Jawaharlal Nehru Technological University Hyderabad B.Tech II PRINCIPLES OF CIVIL AND STRUCTURAL ENGINEERING Previous Question Paper (October - 2020 - R13)

Surveying is primarily classified into two main categories based on the consideration of the Earth's curvature: Plane Surveying and Geodetic Surveying.

1. Plane Surveying

In plane surveying, the Earth's surface is assumed to be a plane surface. This assumption is valid for small areas (typically less than 250 square kilometers) where the curvature of the Earth is negligible. The plumb lines at any two points are considered parallel, and all angles are assumed to be plane angles. It is used for most engineering projects like building construction, road alignment, and land surveys.

2. Geodetic Surveying

In geodetic surveying, the curvature of the Earth is taken into account. This is necessary for large-scale surveys covering extensive areas (greater than 250 square kilometers). The plumb lines at different points are not parallel but converge towards the center of the Earth. Spherical trigonometry is used for calculations, and the lines connecting two points are treated as arcs rather than straight lines. This method is used for establishing national control networks and mapping large regions.

Secondary Classifications

Beyond the primary division, surveying is also classified based on other criteria:

• Based on the Instrument: Includes Chain surveying, Compass surveying, Plane table surveying, Theodolite surveying, Tacheometric surveying, and Photogrammetric surveying.
• Based on the Method: Includes Triangulation and Traversing.
• Based on the Purpose: Includes Land surveying (for boundaries), Topographic surveying (for natural features), Engineering surveying (for infrastructure), Military surveying, Mine surveying, and Hydrographic surveying (for water bodies).

Horizontal angles in surveying are primarily measured using a theodolite. The two main methods employed to minimize instrumental and observational errors are the Method of Repetition and the Method of Reiteration.

1. Method of Repetition:

This method involves measuring an angle multiple times by adding the successive readings on the horizontal circle. The total angle is divided by the number of repetitions to obtain a more precise average value.

• Procedure: Measure the angle, leave the vernier at the reading, transit the telescope, and re-bisect the first station to measure the angle again. Repeat this process for a set number of times (usually three).
• Errors Eliminated: This method effectively eliminates errors due to eccentricity of centers (by taking readings on both verniers) and inaccurate graduation of the horizontal circle (by distributing the measurement over different parts of the scale). It also reduces errors due to imperfect adjustment of the line of collimation and horizontal axis by reversing the telescope.

2. Method of Reiteration:

This method is used when several angles are to be measured around a single station point. It is essentially a series of measurements where the instrument is rotated to close the horizon.

• Procedure: Measure all angles in sequence (e.g., AOB, BOC, COD) and finally close the horizon by measuring the angle back to the starting point. If the sum of the angles is not 360°, the discrepancy is distributed among the measured angles.
• Errors Eliminated: This method eliminates errors caused by instrumental adjustments, such as the line of collimation not being perpendicular to the horizontal axis, and personal errors in reading the verniers. By closing the horizon, it also provides a check on the accuracy of the entire set of observations, allowing for the systematic distribution of closing errors.