By the end of this topic, you should be able to: 1. 2. 3. 4. 5. Outline the basic components of a theodolite; Carry out temporary adjustments of a theodolite; Compute and adjust a theodolite traverse; Compute coordinates for traversing; and Determine the nature of errors affecting the traverse work.
Traversing is a form of a control survey that requires the establishment of a series of stations that are linked together by angles and distances. The angles are measured by theodolites, and the distances are measured conventionally by tapes or electronic distance measuring equipment. The use of theodolite in 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. This topic will describe the construction and use of the theodolite in traversing. It will explain the traverse design and the procedures of computing and adjusting a
traverse. The application of coordinates for point location will also be covered in this topic which will be very useful in civil engineering project.
THEODOLITE PRINCIPLES AND APPLICATIONS
A theodolite is an instrument which is capable of measuring angles to the nearest whole second [Figure 4.1]. 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.
Figure 4.1: Atheodolite Source: SouthGeosystems
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) (ii) Optical theodolite; and 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. EXERCISE 4.1 What is the major difference between the optical and the electronic theodolite? Differentiate between each type in terms of its construction.
Construction of a Theodolite
All theodolites have the same common features [Figure 4.2] which can be described as follows: 1. A 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. The 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 360ºphotographically etched around the edge. 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.
The 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....