Lecture 2: Angles Flashcards
(92 cards)
1
Q
The magnification of a total station is usually between
A
25-30x
2
Q
The field of view ot a total station is usually
A
1-2 degrees
3
Q
The field of view of a total station is useful to know for
A
Rough estimates: observing two points just visible
4
Q
In the optimal design of a total station, the components such as magnification, field of view, brightness and resolution are all
A
Interdependent, therefore thre are tradeoffs involved.
E.g low magnification may be an advantage while shimmer is a problem
5
Q
Theodolite stability is retained through
A
- Tightening the bolts of tripod head occasionally
- Check optical plummet for parallax
- Between sets, tap the legs to remove strains, and re-level if necessary
- Wooden tripods should be oiled/painted occasionally
- Shading the instrument and legs and protection from the wind
- Keep footscrews near their central position
6
Q
Measuring horizontal angles process
A
- Identify all targets to be sighted
- Identify the furthest distinct object as your RO (reference object)
- Set (approximate) join value
- Rotate instrument through 360 degrees before sighting and reading
- Observe left face clockwise then transit (change face) and observe right face anticlockwise
7
Q
Why should all targets be identified to be sighted
A
We want a smooth flow of observations
8
Q
Why should the furthest distinct object be the RO
A
If too close, you won’t be able to tell if the T/S twists
9
Q
Starting with left face is mandatory. True or False
A
False. No advantage either way, but should keep going the same way in case of looseness in any component.
10
Q
Is set comprises ____ rounds of observations
A
Two (LF and RF)
11
Q
A set is sometimes also called
A
An arc
12
Q
Rule of thumb for observations
A
Observe “one set per mile” (1.6km) for a robust mean
e.g A local prac with 3km between targets should do a minimum of 2 sets.
13
Q
For optical theodolites, what should be done in terms of the number of sets
A
Change circle setting each time (divide 180 by number of sets, as the circle is usually read in two places, 180 degrees apart)
e.g for three sets, 0, 60 and 120 degrees.
14
Q
Measuring vertical angles process
A
- Search and identify all targets to be sighted
- Note the point observed to (sketch in field book)
- Observe LF and RF, then next target LF and RF (may do distance at the same time)
- Repeat until all targets have been observed
15
Q
Vertical observations are made ____ of the horizontal observations
A
Independent
16
Q
Bookers checks for measuring angles
A
- Are LF and RF (horizontals) approximately 180 degrees different
- Is there a consistent horizontal collimation error
- Do LF and RF (verticals) sum to approximately 360 degrees
- Is there a consistent vertical circle index error (vertical collimation) e.g if LF+RF = 360.00.16, this would be approximately consistent
- With long lines (or accurate work) include a second pointing back on the RO (last LF pointing and first RF pointing, and check difference between first and last RO pointing is small)
17
Q
What should the booker do in relation to the observer
A
Keep up with the observer, and query any anomalies.
18
Q
A booker shoul not say what a reading should be but should
A
Request a repeat
19
Q
How should clamps be tightened
A
Just enough to prevent movement and no more
20
Q
When observing, touch the instrument lightly and dont touch
A
The tripod at all
21
Q
You should set up with what in mind
A
Observers height and the direction of targets
22
Q
Never re level during
A
A round
23
Q
The official techniue for checking parallax
A
Move your head side-to-side to see if crosshairs move in relation to the peg: pull in, pull out, or seperate focus
24
Q
Eliminate ____ before commencing observations
A
Parallax
25
Aim to take observations ____ to give a better result than agonising over bisection or reading
Smoothly and quickly
26
Horizontal observations are best done when targets are
Clear and steady (early or late in the day are best)
27
Distant targets are more easily seen with the sun
Behind the observer (observe west targets in the morning and vice versa)
28
Verticals are best done during
The middle of the day, 11-3, as k (coefficient of refraction) is the most stable
29
What is the trade off for observing verticals in the middle of the day
