THEODOLITE TRAVERSE
1.0 INTRODUCTION
A traverse consists a series of straight line of known length related one
another by known angles between the line. The points defining the ends of
the traverse lines are called traverse stations.
Theodolite traverse survey is a method of establishing control
points, their position being determined by measuring the distances between
the traverse station which serve as control points and the angles subtended
at the various stations by their adjacent stations. The angles are measured
using theodolite and the distance measured by the method of traverse. We
also measured the distance used the measurement tape.
There are two(2) types of traverse, namely the open traverse
and the closed traverse [Figure 1.1]. An open traverse originates at a
point of known positions and terminates at a point of unknown position,
where as a closed traverse originates and terminates at the same points and
we were using an closed traverse in this survey. For establishing control
points, a closed traverse is preferred since it is provides different
checked for included angles, deflection angles and bearing for adjusting
the traverse. That why we use this method in this project. When an open
traverse is used the work should be checked by providing cut off lines and
by making observations on some prominent point visible forms as many
stations as possible.
Traversing surveys is very fundamental and has become one of the
most common methods in geomatic engineering work such as:- general purpose
angle measurement, provision of control surveys, contour and detail mapping
and setting out and construction work.
1.1 OBJECTIVES
To make a traverse survey, reduce the field data, adjusting a traverse and
plot the results graphically.
Figure 1.1 : Open travers & Closed traverse
2.0 THEODOLITE PRINCIPLES AND APPLICATIONS
A theodolite is an instrument which is capable of measuring angles to the
nearest whole second [Figure 2.1]. This can be done for both vertical and
horizontal angles. Vertical angles are required for the calculation of
elevation of points for example the reduction of slope distance to the
horizontal.
Horizontal angles are required to obtain the relative direction to a
survey control station or points of detail. Basically there are two types
of modern theodolite which are in use today. These are the:
(i)Optical theodolite; and
(ii)Electronic Digital theodolite.
Both types of instrument can be made to read to the nearest whole 1"
which is considered accurate enough for most engineering purposes. With the
advancement of modern electronics, most of the theodolites made today are
of the electronic digital type. But the older optical types are still being
used except that it will take longer time to read the angles than with an
electronic one.
The value of the angle observed however will be the same. Electronic
theodolites are more versatile than the optical type. Useful features in
the form of software can be added to an electronic theodolite. Thus modern
instruments can be used in a variety of surveying situations.
Optical theodolite
Electronic Digital theodolite
Figure 2.1 : Optical theodolite & Electronic
Digital theodolite
3.0 CONSTRUCTION OF A THEODOLITE
All theodolites have the same common features [Figure 3.1] which can be
described as follows:
1.Tribrach
Allows the instrument to be connected to the top of a tripod and also
allows the instrument to be levelled with respect to a plate bubble.
2.Horizontal Circle Compartment
This compartment is comprised of:
i) The lower plate that carries the horizontal circle. In most
instruments it is made of glass with the graduations from 0ºto
360ºphotographicallyetched around the edge.
(ii) The upper plate that carries the horizontal circle indexing device
and fits concentric with the lower plate. Attached to the upper plate is
the plate bubble. When centered, the plate bubble ensures that the
instrument axis is vertical. In modern electronic theodolites, the spirit
bubble has been replaced with an electronic one. This electronic means of
levelling has made initial levelling of the instrument a less time
consuming task.
3.Vertical Circle Compartment
The vertical circle is similar to the horizontal circle but is fixed to the
telescope. Thus it revolves with the rotation of the telescope. This
compartment has the vertical circle index. Most modern theodolites employ
an automatic compensator but some of the more elderly instruments use an
altitude bubble.
4.Horizontal Motion Clamp/Vertical Motion Clamp
This clamp ensures that when engaged the horizontal circle is fixed. The
slow motion screws allow for the movement of the instrument around the
horizontal axis. There is also a similar slow motion screw and plate clamp
for the vertical circle.
