DEPT. OF GEOGRAPHY & RESOURCE DEVELOPMENT

GEOG 204

LECTURE NOTES

Principles of Theodolite and

Total Station Survey

Introduction

In comparison to Chain and Compass, Theodolite and Total Stations

represent more modern and accurate forms of Survey. Theodolite survey

which, though becoming obsolete, forms the basis for the modern electronic

based Total Station technique which has become a more modern and

acceptable accurate technique for mapping.

Key topics:

Introduction to the Theodolite

Description and use of the Theodolite

Introduction to the Total Station

Using the Total Station

Fieldwork

Introduction to the Theodolite Introduction The theodolite became the favourite of surveyors because it could be used to

measure horizontal angles fairly accurately.

The accuracy of triangulation depends on the accuracy of the instruments used. The theodolite is used by surveyors to give accurate measurements. Essentially this instrument is a telescope designed to rotate horizontally and in

the vertical plane. As the instrument is moved in the vertical plane a pointer

moves with it in the horizontal circle calibrated with 360 degrees of a circle.

Vertical movements are recorded on a vertical circle graduated from 0 to 90.

There are different types of theodolites. Each is best used for a particular

purpose.

Low-order instruments. These read directly by optical scales or single reading micrometers 20 or less. Such instruments are used for minor detail surveys, general engineering and building works and setting out where great accuracy is not required.

Universal theodolites. This instrument reads directly to a single second by means of a coincidence optical micrometer. This is one of the land surveyors theodolite being used in all classes of work from large-scale surveys to third-order triangulation. These theodolites are single-axis theodolites as the accuracy is such that the repetition method of observing is never required. The horizontal circle can, however, be rotated so that it may be set in any required position.

Geodetic theodolites. These form the most precise type of theodolite and usually read directly to at least one-fifth of a second of arc by means of the coincidence optical micrometer. Their circles are larger and are more finely divided. Their telescopes are more powerful and the instruments are larger and heavier. They are used mainly for primary and secondary triangulation, first-order precise traversing and geodetic astronomy,

Description and use of the Theodolite Figure 5.2 The labelled T16-84 Theodolite

The earlier type of Theodolite

Description of the Theodolite The Theodolite is also mounted on a tripod when in use. It consists basically of the following parts;

a fixed base (called trivet) with a tribrach, the alidade which is the movable upper part a telescope, the base with the tribrach is fixed tightly to the top of the tripod and it is levelled by means of the three foot screws as well as adjusting the legs of the tripod.

The telescope also contains the

eyepiece lens and the object lens. The line joining the centres of these lenses is

called the sighting axis (line of sight). The instrument is roughly levelled on the tripod by adjusting the legs of the tripod and using the foot screws to bring the circular bubbles fitted to the base to the centre.

Accurate levelling is then achieved by using only the foot screws to bring

tubular bubble (plate level) to the centre of its run. The telescope can be

turned to any target by rotating about its horizontal and vertical axis. When the

rough position to the target is obtained, the alidade is clamped using the

horizontal clamp. This prevents the alidade from rotating. The telescope is

also clamped using the vertical clap.

Fine sighting is then achieved by using the horizontal and vertical slow motion

screws respectively. The instrument must however be set over the point by

means of a plumb bob or the optical plummet.

The eyepiece of the telescope can be adjusted to make the crosshair clear.

The focusing screw is then used to ensure clear of the target through the

instrument. The angle is measured by selecting on the target as Reference

Object (R.O) and an initial reading made to it. The instrument is then swivelled

to the other target and the reading is made again. The angle is then obtained

by subtracting the initial reading from the second reading. This can be

repeated several times and the mean calculated. As you can see these

procedures can be rather complicated

Booking In booking, the reference object is used as back station and the other target

used as forward station. As in series levelling (UNIT 4), you could also stand

at one point and take readings to several points around. This may be called

round rays or radiation. The instrument must always be set up over a point

that is fixed. The reading to the target is also always bone on both sides of the

circle.

Theodolite booking as illustrated in Table 5.1 below involves careful

identification and recording of

Instrument station Back station Forward station Face and swing HCR Include angle Difference Remarks

Table 5.1 Booking of Station Data Instrument

station

Back

station

Forward

station

Face and

swing

H.C.R Include

angle

Distance

(ft)

Remarks

Introduction to the Total Station Introduction In the light of the rapid technological advances particularly in electronics, a

number of cartographic and surveying equipments have been more

acceptable as more modern, accurate and more common for surveying. One

of these is the total station.

