Distance Measurement Flashcards
Give 3 reasons why we measure distances?
1) Need to establish control station coordinates, e.g. from traversing.
2) Usually measure slope distances
3) Used in height traversing (trig heighting)
Name 4 distance measuring method.
1) Physical
2) Optical
3) EDM
4) GNNS
What is the traditional distance measurement technique?
Physical distance measurement.
What is physical distance measurement useful for?
Infilling detail, checking measurement and inexpensive surveys.
What are the problem with physical distance measurement?
Labour intensive, difficult in steep/broken terrain and low accuracy.
Name a piece of physical measuring equipment for pacing.
Odometer (D=n*2πR)
What are the features of a Fibron/fibreglass tape? (4)
1) Various lengths 5-50m
2) Cheap, light bu stretch and break
3) Graduated to 10mm
4) Cost £10-40
What are the features of a steel tape? (4)
1) Various lengths e.g. 20,30,50 or 100m
2) Graduated to 1mm
3) Range is 250m
4) Cost is £15-200
What are ranging rodes used for?
For aligning tape along line measured.
What are arrows used for?
To mark ends of each full tape length.
What are spring balances used for?
To apply a standard tension.
What is the formula for d when stepping and draw a diagram?
d=d1+d2+d3
What is the formula for d when vertical angle measurement and draw a diagram?
d= Lcos(θ)
What is the formula for d when measuring height difference in levelling and draw a diagram?
d=l- Δh^2/2l
What is the equation for correct distance L’?
L’=(L*I)/I’ where;
L is the measured
I’ is the nominal tape length
I is the actual tape length
If a measured distance L is 89.991m, I’=30.000m but I=30.003 what is the correct distance L’?
L’=90.000m
What does not equal standard temperature?
Temperature at time of measurement.
What does an increase of temperature cause/
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.
What is the coefficient of thermal expansion for steel?
0.000012 per unit length per degree C.
What does a temperature change of 9 degrees C give?
10mm/100m
What is the correction relating temperature equal to?
Correction=k(Tm-Ts)L where:
Tm= temperature at time of measurement
Ts= temperature of calibration
k= coefficient of thermal expansion
When is the distance measurement positive and negative?
Positive: Tm>Ts
Negative: Tm
What is the standardisation tension?
50 Newtons
How can correction be obtained for tension?
From Hooke’s Law
How do you apply standard tension?
Use a spring balance
For optical distance measurement what is the equation for distance D?
D=ks
where k is tacheometric constant in most theodolites and levels k=100 and therefore D=100*S.
How do you define tacheometric constant?
The ratio f/i where: f is the local length and i is the separation of the stadia hairs.
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?
S= top-bottom=0.065m
D=ks
D=100*0.065=6.5m
When do we use EDM?
For longer distances (few km).
Describe the EDM process.
Signal is transmitted from total station and reflected (optically) back from target.
What is the EDM time of flight method?
Measure the time taken for a short pulsed signal of electromagnetic radiation to travel to reflector and back.
What is the equation for the EDM time of flight method?
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
What is the velocity of EM radiation?
3x10^8 m/s
Define amplitude.
Height of each peak above or below the mean.
Define phase.
Angle of rotation, changes from 0-306 in one cycle.
Define wavelength.
It is the distance between successive repeating points on the wave.
Define frequency.
The number of oscillations per second, v=fλ.
What method is used in the majority of modern EDM instruments?
Phase difference method.
What is the phase difference method?
Measure the amount by which the received signal is out of phase with the emitted signal.
What must be know in the phase difference method?
The wavelength
What is the phase difference method equation?
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
What are the features of hand held electronic devices?
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.
