Week 10: Techniques III

Question 1

GNSS is handy for many purposes but it is likely that traversing will be with us for some time because of problems with:

Answer 1

1. Multipath
2. Tree cover
3. Building interiors
4. Cost

Question 2

In traversing, for reconnaissance and layout, what survey control is available

Answer 2

1. Control marks, which may be need for:
   - orientation / origin of bearings
   - origin of coordinates (e.g for local transformations for GNSS)
2. Seach survey records in LINZ
3. May need to establish new survey control (with connections to existing survey control)
4. As well as the traverse accuracy, we need to bear in mind the quality of available control, what order is it?

Question 3

Good survey practice requires that new traverses need to incorporate

Answer 3

Checks

Question 4

Traverses should be

Answer 4

Closed (open/hanging traverses are not checked)

Question 5

When siting marks, what seven present things should be thought of

Answer 5

1. Intervisibility of marks
2. Where positions need to be set out / pegged
3. Maximising length of traverse lines
4. Inclusion of external orientations
5. Safety
6. Traffic management
7. Access

Question 6

When siting marks what three future things should be thought of

Answer 6

1. Intervisibility of marks
2. Permanence
3. Ease of relocation/re-finding

Question 7

Orientation / origon of bearings may be obtained from

Answer 7

Existing vectors (e.g bearings and distances on an old cadastral datum (OCD)), joins between coordinated points (including trigs), or even sun/star azimuth, compass bearings, gyro azimuths

Question 8

Cadastral regulations generally call for new cadastral surveys to be connected to

Answer 8

At least 3 existing survey marks, on the national survey control system or a previously approved survey, adequate to prove reliability, orientation and scale

Question 9

2 marks only will not prove reliability, and nearby marks cannot give

Answer 9

Good scale and orientation

Question 10

Even for non-cadastral purposes it is still a good idea to orient to at least

Answer 10

Two marks botha t the start and end of a traverse
- with only one mark it is difficult to prove non-disturbance

Question 11

It is usual to carry an approximate orientation forward, which involves

Answer 11

Orienting at the start of a traverse then add or subtract 180 degrees to the previous forward bearing to make a back bearing and orient to the previous traverse station

Question 12

At the end of a traverse, what is done to average out the inevitable small errors

Answer 12

A bearing adjustment

Question 13

Seven components of ideal traverse geometry

Answer 13

1. Similar length for each traverse line
2. No more than ten lines between orientation checks
3. Few traverse points better than many
4. Avoid (very) short lines
5. Ensure you can observe well clear of the ground and avoid “grazing rays” (refraction effect)
6. Ensure there are adequate checks for scale error and zero errors when a “circular”, “slim” or “scissor” traverse layout is impossible to avoid
7. No open (hanging) traverses

Question 14

Three errors that may come to light with circular traverses

Answer 14

1. A wrong initial coordinate (either misidentified start point or transcription error) may remain unnoticed
2. A scale factor will cancel out and, if symmetrical, a zero error (e.g a wrong additive constant) will cancel
3. A mistake in the orientation may go unnoticed

Question 15

The ideal traverse contains these five components

Answer 15

1. Closed
   - Known coordinate to known coordinate established independently and ideally more accurate: “whole to part”
2. Legs of roughly equal length
3. Two orientations to begin and end with
4. Roughly following a straight line
   - not circular, in which case scale erros or instrument/prism constants may cancel
5. Permanent, thoughtfully sited traverse marks

Question 16

Ground / traverse marks come in many different forms, for example

Answer 16

Iron tubes, iron spikes, lead plugs, plaques (set in kerb or cement block) etc

Question 17

Purpose of ground/traverse marks

Answer 17

Their establishment and placement are described by regulation or government guidlines, e.g permanent reference marks (PRM), officially recognised survey control or bench marks, boundary marks, horizontal control marks verses benchmarks (vertical)

Question 18

What to think of/determine around ground/traverse marks

Answer 18

1. Decide what is appropriate to the site (e.g while a nail/metal pin may be inappropriate for loose sandy soil it may be perfect for a footpath)
2. Ensure there are no underground services under or near position of new mark
   - Consider contacting electricity/telcom companies - “ring before you dig”
3. Think about future re-surfacing and street works
4. Think about finding the marks in the future
   - include finder diagrams with measurements to finder features in your field notes + photos
5. In different situations, a mark that is flush with the ground surface is optimal, or proud or esle buried at a suitable depth
   - record in field nots in finder diagrams + photos

Question 19

Where there are many legs/lines to your traverse or short lines, do

Answer 19

Orientation checks (check rays)

Question 20

Read mean, correct for orientation, to the

Answer 20

Least count of TS or theodolite and EDM, and only round bearings later (not when booking)

Question 21

Keep traverse observations seperate from

Answer 21

Other observations (e.g separate from topo detail

Question 22

Three things that should be done while measuring distances

Answer 22

1. Normally measure to the forward mark only
2. Check or repeat distances in some way (LF, RF, m/ft, HD and SD plus verticals on both faces, forward/back)
3. For a 3D traverse, also need to measure instrument height: get into the habit of doing this first, straight after set up

Question 23

Field notes should contain

Answer 23

1. Must be originals
   - If you have to copy, then doso in poen, write “copy” and attach to originals
2. No erasures; cross out neatly and book again
   - as there can be legal issues with this, there must be no possibility of a figure contested in court
3. Title page: to answer “what, where, when, how questions
   - include: instrument serial numbers, calibration values used, signature of surveyor carrying out the work, usually a sketch diagram
4. Calculations in general done on calcs pages, in pen (or in excel etc) not in field notes