The Total station is one of the modern equipments which is based on these

methods. As you will see in Figure 5.3, this equipment looks similar to the

manual theodolite and the tacheometer and therefore shares with them some of their operating principles. It is by no means a replacement of the old

methods but rather, the old familiar procedures have been integrated, automated and introduced into the new systems thereby making them more

efficient and also less time consuming

Components of the Total Station The Total Station is modern equipment for making very accurate surveys. We

can describe it as a tremendous improvement over preceding equipment such

as the compass, and theodolite because it makes the work of the surveyor

very convenient and pleasant. It is an optical instrument used in modern

surveying. It is a combination of an electronic theodolite (transit), an electronic distance measuring device (EDM) and software running on an external computer.

With a total station one may determine angles and distances from the

instrument to points to be surveyed. The data may be downloaded from the

theodolite to a computer and application software will generate a map of the

surveyed area. Some total stations also have a GPS interface.

Figure 5.3 The Total Station

Source: Compiled from the web

Principles of Operation The distinguishing feature of the Total Station as you can see in Figures 5.4

5.5 is that it integrates into one single functioning unit/system the following

essential components of surveying into hybrid equipment which is also

commonly described as electronic tacheometer.

An electronic theodolite with telescope for measuring angles between stations.

An electronic distance measuring (EDM) system which enables accurate measurement of distances

Computerized input/output system/interface (keyboard, in-built rechargeable battery, software for editing, download, storage and display)

A total station instrument is therefore the central component in modern field-

to-finish surveying systems

Figure 5.4 A diagram showing the front view of a Total Station

Source: Compiled from the web

Figure 5.5 The rear view of a Total Station

Source: Compiled from the web

A total station is a combination electronic transit and electronic distance measuring device (EDM) and computerized input systems/interface (keyboard, software for editing, download, storage and display). With this device, as with a transit and tape, one may determine angles and distances

from the instrument to points to be surveyed. With the aid of trigonometry, the angles and distances may be used to calculate the actual positions (x, y, and z or northing, easting and elevation) of surveyed points in absolute terms.

The Telescope System

A standard transit is basically a telescope with cross-hairs for sighting a target; the telescope is attached to scales for measuring the horizontal angle of rotation of the telescope (normally relative to north as degrees) and the angle of inclination of the telescope (relative to the horizontal as degrees). After rotating the telescope to aim at a target, one may read the angle of rotation and the angle of inclination from a scale.

The electronic transit provides a digital read-out of those angles instead of a scale; it is both more accurate and less prone to errors arising from interpolating between marks on the scale or from mis-recording. The readout is also continuous; so angles can be checked at any time.

The EDM System

The other part of a total station, the electronic distance measuring device or EDM, measures the distance from the instrument to its target. It is normally screwed into a target/bracket on the top of a pole; the pointed tip of the pole is placed on the points to be surveyed. The EDM sends out an infrared beam which is reflected back to the unit, and the unit uses timing measurements to calculate the distance traveled by the beam. With few exceptions, the EDM requires that the target be highly reflective, and a reflecting prism is normally used as the target.

Calculating System

The total station also includes a simple calculator to figure the locations of points sighted (Can you identify the keyboard and display panel in Figure 5.5). The calculator can perform the trigonometric functions needed, starting with the angles and distance, to calculate the location of any point sighted.

Data recorders

Many total stations also include data recorders. The raw data (angles and distances) and/or the coordinates of points sighted are recorded, along with some additional information (usually codes to aid in relating the coordinates to the points surveyed). The data thus recorded can be directly downloaded to a computer at a later time. The use of a data recorder further reduces the potential for error and eliminates the need for a person to record the data in the field.

The determination of angles and distance are essentially separate actions. One aims the telescope with great care first; this is the part of the process with real potential for human error. When the telescope has been aimed, the angles are determined. Only then does one initiate the reading of the distance to the target by the EDM. That takes only a few seconds; the calculations are performed immediately.

Function of the Total Station

The total station, like the theodolite has a face left or right depending on the

position of the station to be sighted. This is illustrated in Figure 5.4

It also has a rechargeable battery which enables continues field work without having to break to charge the battery.

The total station unlike the theodolite also has a keyboard which enables digital/automatic input of back and forward stations.

It has an on-board software makes the necessary adjustments to the calculations to reduce slope distances to their horizontal and vertical components.

The calculation of included angles is done automatically by the total station.

The keyboard also serves an additional purpose of editing when necessary.

It also has an input memory system which enables it to store information to be used at a later date.

