CIVL 102 Leveling Lab Briefing on Field Work

Instrument error  suppose Instrument error  suppose line of sight not truly horizontal as telescope is rotated about vertical axis  ”horizontal line of sight” is always tilted upward (or downward)  sweeps out a cone, not a horizontal plane   h, the rise from A to B is found as (a’ – b’) = (a + aa’) – (b + bb’) = (a – b) + (aa’ – bb’) Using backsight distance ~ foresight distance, then errors aa’ & bb’ approximately cancel out  we can still obtain (approximately) the correct result (a – b) Sources of Error & Precautions

Differential settlement of staff or tripod: Firm, stable & well-defined turning points (change points) needed If leveling over soft ground: use ground plate (change plate): triangular metal plate, corners folded down, dome raised at center Stamped firm on ground with dome head providing a stable point If tripod is on soft ground: plant the metal shoes firmly into soil by stepping on them

Staff tilting sideways: Attach staff level ( “bull’s eye bubble”) to ensure correct staff plumbing Observer can check staff’s coincidence with vertical crosshair, and signal staffperson to correct if needed Staff tilting towards / away from observer: Check staff level Look from side of staff & align with vertical object

Bubble not central: Centralize circular bubbles (on auto. level & the staff) before measurement Use 2 bubbles on staff (detects any malfunctioning bubble) if available Incorrect reading of staff: second observer to double-check all readings familiarize with staff & examine it close-up beforehand

Mishandling of staff: When extending telescopic staff: Extend the lowest section first Ensure no section is left partially extended (this would be like a kink in tape) Look out for overhead power cables

Setting staff on sloping ground: Correct: Fig. 2.11(a): bottom plane (0) flush against leveling station Incorrect: “center the staff over the point”  offset error (Fig. 2.11(b)) Fig. 2.11

Parallax: Parallax: relative movement between image & cross hairs as eye moves Rotate eyepiece for sharp cross hairs, focus on staff until image clear & parallax eliminated Adverse weather conditions: Bring umbrella to protect level from extended exposure to sun / unexpected showers

Booking & Calculations Staff readings: level book / booking form Processed readings -> RL ’ s Use hand-held calculator / notebook with spreadsheets

–Compute all rises & falls  Start at a BM with known RL To get RL of next station: add rise to previous RL, or subtract fall from previous RL  Repeat for all subsequent stations Rise & fall method

Example 2.1. Rise & fall method: CP2 B CP1 BM Fig Table 2.2 subtract

From (2.3), (2.4) & (2.5), = Total rise – total fall = Last RL – first RL i.check equalities in last row, Table 2.2. ii.discrepancy -> arithmetic mistake(s) (unrelated to accuracy of measurements).

Example TBM m above MSL A B C D Table 2.3 spreadsheet method: can use IF & MAX commands to automate

Table 2.4 = Total rise – total fall = Last RL – first RL, no arithmetic mistake. subtract

Always close on the starting (or another) BM Check accuracy & detect blunders Misclosure (at closing BM):  = measured RL of BM  known RL of BM (2.9) If  acceptable (i.e. <= max. permitted error E):  apply least squares adjustment  closing BM must have the correct RL Closure Error ( “ misclosure ” )

A rule often used in construction leveling with many (n) instrument stations: Maximum permitted closure error E : E =  Dn 1/2 (2.10) D = 5 mm & D = 8 mm: commonly adopted (8mm used for CIVL 102 lab)

Things to think about before the leveling lab: What is the approximate height difference between points? Where should the level be placed to ensure –Visibility of back and forward stations –efficiency of work Where should the ground plate (i.e. additional change point) be placed, if needed