Shimmer
30
Aim close to the
Centre of the field of view
31
Why should we aim close to the centre of the field of view
Lens distortion smallest and minimise any error through non-true crosshair
32
What should be avoided during observations
Large changes of focus, should instead observe nearby objects in seperate sets immediately after closing back to RO
33
Verticals are most stable at around
Midday
34
How should an instrument be stored if wet
Left out of box in the dry room, otherwise mildew may form on lenses
35
Instruments should always be transported
In its case
36
After removing the instrument the case should always be
Closed, so neither grit nor moisture get into the box
37
Allow the instrument to ____ before observing commences
Take up ambient temperatures
38
The 7 tests and adjustments in total stations
1. Optical plummet
2. Plate bubble
3. Crosshair alignment
4. Horizontal collimation
5. Vertical circle index error (vertical collimation)
6. Trunnion axis inclination
7. Circle graduation errors
39
Optical plummet condition
Plummet axis is not parallel to XX'
40
Optimal plummet effect
Instrument not set up vertically over survey mark
41
Optical plummet test cases
1. Plummet in instrument - plumb instrument then rotate 180 degrees
2. Plummet in tribrach (e.g tribrach mounting for a measuring prism
42
Optical plummet field remedy cases
A)
1. Move instrument by half the error
2. Adjust plummet so it points to the mark
3. Re-test
B)
1. Place field book on ground below instrument/tribrach
2. Plumb over mark (as pre-marked) on FB page
3. Draw outline of tribrach on tripod top
4. Rotate tribrach by 120 degrees
5. Re-level circular bubble
6. Mark plumb point on FB page
7. Repeat for a third position (240 degrees)
8. Adjust plummet so it points to centre of the three marks on the page
9. Re-test
43
Plate bubble condition
Bubble BB' not perpendicular to primary ("vertical") axis XX'
44
Plate bubble effect
Plate bubble off-centre when instrument is level (i.e when the primary axis is truly vertical)
45
Plate bubble test
Centre the bubble and rotate 180 degrees - watch how the bubble behaves
46
During a plate bubble test, if the bubble is in adjustment and centred, when you rotate 180 degrees, the bubble will
Remain centred
47
During a plate bubble test, if the instrument is level but the bubble out of adjustment, then the bubble will
Stay in the same off-centre position when rotated
48
Plate bubble field remedy
1. Use footscrews to move the bubble back by half its movement
2. Rotate the instrument to check the bubble stays in a constant position
3. Use a screwdriver or "tommy-bar" to adjust bubble so that it is in the centre of the graduation
4. Re-test
49
For plate bubble dislevelment, error in the horizontal reading depends on
1. Magnitude of dislevelment
2. Slope of line being observed (very important on steep sights) (ZA)
3. Bearing of the line relative to the bearing of dislevelment
50
For plate bubble dislevelment, why can the horizontal error not be corrected by LF and RF observations
Because angles are not measured in the horizontal plane
51
For plate bubble dislevelment, how can the error in the horizontal reading be minimised
1. Level carefully
2. Keep bubble in adjustment
3. Check between rounds
4. Shade tripod legs for accurate work
5. Maintain tripod well
6. For accurate work with steep sights, level carefully
52
Crosshair alignment Condition
Occurs when vertical crosshair CC' and the vertical axis XX' are not coplanar
53
For modern instruments, you can assume CC' is perpendicular to
DD'
54
Crosshair alignment effect
Errors in pointings when target is not centre of crosshairs
55
Crosshair alignment test
Bisected object should remain on crosshair as telescope is moved up and down
56
Crosshair alignment field remedy
Move crosshairs with adjusting screws (leave to instrument technician)
57
How to minimise the crosshair alignment effect in the field
Observe at the same position on crosshairs (preferably near centre) and mean LF and RF
58
Horizontal collimation condition
Line of sight AA' is not perpendicular to trunion axis YY' (manifested as LF ≠ RF, but not the only cause of this)
59
Horizontal collimation effect
There is an error in the horizontal reading that applies to all horizontal readings.
