backsight
30Systematic errors in levelling Settlement of the staffSolution: all lines should be run twice in the opposite directions.
31Procedure 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 readings4. The bubble tube must not be affected by strong heat.5. Readings must be taken 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.
32Procedure 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.