Week 5: Distance Flashcards
(82 cards)
1
Q
Two types of distance measurement
A
Manual and Remote
2
Q
Examples of manual distance measurement
A
Taped, Pacing
3
Q
Examples of remote distance measurement
A
EDM, Hand-held laser, Acoustic, GNSS
4
Q
The wavelength, range and accuracy of both Laser and Infrared (or near infrared)
A
- Laser
- Wavelength = 0.6 micron
- Range = 60 km
- Accuracy = High - Infrared
- Wavelength = 0.9 micron
- Range = 10 km
- Accuracy = High
5
Q
Most common source of EDM instruments
A
Infrared, although laser becoming more common
6
Q
A micron is
A
One thousandth of a mm
7
Q
In the past, microwave and radio instruments were used for longer distances (e.g the Tellurometer and Tellumat), but with the advent of ____, their importance declined
A
GNSS
8
Q
Two methods of measuring with EDM
A
- Phase shift measurement - Been the norm until quite recently because time could not be measured to the required accuracy whereas phase shift could
- Pulsed laser distance measurement - Partly superseded today, but phase shift is still more common and better suited to certain tasks
9
Q
What method of measuring with EDM is used in the survey school.
A
Phase shift - used by trimble laser scanner
10
Q
The phase shift method uses continuous
A
Electromagnetic waves
11
Q
Electromagnetic waves are complex but can be represented by
A
Periodic sinusoidal waves
12
Q
Electromagnetic waves complete a cycle when
A
Moving between identical points on the wave
13
Q
The wavelength is the distance which seperates
A
Two identical points on the cycle
14
Q
The frequency in electromagnetic waves is
A
The number of cycles per second
15
Q
Frequency in waves measured in
A
16
Q
1 HZ equals how many cycles per second
A
One
17
Q
The speed of light in a vacuum
A
299,792,458 m s-1
18
Q
The speed of propagation of an electromagnetic wave depends on
A
The medium through which it is travelling
19
Q
Practical implications of the speed of propagation of an electromagnetic wave
A
- Distance measured depends on temperature, humidity and pressure
- For long ranges and precise work, the surveyor should measure pressure, wet and dry bulb temperature etc
20
Q
Equation of a wavelength
A
Velocity divided by frequency
21
Q
The phase (or phase angle) of the wave is a convenient method of identifying
A
Fractions of a wavelength
22
Q
What wave is good for measuring but bad for transmission over long distances
A
Measuring wave
23
Q
The wave that is somewhat good for measuring and good for transmission over long distances
A
Carrier wave modulated by measuring wave
24
Q
In pulsed measurement, distances are obtained by
A
Measuring the time taken for a pulse of laser radiation to travel from the instrument to a prism (or target) and back
25
The reason for pulsed measurement only now beginning to supercede the phase shift mwthod
When phase shift method was developed (50 years ago) time could not bemeasured with the required accuracy
26
Three types of reflectors
1. Prisms
2. Flat reflectors: stick on reflecting targets
3. Natural surfaces
27
Changing the prism mounting may change the ____ and therefore must be ____
May change the prism constant and therefore must be put back together correctly after cleaning
28
How far Edms can reach is quoted by
The number of prisms or reflector strip
29
Different EDM measurement modes range from
Precise (fine or standard) to rapid (coarse or fast)
30
Rapid EDM measurement modes provide
Faster measurement time but less accuracy
31
The EDM tracking mode provides
Measurements updated very quickly but not as accurate
32
The EDM tracking mode is useful for what
Setting out
33
EDM calibration in prisms is done by
Physical measurement and using manufacturers figures while being careful of loose mountings and swapped mountings
34
EDM measures a known baseline by
Direct comparison with known distances to determine additive constant and scale
35
Long EDM baselines are good, but what increases as a result
Atmospheric effect
36
If there is no access to a measured base, may have to settle for
Existing survey lines
37
When doing a 3, 5 or 7 peg test to determine AC, if prisms are in detachable tribrachs with built in optical plummets, what should always be done
Always keep optical plummet in the same direction (any error will cancel)
38
If tribrachs are positioned and levelled with a TS prior to fitting a prism, what should be done to check it is in adjustment
Look through the optical plummet 180 degrees apart
39
What is forced centring on a tribrach
Pillars with threads onto which tribrachs can be screwed directly
40
Overall combined instrument constant (IC) and prism constant (PC) gives out
Additive contant and zero error
41
The most important check once PC and IC (or AC) have been set
Measure a taped distance (e. 4-5m with a small tape) to confirm that nothing has been set wrong
42
EDM field practice calbration checks
1. 3 peg test regularly, so set AC and then repeat the test as a check, and/or preferably measure a taped distance at see AC has been applied correctly
2. Measure known line, at least once a year (to check SF)
43
The limiting accuracy factor on medium to long lines
Atmospheric correction
44
How to limit atmospheric effects
1. Use of a good thermometer
2. Avoid grazing rays
45
