how to: some basic principles for leveling tilo schöne gfz, germany [email protected]

How to:Some Basic Principles forLeveling

Tilo SchöneGFZ, [email protected]

ODINAFRICA/GLOSS Sea Level Training Course - Oostende, Belgium - 13-24 Nov. 2006

Lecture Overview

•Equipment

• Introduction to Leveling

•Observation, Field Notes, and

Computation

•Errors and their effects


Equipment


Equipment

• Level Instrument• Tripod• Staff/Pole• Change plate (German:

Frog/Frosch)• Pole staff bubble (bull eye)• Marker


Equipment: Level Instrument

• Automated Levels Easy to use (not power!) Needs experience Robust even in hostile environment

• Digital Levels Push-button technique No reading errors, special staff Readings are stored and analyzed digitally


Automated Levels (Compensator)

Pendulum

Tribrach

Courtesy: Deumlich, Vermessungskunde

Bull Eye


Digital Levels• Uses Barcode staffs• Internal storage of data

Download to the computer Automated height computation +

adjustment No feeling for quality anymore You frequently need power plugs


Equipment




Equipment: Tripod• Wooden design or aluminum

From “easy to sit” to “ops, this is high”


Equipment




Equipment: Staff/Pole

• Wood, aluminum• INVAR type for high precision leveling

Conventional (“E”-type) Barcode for Digital Levels


Equipment




Equipment: Change Plate

• For long survey lines• Allows change of instruments

Best is a metal change plate Screws e.g. at fences Sharp stones or nails

• Beware of dark colors

It’s not the Indonesian-German Dictionary, It’s the nail!


Equipment




Equipment: Bubble

• Keep the pole upright Any tilt will disturb your readings


Equipment




Survey Markers

• Gives you a fixed point Should be of good quality Should be long-term Preferable in bedrock, settled buildings, or

bridges Do not use fences or walls


Introduction to Leveling


Some Basic Definitions

• Level surface (e.g. the geoid) A water surface with no motion Gravity gradient is the normal to the level surface The Instrument’s Bubble is in the normal (!)

• Horizontal surface At the instruments axis, the horizontal surface is

tangent to the level surface Over short distances (<100 m) the horizontal

surface and the level surface will coincide For long leveling lines the effects of the gravity

field must be considered


Leveling rods

Line of sight

Back sight Fore sight

Gravity Gradient

Basic Principle of Leveling• Measures height differences between

points Along a line Several points from one occupation

bs

fs

h = bs - fs


Definitions

• Back sight (BS) The first reading from a new instrument

stand point (i.e. take the height to the instrument)

• Fore sight (FS) The last reading from the current instrument

station (i.e. give the height to a benchmark)

• Intermediate sight (IS) Any sighting that is not a back sight or fore

sight


Reading a Staff

1422

• Read the [m], [dm] & [cm]

• Estimate the [mm]

• Check yourself for frequent used numbers (2/3) or (7/8)


Basic Rules for Leveling• Always start and finish a leveling run on a

Benchmark (BM or TGBM) and close the loops

• Keep fore sight and back sight distances as equal as possible

• Keep lines of sight short (normally < 50m)

• Never read below 0.5m on a staff (refraction)

• Use stable, well defined change points

• Beware of shadowing effects and crossing waters


Observation, Field Notes, and Computation


How to: A sample loop

S2

S1

New BenchmarkNB1

Tidal HutTH

New BenchmarkNB2


How To: Field Notes

Back Inter Fore Point

TH

NB1

NB2

NB1

TH

1327

3982

S2

S1

New BenchmarkNB1

Tidal HutTH

New BenchmarkNB2

2365

09862347

37243753

1101


Date, Observer,Instrument

Instrument Check

Fore

Back


Compute levels

Back Inter Fore dh H Comment

TH

BM1

BM2

BM1

TH

IST

SOLL

1327

3982

0986

3724

2365

2347

3753

1101

1379

2652

9792 9793

-0001

+1

100 000

100 000

0000

0001 (SOLL – IST)

´7345

´8624

0

97 345

98 724

97 348

?

