1.18.4. Measurement of horizontal angles

Theodolite is generally used for accurately measuring the horizontal and vertical angles. For this, the theodolite is centered on the desired station point, levelled and telescope is focused. This process of centering, levelling and focusing is called temporary adjustment of the instrument, and it is similar to any optical telescope, such as levels. Now the object is bisected at the intersection of cross-hairs so that both the horizontal and vertical angle readings are taken.

To measure angles accurately, the theodolite must first be positioned correctly over a survey point. This involves centering the instrument, which means placing its vertical axis directly above the specified point on the ground. After centering, the next step is leveling, where the instrument is adjusted to ensure it is horizontal. Finally, the telescope is focused on the target object. This alignment process is essential because any slight misalignment can lead to errors in angle measurement.

Think of setting up a camera on a tripod. Just as you would ensure the camera is directly over the point you want to photograph, level it to avoid skewed images, and focus it for clarity, setting up a theodolite requires precise positioning to get accurate measurements.

The reading is read from the main scale as well as vernier scale, and both are added together to get final reading. The vernier scale is a small scale used for determining the fractional parts of the smallest division of the main scale more accurately than it can be done by simply estimating with eye.

Once the theodolite is set up and aimed at the target, the angles are measured using two scales on the instrument: the main scale and the vernier scale. The main scale provides the whole number of degrees while the vernier scale allows for measuring smaller angles between those whole numbers. By taking readings from both scales and adding them together, the surveyor obtains a precise angle measurement.

Imagine a regular ruler combined with a fine-adjustment scale. The ruler gives you whole centimeters, while the fine scale allows you to measure millimeters more accurately. This combination helps you determine the length or angle with maximum precision, just like using both scales on the theodolite.

There are two methods of measuring the horizontal angles: (a) Reiteration method, and (b) Repetition method.

To measure angles, surveyors generally use two methods: the reiteration method and the repetition method. The reiteration method involves measuring multiple angles in succession and returning to the starting point (closing the horizon). This allows for error checking and distribution. In contrast, the repetition method focuses on measuring a single angle multiple times to enhance precision, averaging the results to get a more accurate final angle.

Think of playing a game where you need to measure how far you've thrown a ball. If you throw it multiple times and average the distances, you'll get a better estimate of your capabilities than just one throw. This is similar to how surveyors measure angles more accurately by repeating and averaging their readings.

Reiteration method is generally preferred for measuring several horizontal angles around the instrument station. It measures several angles in continuation, and finally closes the horizon at the starting point. These angles are measured on both left face and right face. The final readings of the verniers should be almost same as their initial readings. If error is observed, it is equally distributed among all the measured angles. The steps involved are: (i) Set up the instrument over station O and centre it and level it accurately. (ii) Make sure that the instrument face is left face....

The reiteration method allows surveyors to measure multiple angles in a sequence, making it more efficient for various applications. Steps include setting the theodolite on a designated station, ensuring it is level, and taking angle measurements in both clockwise and counterclockwise directions. After measuring, surveyors check the closing of the angles by returning to the starting point to verify accuracy. If there are errors in measurements, they are distributed across the collected angles to minimize their impact.

Picture following a trail that loops back to your starting point. If you measure the distance gone at various points along the loop, you'll not only keep track of your total journey but also identify any discrepancies effectively, just like how the reiteration method checks for errors in angle measurements.

This method is used to determine very accurate horizontal angle. It is preferred when only one or two angles are to be measured around the instrument station. Here, the same angle is measured several times and the correct value of the angle is obtained by dividing the accumulated reading with the number of repetitions.

In the repetition method, surveyors focus on measuring specific angles multiple times for enhanced accuracy. By taking repeated measurements of the same angle and then calculating the average, they can achieve results that surpass the limitations imposed by the instrument's least count. This method is beneficial for critical measurements where precision is vital.

Imagine taking a shot at a basketball hoop. If you shoot multiple times, noting how many times you score, you can get a better idea of your shooting accuracy versus just one shot. The repetition method works similarly, allowing surveyors to refine their angle measurements for improved reliability.

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