In This Article, We Explained About Errors And
Precautions For Plane Table Surveying, Procedure Of Plane Table And Advantages
And Disadvantages
ERRORS AND PRECAUTIONS FOR PLANE TABLE SURVEYING
The following are the common errors in plane tabling:
A. Instrumental Errors
1. The surface of the table may not be perfectly level.
2. The fiducial edge of the alidade might not be
straight.
3. The vanes may not be vertical.
4. The horsehair may be loose and inclined.
5. The table may be loosely joined with the tripod
stand.
6. The needle of the trough compass may not be perfectly
balanced. Also, it may not be able to move freely due to the sluggishness of the
pivot point.
The above errors are adjustable or
avoidable.
B. Personal Errors
1. The levelling of the table may not be perfect.
2. The table may not be centred properly.
3. The orientation of the table may not be proper.
4. The table might not be perfectly clamped.
5. The objects may not be bisected perfectly.
6. The alidade may not be correctly centred on the
station point.
7. The rays might not be drawn accurately.
8. The alidade may not be centred on the same side of
the station point throughout the work.
C. Plotting Error
1. A good-quality pencil with a very fine pointed end
may not have been used.
2. An incorrect scale may be used by mistake.
3. Errors may result from failure to observe the correct
measurement from the scale.
4. Unnecessary hurry at the time of plotting may lead to
plotting errors.
The following precautions should be taken
while using the plane table:
1. Before starting the work the types of equipment for survey
work should be verified. Defective accessories should be replaced by perfect
equipment.
2. The centring should be perfect.
3. The levelling should be proper.
4. The orientation should be accurate.
5. The alidade should be centred on the same side of the station pin until the work is completed. (Note: If the work was started with
the alidade on the left of the station pin, then the work should be completed by maintaining this position. This is the ideal position of
the alidade and should always be preferred.)
6. While shifting the plane table from one station to
another, the tripod stand should be kept vertical to avoid damage to the fixing
arrangement.
7. Several accessories have to be carried. So, care
should be taken to ensure that nothing is missing.
8. The pencil should have a sharp point.
9. The distances of the objects or lines should be
written temporarily along with the respective rays until the plotting is completed.
10. Only the selected scale should be on the table.
11. Measurements should be taken carefully from the
scale while plotting.
12. The stations on the ground are marked A, B, C, D,
etc. while the station points on the map are marked a, b, c, d, etc.
PROCEDURE OF PLANE TABLE TRAVERSING
A. Equipment Required
1. Plane table with tripod stand — 1
2. Alidade (plain or telescopic) — 1
3. Trough compass or circular box compass — 1
4. Spirit level — 1
5. U-fork with plumb bob — 1 set
6. Metric chain (20 m) — 1
7. Metallic tape (15 m) — 1
8. Arrows — 10
9. Ranging rods — 3
10. Wooden pegs — 10
11. Mallet — 1
12. Drawing sheet (good quality) — 1
13. Board pins or clips — 4
14. Cardboard scale (set of 8 scales) — 1 set
15. Good pencil — 1
16. Eraser — 1
17. Knife — 1
18. Pins — 5
19. Set-square (45°, 60°) — 2
20. Duster — 1
B. Procedure of Field Work
1. Reconnaissance
The area to be surveyed is thoroughly examined to find
the best possible way for traversing.
The traverse stations should cover the whole area and should be intervisible.
The provisions for check lines should be kept in mind.
2. Marking
the Stations
The selected stations are marked on the ground by wooden
pegs. Reference sketches should be prepared for the stations so that they can
be readily located in case the station pegs are removed.
3. Connecting
the Traverse Legs and Marking Details
A detailed description is given in the previous post. Simply
follow the procedure outlined there.
ADVANTAGES AND DISADVANTAGES
OF PLANE TABLING
A. Advantages
1. It is the most rapid method of surveying.
2. There is no need for a field book as plotting is done
along with the fieldwork. So, the problem of mistakes in booking field notes
does not arise.
3. Plotted work can be compared with actual objects
regardless of whether or not they are properly represented.
4. There is no possibility of overlooking any important
object.
5. There is no possibility of overlooking any
measurement as plotting is done in the field.
6. Irregular objects may be represented accurately.
7. It is suitable in magnetic areas.
8. The map can be prepared easily and does not require
any great skill.
9. Errors in measurement and plotting can be detected by
checking lines.
10. Inaccessible points can be easily located by
intersection.
B. Disadvantages
1. The plane table is not suitable for accurate work as
the fitting arrangement is not perfect.
2. Plane table surveying is not suitable in a wet climate,
in the rainy season, on foggy mornings and in windy weather.
3. The number of accessories required in such a survey is
large, and they are likely to be lost.
4. The instrument is very heavy and difficult to carry.
5. The map cannot be replotted to a different scale as
there is no field book.
READ ALSO: Plane Table Surveying | Principle | Orientation | Procedure Of Setting Up | Methods
FAQ
1. One
month after the completion of a plane table survey, it is detected that one
important object was not plotted. How will you plot the object on going to the
field?
A new station has to be established with the help of the
two-point or three-point problem. The object can then be located with reference
to that new station.
2. What
type of orientation would you prefer and why?
Orientation by backsighting is always preferred because
it is more reliable than the magnetic needle method. In this method, magnetic
substances do not affect the work.
3. What
method would you apply for locating inaccessible points?
The method of intersection should be applied in order to
locate inaccessible points.
4. What
do you mean by the ‘fiducial edge’ of the alidade?
The working bevelled edge of the alidade is known as the
fiducial edge.
5. What
are the different types of alidade?
There are two types of alidade—plain and telescopic.
6. What
do you mean by ‘strength of fix’?
In the three-point problem, the relative positions of A,
B, C and the required point P should be such that the required point can be located very quickly and accurately. The
accuracy with which the point can be
fixed is termed the strength of fix.
7. What
are the precautions you have to take while centring the alidade with the
station pin?
The alidade should be centred on the same side of the station pin throughout the traverse. Keeping the alidade on the left of the station
pin is ideal.
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