The total station also has an external part which enables a computer to be connected to it back at the office to enable further calculations and plotting. Thus, data from the total station are down-loaded into a laboratory computer that possesses data analysis, editing and plotting capabilities.

By combining total station data with information held in existing spatial databases, you have the power to produce a variety of finished maps.

The total station with the above features enables accurate field-to-finish survey where angles can be measured and included, angles calculated even on the fields.

It automatically measures and displays distance and direction data (both horizontal and vertical angles), and transmits the results to its computer.

It also facilitates the measurement of distance from the instrument station to the back or forward station, an activity that would have been done using a chain if the theodolite was being used.

The total station also has an additional advantage of ensuring more accurate traversing to be done in the end and further changes made when necessary and at a later date.

In conducting a land survey the feature and in-built abilities of the total station

makes work more flexible and less tedious.

Using the Total Station The TS makes surveying less time consuming and has a very high degree of accuracy compared to the other instruments used in land surveying.

Field Principles and Methods of the Station

A total station automatically measures and displays distance and direction data (both horizontal and vertical angles), and transmits the results to its in-built computer. The on-board software then makes the necessary adjustments to reduce slope distances to their horizontal and vertical

components. If a station coordinates (x, y, z) and a back sight azimuth are

entered into the instrument, the coordinates of the sighted point can be

computed and displayed.

Data from the Total station are downloaded into a laboratory computer that possesses data analysis, editing, and plotting capabilities. By combining total station data with information held in existing spatial databases, you have

the power to produce a variety of finished maps.

Operational Technique of the Total Station The following activities are done when using a total station.

Set instrument on a tripod at a known TBM to serve as instrument station. You are already aware of the tripod and how it is used Identify the Back station (A) and Forward station (B)

Locate a back station and face the telescope towards the station (face left or face right)

The position of the Total Station and the survey stations is illustrated in Figure

5.6 below. With included angle ()

Figure 5.6 Illustration of the use of the Total Station

Source: Author

From the above, A is the back station and B the forward station.

The angle (?) which is known as the included angle is calculated by the total station by subtracting B from A. The coordinates of A and B are measured by the total station in degrees, minutes and seconds.

In-put the angle of the back station into the system using a keyboard after reading from the graduated card at the base of the instrument through a lens (telescope)

Turn the instrument without moving towards the forward station and record the angles also.

Command the system to calculate the angle between the two stations using the keyboard and record distance between instruments station and back or forward station.

STATION B

STATION A

INCLUDED ANGLE SURVEY STATION

(TEMPORARY BENCHMARK)

KEY

The same procedure can be repeated until all the recordings in

the series are completed.

Set up on tripod Mount/Set up the prism Sighting with the telescope Take readings Make recordings Identify and address field problems and challenges Download data and use

Using the Total Station (Field application and challenges)

Setting up the Total Station

The total station is mounted on a tripod and leveled before use.

Mounting the prism

Meanwhile, the reflective prism is mounted on a pole of known height; the mounting bracket includes aids for aiming the instrument.

The reflective prism is mounted so that its reflection point is aligned with the

center of the pole on which it has been mounted. Although the tip of the pole

is placed on the point to be surveyed, the instrument must be aimed at the

prism. So it will calculate the position of the prism, not the point to be

surveyed. Since the prism is directly above the tip, the height of the pole may

be subtracted to determine the location of the point. That may be done

automatically. The pole must be held upright, and a bubble level is attached to

give the worker holding the pole a check. It is not as easy as one might expect

to hold the pole upright, particularly if there is any wind; as a result, multiple

readings may be required.

Making measurements

When the instrument is set up and turned on, it sets itself to be pointing to

zero degrees (north) when power is first supplied.

There are two adjustment knobs for rotating within the horizontal plane. One

rotates the telescope to make a sighting, with the readout of angles displaying

changes. The other, however, permits the user to rotate the entire instrument

and to keep the current angle unchanged during the process. That effectively

re-orients the zero or north setting. That can be very helpful when setting up

or re-setting the instrument. Thus, it can be devastating if one makes that

adjustment by mistake and thereby changes the north setting.

As I have explained to you the survey information can be recorded by hand,

and the data then entered into the AutoCAD model. Recording could be done

in a field notebook or stored in the system memory to be assessed at a later

date. This process is not necessary if a data collector with the most modern of

capabilities is available. The data collector can automatically orient all new

points to a pre-existing set of survey coordinates.

References Pritchard, J. M. (1984) Practical Geography for Africa, Longman Group Ltd, U.K.

Wilson, R.J.P. (1977) Land Surveying, The M & E Handbook Series

Macdonald and Evans Estover