Angular Measurement 2014

Lect 1:Introduction to theodolites - RMJ 1

ANGULAR MEASUREMENTCovers measurement of angles in Horizontal plane

and Vertical Plane

Vertical angle

Horizontal angle

Vertical a

ngle

A

B


Theodolite

• Designed specifically for the measurement of Horizontal and Vertical angles in surveying and construction work.

• most versatile of survey equipments.• Capable of performing other tasks – setting out of lines and angles,

levelling, Plumbing tall buildings and deep shafts, electromagnetic distance measurement

• Types (i) Optical, the angles are read against graduated scale

(ii) Electronic, the angle value displays in digital form• Extremely accurate piece of equipment • Capable of reading to 20”, 10”, 1”, 0.1”


• Classified according to the smallest angle of reading.• The types of theodolite used depending on its intended application.• The size of an angle subtends over a distance is considered.

– 0.2 secs of arc Precision theodolite– 1 secs of arc Universal Theodolite used in extreme angular

accuracy and over long distance up to 2km– 20 secs of arc General purpose (Ideal for general survey work)

up to 103 m– 1 minute Builder theodolite

(comparatively low order of accuracy)• Which theodolite would you choose if you are measuring angles over

distances of about 50 m and your tolerance is about 5mm?

10 mmDistance

Smallest angle reading


About 400 years ago


BASIC CONSTRUCTIONAL FEATURES OF OPTICAL THEODOLITE


Essential components of optical theodolite


TRIBRACH (Levelling Head)• Prime purpose is to facilitate the levelling up of instrument, so that

readings are taken in truly horizontal and vertical planes. The levelling up operation involves making the vertical axis of instrument properly vertical.

• Triangular bracket at the base of the theodolite• Incorporates footscrews which allows levelling up of the instrument .• Fixed and Detachable ( to permit interchange of instrument and

accessories without disturbing centring over a station).• Optical plummet – a small telescope built into the base of the

theodolite. Provides a line of sight down the vertical axis of the instrument. Replaces plumb bob and line. The horizontal view being deflected through 90°by a prism.


Bottom and Top plate and Horizontal circle


• Bottom plate – the first disc, directly above the tribrach. This plate is equipped with a clamp which is used to clamp the horizontal circle to the bottom plate. Unclamped, the horizontal circle is free to rotate independently of the bottom plate. The clamp is fitted with a tangent or slow motion screw enabling fine adjustment after the clamp is tightened.

• Horizontal Circle –the second disc. Made up of glass with graduation lines and numerals etched on the surface. The circle is divided into conventional sexagesimal degrees and fraction (360°).

• Top Plate (Alidade) is the third disc. This plate is also equipped with a clamp however this does not clamp to the vertical axis but clamps the top plate to the horizontal circle. It is also fitted with slow motion screw. The top plate carries a bubble tube (the plate bubble) which is used for levelling up the instrument.


Upper part of the theodolite - Alidade

• The top plate supports the two frames called standards. • Supporting between the two standards is the trunnion or transit axis.• Attached to the trunnion axis are the telescope, circle reading telescope, micrometer

screw and vertical circle.• Features of telescope similar with the level instrument. It can be clamped in the

vertical plane and a slow motion screw is provided for the fine vertical movement. • Circle reading system consists basically of a microscope eyepiece located alongside

the telescope. A small mirror is located on one standard to direct light into the instrument and, then prisms system to deflect the light to illuminate the graduations on the glass circles.

• Generally one standard is bulkier since it encases the vertical circle. The normal observing position when looking through the telescope is to gave the vertical circle located at the observer’s face left-hand side and this described as observing with face left.


Setting up of theodolite – (i) Centring of Instrument

• Theodolite in use must be centred over ground mark• Traditionally by plumb-bob and string or alternatively

use the optical plummet• Centring is achieved by appropriate movement

(laterally, or by extending or shortening) of one or more tripod legs, until the plumb-bob is exactly over the ground mark.

• The final fine adjustment being made by unclamping the tribrach and sliding it across the top of the tripod head.


Setting up of theodolite – (ii) Levelling up the plate level or fine levelling

Setting up of theodolite – (iii) eliminating parallax


Observing horizontal and vertical angles

• To observe horizontal angles, requires a telescope that can rotate the vertical axis.

• To observe vertical angles, requires a telescope that can rotate the horizontal axis.

• The amount of movement of the telescope in the two planes can be read from the two circular scales.

• To record and measure the amount of movements, the scales must be engraved in appropriate units.


Reading Horizontal angles


Observing vertical angles


Circle reading• Open and turn mirror until illumination of circles as seen in reading

microscope is uniformly bright.

• Turn eyepiece of reading microscope until circle graduation lines and double index lines are in sharp focus. The circle graduation is interval of 1°. In the right hand window is the micrometer. It is digital with numbers every 6“. Upper window is the vertical circle and lower window is the horizontal circle.

• To read the horizontal circle, turn the micrometer knob until a circle graduation line is exactly in the middle of a double line index. Read the degrees graduation line. The centre of the micrometer window is indicated by a black pointer. Read the minutes and seconds against the pointer. It is possible to estimate half a micrometer interval, i.e 3“, if the pointer is between two numbers.

• To read the Vertical readings are taken in exactly in the same way.


Measuring angle AOB• Step 1 – set micrometer to read 00‘00“ and turn FL.• Step 2 – release both upper and lower clamps, turn the horizontal circle

until 0°appears in the micrometer window. Clamp the upper plate and by means of upper plate slow motion screw, set the 0° index mark to its true position.

• Step 3 – While on FL, sight target A by using the lower clamp and tangent screw. ( check to ensure that the micrometer reading for the horizontal circle is still 0° 00‘00“ ).

• Step 4 – release upper plate, turn the instrument and sight target B with the fun sight. Clamp upper plate and bisect target by means of upper plate tangent screw. Book the FL circle readings.

• Step 5 – transit or plunge the telescope to FR position, release the upper plate and resight target A, adjust the micrometer to read 00‘00“ and turn the upper plate tangent screw to set 180°. Book the FR circle reading.

A

O

B


Documents

Angular Measurement 2014