Minimum number of EDM readings
At least 2 complete determinations, pointing and reading
46
Why should one book the whole figure, not just mm
A repeat can give a mean mm and watch for any gross errors
47
Why is conducting a LF and RF always a good thing to do
1. Avoids vertical circle index error (vertical collimation)
2. A good check, since it entails a complete new pointing
3. Separately mounted EDMs may need to be detached for RF readings - historical
48
What should always be booked in EDM field practice
All instrument and prism serial numbers and settings
49
What checks should be made in field practice
1. Metres/feet
2. SD/HD
3. Read backwards
4. The best check of all is an independent fix from another mark (a double tie)
50
EDM field practice precision entails
Taking several readings and mean them (don't just repeat mm)
51
EDM field practice accuracy entails
Least count of EDM, reduction formulae, prism and instrument constants
52
Six general think-abouts in EDM field practice
1. Must know what the instrument is doing
2. What sort of reading (coarse/fine/tracking)
3. What reductions are being applied (ppm, AC, slope, curvature and refraction, sea level)
4. Don't just accept readings, be sceptical
5. Analyse the readings: do they paint a believable and consistent picture
6. Reductions are repetitive and ideal for a calculator program
53
Does sound, light or other electromagnetic radiation travel better underwater
Sound
54
What does sound require to pass through a medium
Vibration, of solid liquid or gas
55
Sound can travel through non-transparent materials depending on
The frequency used
56
Longitudinal waves are when
The motion of the particle is back and forth in the direction of travel of the wave
57
What type of waves are sound waves in water
Longitudinal waves
58
What is an echo sounder
A downwards looking sonar that transmits/recieves sound wave
59
The distance between the floor and the echo sounder can be calculated if
The velocity of the soundwave is known
60
Equation for depth from an echo sounder
Depth = (Velocity of sound in seawater x return travel time) / 2
61
Three factors speed of sound in water (SV) is affected by
Temperature, Salinity and pressure
62
How does temperature affect the speed of sound in water
An increase of 1 degree increases the speed of sound in seawater by about 2.5 m/s
63
How does salinity affect the speed of sound in water
A 1o/oo increase will increase the speed of sound by 1.3m/s.
64
How does pressure affect the speed of sound in water
Speed of sound increases 1.7m/s fro every 100m increase in depth
65
Direct measurement of speed of sound in water is done by
Transmission of acoustic energy over a known distance, measurement of the time taken
66
Indirect measurement of speed of sound in water is done by
Measurement of the physical properties that affect SV
- Temperature, salinity (conductivity), depth (pressure)
- then use formula to calculate SV
67
Incorrect SV is a result of
Incorrect depth calculation which gets worse as depth increases
68
Echo sounder calibration process
1. Measure or derive SV and enter into instrument or data collecion software
2. Measure or derive draft of echo sounder (TX = how far below the surface of the water the tool is) and enter into instrument or data collection software
69
The three categories of corrections to EDM measurements
1. Instrumental corrections (IC and PC)
2. Atmospheric corrections (Temp and pressure)
3. Geometrical corrections (Slope, Height above ellipsoid, projection)
70
Two EDM instrumental errors
1. Zero errors in instrument and prism
2. Scale error (or instrument Scale Factor)
71
Zero errors in instrument and prism can be determined with
A 3, 5 or 7 peg test
72
Zero errors in instrument and prism includes determination of both ____ combined as an ____
Determination of both instrument constant and prism constant combined as an additive constant
73
Ideally, manufacturers try to make the additive constant what value
0
74
The instrument varies with factors such as
Age, repairs, knocks and signal strength
75
Instrument constant is often valued at
30 mm
76
Prism constant is often valued at
-30mm
77
Scale error in EDMs are often due to
Variation in modulation frequency, and not as simple as instrument and prism errors
78
Scale error mitigation process
1. Should measure a known distance after setting AC (e.g a standard baseline measured accurately with a mekometer or invar band)
2. In the absence of a standard baseline, measure a previously measured distance at least once a year
79
Atmospheric correction to EDM measurements process
1. We measure in atmosphere not a vacuum
2. Need to determine the refractive index
3. Refractive index = speed of light / velocity in the atmosphere
80
Refractive index depends on
Temperature, pressure, humidity and carrier wavelength
81
Before measuring distances, get into the habit of
Checking T, P and set corresponding ppm on your EDM/TS
82
Some ballpark correlations:
Error of 1 degree in temp ->
Error of 1mb in pressure ->
Error of 1 degree in wet-dry bulb ->
Error of 1 degree in temp -> error of 1ppm distance
Error of 1mb in pressure -> error of 0.3 ppm
Error of 1 degree in wet-dry bulb -> error of 0.03 ppm (for near infrared, not microwave, where humidity is a lot more critical)
