Distance Measurement

Question 1

Give 3 reasons why we measure distances?

Answer 1

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

Question 2

Name 4 distance measuring method.

Answer 2

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

Question 3

What is the traditional distance measurement technique?

Answer 3

Physical distance measurement.

Question 4

What is physical distance measurement useful for?

Answer 4

Infilling detail, checking measurement and inexpensive surveys.

Question 5

What are the problem with physical distance measurement?

Answer 5

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

Question 6

Name a piece of physical measuring equipment for pacing.

Answer 6

Odometer (D=n*2πR)

Question 7

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

Answer 7

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

Question 8

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

Answer 8

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

Question 9

What are ranging rodes used for?

Answer 9

For aligning tape along line measured.

Question 10

What are arrows used for?

Answer 10

To mark ends of each full tape length.

Question 11

What are spring balances used for?

Answer 11

To apply a standard tension.

Question 12

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

Answer 12

d=d1+d2+d3

Question 13

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

Answer 13

d= Lcos(θ)

Question 14

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

Answer 14

d=l- Δh^2/2l

Question 15

What is the equation for correct distance L’?

Answer 15

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

Question 16

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

Answer 16

L’=90.000m

Question 17

What does not equal standard temperature?

Answer 17

Temperature at time of measurement.

Question 18

What does an increase of temperature cause/

Answer 18

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.

Question 19

What is the coefficient of thermal expansion for steel?

Answer 19

0.000012 per unit length per degree C.

Question 20

What does a temperature change of 9 degrees C give?

Answer 20

10mm/100m

Question 21

What is the correction relating temperature equal to?

Answer 21

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

Question 22

When is the distance measurement positive and negative?

Answer 22

Positive: Tm>Ts
Negative: Tm

Question 23

What is the standardisation tension?

Answer 23

50 Newtons

Question 24

How can correction be obtained for tension?

Answer 24

From Hooke’s Law

Question 25

How do you apply standard tension?

Answer 25

Use a spring balance

Question 26

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

Answer 26

D=ks

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

Question 27

How do you define tacheometric constant?

Answer 27

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

Question 28

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?

Answer 28

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

Question 29

When do we use EDM?

Answer 29

For longer distances (few km).

Question 30

Describe the EDM process.

Answer 30

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

Question 31

What is the EDM time of flight method?

Answer 31

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

Question 32

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

Answer 32

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

Question 33

What is the velocity of EM radiation?

Answer 33

3x10^8 m/s

Question 34

Define amplitude.

Answer 34

Height of each peak above or below the mean.

Question 35

Define phase.

Answer 35

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

Question 36

Define wavelength.

Answer 36

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

Question 37

Define frequency.

Answer 37

The number of oscillations per second, v=fλ.

Question 38

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

Answer 38

Phase difference method.

Question 39

What is the phase difference method?

Answer 39

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

Question 40

What must be know in the phase difference method?

Answer 40

The wavelength

Question 41

What is the phase difference method equation?

Answer 41

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

Question 42

What are the features of hand held electronic devices?

Answer 42

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.