5.Optical Plummet
Built in the base of the instrument to allow the instrument to be
precisely centered over the station. The line of sight through the optical
plummet is exactly the same as the vertical axis of the theodolite.
Figure 3.1 : Theodolite construction
4.0 TEMPORARY ADJUSTMENTS OF THEODOLITE
The temporary adjustments are steps that must be carried out every time at
theodolite is used. It is a procedure of setting up a theodolite that
involves the following process:
a) centering;
b) leveling; and
c) removing parallax.
4.1 CENTERING THE THEODOLITE
The instrument must be vertically above the survey station to ensure that
horizontal angle observations are correct. The steps are as follows:
1.Start with a plumbob to get it approximately right above the survey
station [Figure 4.1].
2.Using the foot screws, move the optical plummet cross hairs on to the
survey station.
3.Roughly level the instrument using the legs of the tripod – the
theodolite should stay almost on target.
4.Level with foot screws. Move instrument above target; repeat level and
move until done.
4.2 LEVELLING THE THEODOLITE
1.Turn bubble parallel to two foot screws A and B [Figure 4.2], to bring
the horizontal bubble to the centre of its run by moving the foot screws in
opposite directions (the bubble moves in the direction of your left humb).
2.Turn the instrument through 90º and bring the bubble to the centre of its
run by adjusting the third foot screw C only.
3.Turn the instrument through a further 90º to check the adjustment of the
plate bubble.
4.If the bubble remains in centre, then it is adjusted.
5.If not, move it back one-half of the movement from the centre and re-
adjust for a further 90º turn.
6.Repeat the whole procedure; assuming this is the correct, the bubble will
stay in a stationary position.
7.The bubble must remain in the same place in the tube during a
360ºrotation of the instrument.
8.If the stationary position of the bubble is still off the centre, then a
permanent adjustment should be made.
4.3 REMOVING THE PARALLAX
Parallax is a condition happen when the image formed by the objective is
not in the plane of the cross-hairs. Parallax should be eliminated in order
to have accurate sighting. There are two ways to overcome or eliminate the
parallax. There are by accurately focusing the cross-hairs against a light
background and focusing the instrument on a distant target or by focusing
the eye-piece for distinct vision of the cross hairs.
Figure 4.1 : Centering the theodolite
Figure 4.2 : Levelling the theodolite
5.0 PERMANENT ADJUSTMENTS OF THEODOLITE
These adjustments are carried out once and will not alter unless it is
being roughly handled or tampered with. There are certain basic
requirements for a theodolite that must be established particularly when
using it. The basic requirements are as follows:
(a) The vertical axis of a theodolite should be truly vertical.
(b) The line of sight should be perpendicular to the horizontal axis.
(c) The horizontal axis should be truly horizontal.
(d) The cross hairs should be truly vertical and horizontal.
(e) The vertical circle should be at zero when the line of sight is
horizontal.
For this study it is appropriate to know only the basic requirements for
permanent adjustments. The steps in carrying out the adjustments should be
handled by the qualified person at the laboratory.
6.0 TRAVERSE FIELD WORK
Traversing is carried out with one(1) tripod and two(2) wooden stands with
plumbob. Tripod is for the instrument and the other two(2) wooden stands
are for the back and front stations. A minimum of three people is required
in a traversing team.
The leader of the team, setting up and reads the instrument, while
the 2nd person has the important job of recording the readings on the
booking sheet. The 3rd person has the task of moving and setting up the
wooden stands with plumbob as the traverse progresses. There are several
steps which should be followed that will lead to a smooth traverse [Figure
6.1].
1.Three picket(station) were established at the proposed site.(peg 1, peg
2 & peg 3).
2.The theodolite was plumbed over peg 2 and accurately leveled. Wooden
stands were plumbed over peg 1&3.
3.Level and center the instrument.
4.Set the theodolite to read zero.
5.Distance was measured by collimating the center of peg 1 by measuring
tape. The reading was taken and entered in the field book. Distance between
peg 2 and 3 also done with the same technique.