Question 24

Six essential elememts of good field notes

Answer 24

1. Date all field note pages
2. Sketches (may be on the back of pages)
3. Naming/numbering of survey marks, and whether existing marks are not found, gone, disturbed or replaced (OIT, N/F, G, Dist, replaced)
4. Comprehensive descriptions of all marks, including depth buried, finder sketches, photos etc.
5. Use plenty of room with your field notes and, do not crowd
6. Field pages have value even where data capture is electronic

Question 25

Why do LINZ consider electronic data capture affidavit:

Answer 25

1. Surveyors are swearing to have placed new marks, looked for/not found/judged as gone or replaced existing marks, verified descriptions and made measurements 2. Often each page is initialed by surveyor as well as the title page being signed

Question 26

Distances are corrected for

Answer 26

Instrument corrections, prism, constant, ppm factor (temp and pressute) slope, height above ellipsoid and projection

Question 27

In NZ, coordinates (on a traverse sheet) are corrected for

Answer 27

Projection

Question 28

In NZ, distances on title plans and survey plans are reduced to

Answer 28

The ellipsoid (but are not corrected for projection)

Question 29

In NZ, bearings are expressed in terms of

Answer 29

The projection

Question 30

In NZ, areas are expressed in terms of

Answer 30

The ellipsoid

Question 31

For shorter traverses, equal legs are where accuracy is

Answer 31

Not critical, the bearing adjustment distributes the error evenly among the traverse legs

Question 32

For longer traverses, hight accuracies are where some legs are

Answer 32

Shorter than others, it may be worth distributing error more rigorously, according to the inverse of the length of traverse legs

Question 33

For a long traverse, reorientation with check rays is advisable, often to

Answer 33

A distant trig and ideally these check rays should involve observations from both ends

Question 34

In some circumstances, an appropriate orientation check can be

Answer 34

A daylight star or a pair (AM and PM) of sun observations

Question 35

When adjusting, as pointing errors are larger for shorter legs, typically more of the error compensating adjustment is put into

Answer 35

Shorter legs

Question 36

Once a bearing adjustment has been done, the simplest method to use (and to prgram) to adjust traverse coordinates would be

Answer 36

The bowditch adjustment

Question 37

Principle of the bowditch adjustment

Answer 37

The correction to individual E or N coordinates is in the same proportion as the length of the leg bears to the total length of the traverse

Question 38

Another traverse adjustment method along with bowditch

Answer 38

Least squares in a SNAP network adjustment: can include multiple data types

Question 39

If the bearings have closed satisfactorily but a large error is found in the closing join (traverse adjustment), this can suggest

Answer 39

An error in one of the traverse distances

Question 40

The bearing of the closing join (the join between the end coords using the traverse data and the control coordinate) often provides

Answer 40

A valuable pointer to which traverse line the distance error occured in (or 180 degrees apart)

Question 41

How to locate a gross error in distance

Answer 41

Check for transposed figures and gross errors like a 10m error

Question 42

If a gross error can be inferred, what should be done

Answer 42

Make a change in your field notes, and make it clear to read, and obvious it is an office change and not a reobserved distance in the field

Question 43

When recording field nots, never

Answer 43

Overwrite and never erase

Question 44

If a gross error in distance is discovered, the line should be remeasured if possible, however this id sometimes avoidable if

Answer 44

You have checked the distance in another way, for example HD and SD+ZA or through external orientations

Question 45

Five steps to locating a bearing error

Answer 45

1. Calculate coordinates from initial point through to final pointusing adjusted bearings 2. Calculate coordinates from final point through to initial point using unadjusted bearings - The point at which the error occured should have similar coordinates from the two calculations 3. Search field notes for differences between LF and RF, or between two sets (redundancy helps) 4. If no error is found, re-observe at the identified point (step 2) 5. Plotting the traverse can be useful to identify where the bearing error was made

Question 46

Observing intermediate orientations can:

Answer 46

1. Assist in finding where errors have occured (e.g you know in which part of a traverse an error lies) 2. Strengthen a traverse with a short traverse leg or legs

Question 47

In trig heighting, for longer lines, we need to consider

Answer 47

Curvature

Question 48

For trig heighting, we need a datum of

Answer 48

A vertical reference system e.g NZVD2016, Dunedin VD1958

Question 49

For EDM distances, you must know if the C and R (curvature and refraction) correction is

Answer 49

On or Off

Question 50

Most TS require an independently measures what to be keyed in

Answer 50

Temperature and pressure

Question 51

What tells you what formula is used for height reductions

Answer 51

TS manual

Question 52

Must observe on both LF and RF and mean wherever

Answer 52

Vertical collimation is incompletely corrected

Question 53

TS giving you change in height only applies when

Answer 53

The distance is measured

Question 54

What measurement systems give heights relative to the ellipsoid (ellipsoid heights)

Answer 54

1. GNSS receivers 2. LIDAR data sets (unless corrected)

Question 55

Ellipsoidal heights are geometrically correct with respect to

Answer 55

The coordinate frame and makes the mathematics straightforward

Question 56

Drawback to ellipsoidal heights

Answer 56

They are at odds with gravity, i.e water will not necessarily flow from a greater ellipsoidal height to a lesser one

Question 57

Water flows from a point greater to lesser gravitational potential (i.e relative to the geoid) and this has traditonally been determined by

Answer 57

Bubble/compensator instruments (levels, theodolites, TS) - These are gravity sensitive heights (orthometric heights) rather than ellipsoidal heights

Question 58

GNSS heights (determined on WGS84 ellipsoid) will uusally be of limted use to

Answer 58

Engineers or farmers who want to know wherher water will flow from one point to another before a canal or pipeline is built e.g irrigation races

Question 59

If GNSS is to be used for heighting, then what has to be applied

Answer 59

Geoid-ellipsoid sepearion has to be applied to the ellipsoid heights i.e GNSS heights