Engineering Surveying IChapter 4: Theodolite

5.1Introduction5.2Classification5.3Components of a Theodolite5.4Reading the Circles5.5Setting Up a TheodoliteContents

In topic of traversing, and tacheometry, the subject of angular measurement will be a common factor.Therefore, it is essential that a surveyor has a sound knowledge of the instruments and methods used in the measurement of horizontal and vertical angles.Theodolite is an instrument that is used to measure angles, with an accuracy varying from 1 to 60 seconds of arc.5.1 Introduction

A theodolite is generally classified according to the method used to read the circles.Broadly speaking, the methods are vernier (now obsolescent);Direct reading;Optical scale;Optical mirometer, and;Opto-electronic.5.2 Classification

The main components of a theodolite include :Tripod (support instrument);Trivet stage;Tribrach;Footscrew (levelling screws for levelling arrangement);Horizontal circle (lower plate) & alidade (upper plate);Lower plate clamp and upper plate clamp (controls for measuring horizontal angles);Telescope;Index marks;Transits axis or trunnion axis (telescope and vertical circle are attached to it);Altitude spirit level (maintain a horizontal position of telescope);Optical plummet (aids in centering the instrument).5.3 Components of a Theodolite

5.3 Components of a Theodolite

5.3 Components of a Theodolite

5.3 Components of a Theodolite

Four methods of reading the graduations on the circles are available.Most of the time, the sexagesimal units are used in the measurement of angles, hence all subsequent examples use degrees, minutes, and seconds.

Direct readingRead through a reading eyepiece attached to one of the standards or alongside the main telescope eyepiece.

Direct scale readingIntermediate order theodolites employ an optical scale to read the horizontal and vertical circles.

5.4 Reading the Circles

Micrometer readingThe higher order of theodolites employ micrometer reading.

Opto-electronicThe automatic display of the circle readings eliminates the need for scales and micrometers, and greatly reduces reading and booking errors.The time required for measuring an angle is also reduced because of zero-set facility on most of these instruments, whereby the circle can set to read zero at the touch of a button.5.4 Reading the Circles

Read the readings :Direct angle readings Vertical / zenithal (left)and horizontal /azimuthal (right)from theodolite5.4 Reading the Circles

Read the readings :5.4 Reading the Circles

Plumb-bob methodSetting the tripodMounting the instrumentLevellingCentringParallax elimination

Optical plumb methodSetting the tripodMounting the instrumentCentringLevellingParallax elimination5.5 Setting Up a Theodolite