0


Loop misclosure

• Misclosure Error The difference of the measured height

difference (Hmeas) to the known height (closed loops = 0, known benchmarks = height difference)

Misclosure = HSOLL – HIST

• Point errors at double observed points


Achievable Accuracy

• Instrument dependent Roughly from the instrument

NI002 = 0,2mm/km (doubled line) NI025 = 2.5mm/km (doubled line)

• Survey line length dependent ms = m1km s, s in km mH = (m1km/2) s, s in km #(middle of the line)


An acceptable misclose?

• Small misclosures in closed level loops are expected because of the accumulation of random errors and can be adjusted

• If the misclosure is large, the loop (or part of it) must be repeated

• Misclosures can also result from errors in published BM levels and from BM instability


Testing the misclose• The amount of misclosure acceptable

using a specific instrument and survey line length

• For our example, a second order leveling standard is adopted*…

misclosure 2,5s mm

• where s is the length of the line in km

*Dependent on your contry’s rules and the instrument used


Our example

• The misclosure is +1 mm

• The length of the loop is 0.4 km

• Acceptable error is

2.5(0.4) = ±1.6 mm

• The misclosure of +1 mm is within the limit

• Mean error for NB1 = 2.5/2* (0.4)


Errors and their effects

(many, but only a few addressed)


Errors in leveling, e.g.

• Collimation, Parallax

• Change point / staff instability

• Instrument or Benchmark instability

• Refraction

• Uncalibrated staff or levels

• Reading, booking, or computation errors

• Fore- and backsight distances different


Systematic and Random Errors

• Earth curvature

• Refraction

• Collimation errors


Effect of Earth Curvature

Curva

ture

effec

t

Distance (s) in m 10 20 50 100 1000

Effect (h) in mm 0,008 0,03 0,2 0,8 80

www.fh-oow.de/institute/ima/personen/weber/VK_12/VL_VK1/geo_niv_6.htm

Horizontal Level

(r +h)2 = r2 + s2

=>h s2/(2r)


Refraction

www.fh-oow.de/institute/ima/personen/weber/VK_12/VL_VK1/geo_niv_6.htm

Mean Gradient: 0,2 °C / m


Collimation error• Occurs when the line of sight (as

defined by the lens axis and cross-hairs) is not horizontal

• Leads to an incorrect staff reading

horizontal line

line of sighterror


Stand point 1

Stand point 2

´ ´

´ ´

Instrument test: Nähbauer

a′1 = a1+e

b′1 = b1+2e

Δh = a1−b1

Δh′1 = a′1−b′1 = a1−b1−e = Δh−e

With Δh′1+e = Δh′2−e

Δh′2−Δh′12

a′2 = a2+2e

b′2 = b2+e

Δh = a2−b2

Δh′2 = a′2−b′2 = a2+e−b2 = Δh+e

Δh =Δh′2−e

e =


Summary


Procedure of leveling1. The instrument must be check before use! (see

lecture)

2. The instrument and level must be stable settled-up

3. The bubble tube must be leveled before the reading

• Beware of sun exposure (will wander)

• Ensure the instruments pendulum is in-limit

4. The instrument must be set up in the middle between two staffs

• Prevents curvature effects

• If impossible, use the same distances, but opposite for the next readings

5. You must not use the parallax screw between the backsight and foresight readings


6. Readings must be taken 30-50 cm above the ground • Surface refractions• Beware also of temperature gradients (inside/outside

buildings) !!!!

7. Staff should be set up vertically

8. A change plate should be used

9. Leveling must be done in two opposite directions but

the same line (beware of gravity gradients)

10. Staff should be calibrated, especially if INVAR

11. Be careful when crossing rivers (large water surfaces)• Use “same-time” (mutual) observations• Repeat it during different times of the day

Procedure of Leveling


An Unhappy Surveyor

… having a 2 centimeter difference

how to: some basic principles for leveling tilo schöne gfz, germany [email protected]

Documents

high slide

effects slide

metal change plate screws

staffpole wood

change of instruments

indonesian german dictionary

special staff readings

tripod wooden design