6.Record face left horizontal reading to back station(peg 1).
7.Turn instrument and sight front station.
8.Record face left horizontal reading to front station(peg 3).
9.Transit the instrument to change to the face right setting.
10.Record face right horizontal reading to front station(peg 3).
11.Turn instrument to face back station.
12.Record face right horizontal reading to back station(peg 1).
13.The theodolite was moved to peg 3. Wooden stands were plumbed over peg 1
and 2. Peg2 was sighted on face left with theodolite set to the reading
taken from step 10.
14.Peg 1 was sighted and the horizontal angle was taken. The instrument was
set to face right and peg 1 was sighted again. Then peg 2 was sighted and
the reading was taken.
15.Step 13-14 was repeated on peg 1.All readings were observed and
recorded.
Figure 6.1 : Field traversing
7.0 TRAVERSING ERRORS
Traversing errors normally falls into three categories, i.e. centering,
angular and distance. By taking precautions during the field work, it is
possible to reduce their effect.
1.Centering
It is important to ensure that the theodolite instrument and targets are
centered correctly over each survey station. Remember that angles and
distances may be required from or to a known station. This will not be the
case if the theodolite or targets are not centered correctly.
2.Angles
When clamping the instrument, apply light clamp to the vertical and
horizontal locks. Hard clamping can affect the pointing of the instrument
and is not necessary. Failure to eliminate parallax and poor focusing can
affect accurate pointing.
Always keep the target in the center of the field of view. All movement of
the theodolite should be kept as smooth as possible and all movement around
the instrument should be kept to a minimum.
3.Distances
When recording these, all distances should be obtained to 3 decimal places
and three readings should be taken and the mean calculated.
There is a possibility that that some of the errors outlined below will
occur from time to time, so be aware of them. Don't rush and hopefully you
will not forget to record any information which is required.
a)Turning the wrong screw.
b)Sighting the wrong target.
c)Using the stadia instead of the cross-hairs.
d)Forgetting to set the micrometer reading before taking a reading.
e)Misreading the circles.
f)Transposing the figures when booking the data.
8.0 DISCUSSION
1. Before doing the survey, there are a few things that must be
considered. There are:
All screw must be parallel to the centre line
Tripod stand must be in a straight line with the
picket(station).
After setting out, wooden adjustment legs and theodolite
must be in a straight line to the picket.
Value from bearing must be entered correctly.
Distance between one point to the others must clearly seen.
2. When survey was taking on, there are several things that must be
clearly taking place which are:
The cross hair must be clearly seen by observer.
The parallax error might be occur when reading was
taken.Therefore the observer must be the same person and very
sure that the reading taken was correctly.
During the changing of left face and right face, be sure
tahat the position of tripod did not change.
Distance between one point to another was taken correctly.
3. After field wprk was complete, proceed with analyzing data. The
correction between latitude and departure must be placed with
correct sign and value.
4. A closed traverse enables a check by plotting or computation with a
gap called the linear misclosure.
9.0 CONCLUSION
Traversing is a form of a control survey that requires the establishment of
a series of stations that are linked together by the angles and distances.
The angles are measured by theodolites, and the distances are measured
conventionally by tapes or electronic distance measuring equipment.
A theodolite is an instrument which is capable of measuring both
vertical and horizontal angles to the nearest whole seconds.
Basically there are two types of modern theodolite i.e. the optical
theodolite and electronic digital theodolite. But both have the same common
features in terms of their construction.
The theodolite system is comprised of the horizontal circle where
it is perpendicular to the vertical axis and the vertical circle where it
is perpendicular to the horizontal axis. Theodolites in correct adjustment
have their axes and line of sight of the telescope mutually perpendicular.
All three should intersect at one point.
10.0 REFERENCE
1. http://www.southgeosystems.com/index.html
2. http://www.geomaticsurveys.systems.com
3. http://www.scribd.com/doc/38971159/Traverse-Survey
4. http://www.ce.memphis.edu/1112/notes/traverse/Surveying_traverse