Site Surveying & Set Out 2
Using the TheodoliteIn Surveying 2 we use a theodolite to determine (read) both horizontal and vertical angles to an accuracy of 20" and mark out the buildings on College ground using theodolite and tape (Module requirements)
A theodolite
is an instrument for measuring both horizontal and vertical angles. It
consists of a telescope mounted movably within two perpendicular axes,
the horizontal and vertical axis. All features of the Sokkia Theodolite are described below:
The axes and circles of a theodoliteA theodolite can measure angles in both the horizontal and vertical planes. How accurately this can be will depend partly on the quality of the instrument, and partly on the competence of the student.A theodolite may not be in perfect adjustment and the lines and planes should be checked. To minimise error as much as possible, an angle is measured a number of times with instrument: ►face left (vertical circle on the left of the telescope) and ►face right (vertical circle on the right of the telescope). Reading an angle face right and face left will eliminate the errors due to the non adjustment of the line of collimation and the trunnion axis. We won't do any adjustment and assume the instrument is calibrated.
Reading scalesTheodolites have different reading scales. A theodolite can read, depending
on the accuracy,
Modern theodolites read the horizontal and vertical circles electronically and display readings digitally as in c) above. How to read the micro-scales on the Sokkia Theodolite The opposite figure shows a typical view through the microscope eye
piece. The microscope eyepiece is located adjacent (right) to the telescope
eye piece. Two set of parallel lines (0° and 360°} are seen in
the H window. If the observed reading in the H-window is 245°
sitting between 50' and 60' divisions you need to turn the mircometer
knob until the parallel 245° lines are biseced by the 50' line as
shown in figure. Then the window above the H window will shows in the
top row the minute and below the seconds. The final reading as shown in
the figure is 245° 53' 18". (The seconds reading is scaled and
not precise.) Using the Vertical Angles and the distances to determine the height. Procedures
to measure vertical and horizontal angles We use the tan-function to calculate the height. As shown in the figure above the height is determined by measuring two angles (angle of elevation & angle of depression). To calculate the Height A multiply the horizontal distance on for your first height reading by the tan of the related angle. Height A = horizontal distance x tan 90°- A° Height B = horizontal distance x tan B°- 90° Now add both distances together. Add this measurent to the known reduced
level (RL) at the bottom to obtain the new height level (RL gutter or
fascia) at the top. Make up a template for recording angles and measurements. The reading precision of the instrument and the accuracy of your angle reading and horizontal measured distance will determine the correct height distance. Please contact me on 9202 4398 if you have any query regarding your work. |