Distance Measurement Flashcards

1
Q

Give 3 reasons why we measure distances?

A

1) Need to establish control station coordinates, e.g. from traversing.
2) Usually measure slope distances
3) Used in height traversing (trig heighting)

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2
Q

Name 4 distance measuring method.

A

1) Physical
2) Optical
3) EDM
4) GNNS

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3
Q

What is the traditional distance measurement technique?

A

Physical distance measurement.

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4
Q

What is physical distance measurement useful for?

A

Infilling detail, checking measurement and inexpensive surveys.

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5
Q

What are the problem with physical distance measurement?

A

Labour intensive, difficult in steep/broken terrain and low accuracy.

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6
Q

Name a piece of physical measuring equipment for pacing.

A

Odometer (D=n*2πR)

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7
Q

What are the features of a Fibron/fibreglass tape? (4)

A

1) Various lengths 5-50m
2) Cheap, light bu stretch and break
3) Graduated to 10mm
4) Cost £10-40

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8
Q

What are the features of a steel tape? (4)

A

1) Various lengths e.g. 20,30,50 or 100m
2) Graduated to 1mm
3) Range is 250m
4) Cost is £15-200

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9
Q

What are ranging rodes used for?

A

For aligning tape along line measured.

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10
Q

What are arrows used for?

A

To mark ends of each full tape length.

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11
Q

What are spring balances used for?

A

To apply a standard tension.

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12
Q

What is the formula for d when stepping and draw a diagram?

A

d=d1+d2+d3

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13
Q

What is the formula for d when vertical angle measurement and draw a diagram?

A

d= Lcos(θ)

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14
Q

What is the formula for d when measuring height difference in levelling and draw a diagram?

A

d=l- Δh^2/2l

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15
Q

What is the equation for correct distance L’?

A

L’=(L*I)/I’ where;
L is the measured
I’ is the nominal tape length
I is the actual tape length

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16
Q

If a measured distance L is 89.991m, I’=30.000m but I=30.003 what is the correct distance L’?

A

L’=90.000m

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17
Q

What does not equal standard temperature?

A

Temperature at time of measurement.

18
Q

What does an increase of temperature cause/

A

Causes a tape to increase in length and therefore distances displayed by the tape will be too small, and vice versa for a decrease in temperature.

19
Q

What is the coefficient of thermal expansion for steel?

A

0.000012 per unit length per degree C.

20
Q

What does a temperature change of 9 degrees C give?

A

10mm/100m

21
Q

What is the correction relating temperature equal to?

A

Correction=k(Tm-Ts)L where:
Tm= temperature at time of measurement
Ts= temperature of calibration
k= coefficient of thermal expansion

22
Q

When is the distance measurement positive and negative?

A

Positive: Tm>Ts
Negative: Tm

23
Q

What is the standardisation tension?

A

50 Newtons

24
Q

How can correction be obtained for tension?

A

From Hooke’s Law

25
Q

How do you apply standard tension?

A

Use a spring balance

26
Q

For optical distance measurement what is the equation for distance D?

A

D=ks

where k is tacheometric constant in most theodolites and levels k=100 and therefore D=100*S.

27
Q

How do you define tacheometric constant?

A

The ratio f/i where: f is the local length and i is the separation of the stadia hairs.

28
Q

If a E-staff reading through a theodolite is 1.965m and the top stadia=1.998m and the bottom stadia is= 1.993m what is the distance?

A

S= top-bottom=0.065m
D=ks
D=100*0.065=6.5m

29
Q

When do we use EDM?

A

For longer distances (few km).

30
Q

Describe the EDM process.

A

Signal is transmitted from total station and reflected (optically) back from target.

31
Q

What is the EDM time of flight method?

A

Measure the time taken for a short pulsed signal of electromagnetic radiation to travel to reflector and back.

32
Q

What is the equation for the EDM time of flight method?

A

2d=v*(tb-ta) where:
d is the distance to be measured
v is the velocity of the pulse through the medium m/s
tb is the time of arrival of return pulse in s
ta is the time of departure of the pulse in s

33
Q

What is the velocity of EM radiation?

A

3x10^8 m/s

34
Q

Define amplitude.

A

Height of each peak above or below the mean.

35
Q

Define phase.

A

Angle of rotation, changes from 0-306 in one cycle.

36
Q

Define wavelength.

A

It is the distance between successive repeating points on the wave.

37
Q

Define frequency.

A

The number of oscillations per second, v=fλ.

38
Q

What method is used in the majority of modern EDM instruments?

A

Phase difference method.

39
Q

What is the phase difference method?

A

Measure the amount by which the received signal is out of phase with the emitted signal.

40
Q

What must be know in the phase difference method?

A

The wavelength

41
Q

What is the phase difference method equation?

A

2D = n*λ+dλ where:
D is the distance to be measured
λ is the measuring wavelength
n is the integer number of whole wavelengths in 2D
dλ is the fractional part of wavelength in 2D

42
Q

What are the features of hand held electronic devices?

A

1) Typically used for low accuracy distance measurement
2) Handled device with a laser beam
3) Often used in building surveys
4) Ordnance Survey data collectors for map updating.