60
Horizontal collimation field remedy
1. Mean LF and RF
2. Adjustment = a) Optical theodolite: move crosshairs
b) Total station: Programmed procedure (a software correction)
3. Not uncommonly there is residual collimation even after testing
4. Field test: to evaluate how big the collimation error is (for ZA approx 90 degrees)
61
Vertical circle index error is also known as
Vertical collimation error
62
Vertical circle index error condition
XX'' not perpendicular to BB' and/or ZZ' not parallel to AA'
63
Vertical circle index error effect
Constant error on all vertical circle readings
64
Vertical circle index error test
Take LF and RF vertical readings to a distinct point: 2V'' = 360 degrees - (LF+RF)
65
Vertical circle index error field remedy
Add V to all LF readings and check: (LF+v) + (RF+v) = 360 degrees
66
Trunnion axis inclination condition
Trunnion axis YY' not perpendicular to vertical axis XX'
67
Trunnion axis inclination field remedy
1. Test: Sight a point on a tall object, lower telescope and read scale at ZA approx 90 degrees. Change face and repeat
2. Mean LF and RF readings
3. Adjustment: only in instrument workshop
68
Circle graduation errors occur when
Graduation marks on horizontal or vertical circles are in erroneous positions
69
Circle graduation errors effect
Varies systematically depending on method of manufacture
70
Circle graduation errors field remedy
1. Test: Many observations on different parts of the circle
2. Adjustment: None
3. Change orientation between sets (a good idea even on a modern total station using a barcode on circles)
71
Micrometer errors: If using an optical theodolite for precise work, you should use
Different parts of the micrometer for different sets
72
Circle eccentricity field remedy
Use a double circle reading theodolite or total station, where readings on opposite sides of the graduated circle are made simultaneously.
73
We can assume all school of survey instruments are
Double reading
74
Keep your total station in good adjustment, but act as if
It was not in good adjustment
75
Simple checks for total station adjustment
1. Observe Lf and RF
2. Observe near centre of field of view
3. Observe on different parts of the circle
76
Three Instrumental sources of error and how to mitigate them
1. Circular bubble - adjust to centre when instrument is levelled with plate bubble
2. Tripod instability - keep hinges firm
3. Parallax - remove
77
When looking through a total station, if the image is not formed in the plane of the reticule, there will be
Parallax
78
If parallax is present, when the observers eye is held in a different position the reading will be
Altered and inaccurate
79
Method for checking for parallax
Focus on the target and move head from side to side to see if the target moves in relation to the crosshairs
80
Staff bubble as a source of error field test
1. Level a prism pole and brace it against a tripod
2. Turn staff bubble 180 degrees - the bubble should not move
3. To adjust, following the test above, move the pole so that the bubble comes back by half (bubble should now stay in a constant position when rotated on the pole)
4. Use screws underneath to adjust bubble housing
81
Staff bubble as a source of error office test
Adjust against a surface proved vertical
82
Four Environmental factors and how to mitigate them
1. Wind/vibration - may need shelter, wind tent
2. Temperature - It may pay to level with the plate bubble in observers shadow, and for accurate work may need an umbrella
3. Refraction - Not only ground but also hills/buildings (grazing rays)
4. Unstable ground - Use of platforms, waratah plus legs set in plaster of paris, observing pegs for each of the tripods legs, don't stand too close to legs
83
Four personal errors and blunders (gross errors)
1. Misidentification of target over mark to be observed
2. Poor focusing (blurred, and parallax introduced)
3. Hesitation: aim for a smooth flow of readings
4. Bumping the instrument and not noticing
84
If a mark is poorly defined, e.g drill hole in concrete, you can
Place a coin at the bottom, or for an OIS use the point of a pencil
85
Error in the horizontal plane depends on
1. Magnitude of offset
2. Length of line being observed
3. Bearing of line relative to bearing of offset
86
Reading circles incorrectly can be resolved by
Checking LF and RF
87
Booking errors for distance cen be kept to a minimal if we record
Measurements in cm and inches to convert, and slope awell as horizontal
87
How to check the additive constant (AC) on a total station is correct
Check on a short, horizontal distance measured with a 5m tape to make sure you have applied the AC correctly
88
Eccentric stations are sometimes called
Satellite stations
89
Cases when using eccentric stations is the best option
1. When a set-up is not on a survey mark
2. A target does not coincide with a mark
90
What situations result in us being unable to set up on a survey mark
1. If you cannot set up a point, e.g a low trig vane, well bolted down, or a cadastral traverse mark immediately beside a wall corner or fence post
2. Your distance observation is from another seperate EDM instrument (historical)
3. If you can set up over a survey mark but a line is not inter-visible so a nearby mark needs placing
91
What situations result in a target not coincideing with a mark
1. Can happen unintentionally (e.g if an observed target is found to be mis-plumbed, in which case observations need to be corrected to account for the mis-plumbing)
2. If a target point is just not visible and a prism pole/target is deliberately set up off the true position and a correction made (e.g a tape measurement is made to the correct mark)
