Samacheer Kalvi 12th Geography Surveying Exercise Solutions

Surveying is the art and science of measuring distances, directions, and elevations on the surface of the Earth, and presenting the data in the form of a map, plan, or section. It provides the foundation for all engineering and construction works.

1. Work from whole to part: First establish a framework of control points covering the entire area, then fill in the details. This prevents accumulation of errors. 2. Locate a point by at least two independent measurements: A point should be fixed by measuring from at least two known reference points so that errors can be detected.

Chain surveying is the simplest method of surveying in which only linear measurements (distances) are taken using chains and tapes. No angular measurements are made. It is suitable for: - Small areas (preferably less than 10 hectares) - Relatively flat ground - Areas with simple details and no obstacles - Sites where a rough plan is sufficient

1. Chain — For measuring distances (Gunter's chain, engineer's chain, metric chain) 2. Tape — For accurate short-distance measurements 3. Arrows (ranging rods) — For marking chain lengths and aligning 4. Cross-staff — For setting out right angles 5. Offset rod — For measuring short lateral distances (offsets) 6. Plumb bob — For transferring points to the ground

| Base Line | Check Line | |---|---| | Longest and most important chain line | Line connecting two points on adjacent chain lines | | Passes through the centre of the area | Verifies the accuracy of the framework | | Measured most carefully | Its measured length is compared with the plotted length | | Used as reference for all other lines | Used to detect errors in the survey |

A prismatic compass is a magnetic compass with a prism for reading bearings. It measures whole circle bearings (WCB) — the angle measured clockwise from the magnetic north (0°) to the survey line. The bearing ranges from 0° to 360°. The compass is held in hand or mounted on a tripod at the station point.

Plane table surveying is a graphical method of surveying in which a drawing sheet is fixed on a plane table, and the map is drawn directly in the field as observations are made. Using an alidade for sighting and a magnetic compass for orientation, the surveyor plots points and lines by radiation, intersection, traversing, or resection methods.

Levelling is the process of determining the relative heights (elevations) of different points on the Earth's surface, usually with reference to mean sea level as the datum. Instruments used: 1. Dumpy level — The levelling instrument 2. Levelling staff — Graduated rod placed at points whose elevation is to be determined 3. Tripod — Supports the dumpy level 4. Plumb bob — For centring the instrument

Definition: Chain surveying is the simplest method of surveying where only linear measurements are taken using chains and tapes. No angular measurements are involved. Principle: The area to be surveyed is divided into a network of triangles (the simplest geometric figure that can be plotted from its sides alone). The survey framework consists of main survey lines (base line, tie lines, and check lines), and details are filled in by offsets. Equipment: 1. Chain — Metric chain (20 m or 30 m) for measuring distances 2. Arrows — Iron pins (usually 10) for marking chain lengths 3. Ranging rods — Wooden poles for aligning chain lines 4. Cross-staff — For setting out right angles to chain lines 5. Offset rod — For measuring short lateral distances 6. Tape — Steel or linen tape for precise measurements Procedure: 1. Reconnaissance: Walk over the area, select stations, and prepare a rough sketch. 2. Marking stations: Fix ranging rods at selected stations. 3. Running chain lines: Measure distances along base line and other chain lines. 4. Taking offsets: Measure lateral distances from chain lines to features (perpendicular offsets). 5. Check measurements: Measure check lines to verify accuracy. 6. Plotting: Draw the framework to scale and fill in details from offsets. Suitability: Small areas, flat ground, simple details. Limitations: Not suitable for hilly terrain, large areas, or areas with many obstacles.

Definition: Compass surveying uses a magnetic compass to measure directions (bearings) of survey lines, combined with chain measurements for distances. Principle: At each station, the surveyor measures the whole circle bearing (WCB) of each line using a prismatic compass and the distance using a chain. The survey proceeds by traversing from one station to the next. Types of Compass: 1. Prismatic compass: Handheld or tripod-mounted; reads WCB (0°–360° clockwise from magnetic north). 2. Surveyor's compass: Reads quadrantal bearings (NE, SE, SW, NW from north or south). Procedure: 1. Set up the compass at the first station. 2. Sight the ranging rod at the next station and read the fore bearing. 3. Measure the distance with a chain. 4. Move to the next station, set up the compass, and read the back bearing to verify. 5. Continue around the traverse, closing at the starting point. Bearings: - Fore bearing (FB): Bearing measured in the direction of survey. - Back bearing (BB): Bearing measured in the reverse direction. - Relation: BB = FB ± 180° Advantages: - Quick and simple method - Suitable for traversing large areas - No elaborate equipment needed - Can be used in uneven terrain Limitations: - Magnetic compass is affected by local attraction (iron, steel, power lines) - Less accurate than theodolite surveying - Errors due to magnetic variation - Not suitable for very precise work

Definition: Plane table surveying is a graphical method in which the map is drawn directly in the field on a drawing sheet fixed to a plane table, eliminating the need for separate office work. Equipment: 1. Plane table — A drawing board mounted on a tripod 2. Alidade — A straight edge with sight vanes for sighting and drawing 3. Trough compass — For orienting the table to magnetic north 4. Spirit level — For levelling the table 5. Drawing sheet — Fixed to the table with clips 6. U-fork and plumb bob — For centring the table over the station Setting up the plane table: 1. Centring — Position the table so the plotted point is vertically above the ground station (using U-fork and plumb bob). 2. Levelling — Use the spirit level to make the table horizontal. 3. Orientation — Align the table with magnetic north using the trough compass. Methods of Plane Table Surveying: 1. Radiation Method: - Suitable for surveying a small area from a single station. - Set up the plane table at a central station O. - Mark point 'o' on the sheet representing station O. - Sight each feature A, B, C, D... with the alidade and draw rays. - Measure distances OA, OB, OC, OD... with a chain or tape. - Plot the points a, b, c, d... to scale along the respective rays. - Join the plotted points to complete the map. 2. Intersection Method: - Suitable for surveying inaccessible points (e.g., across a river). - Set up the table at station P, orient it. - Sight point A and draw a ray pa. - Shift the table to station Q, orient it. - Sight point A again and draw ray qa. - The intersection of rays pa and qa gives the position of point A on the map. - Repeat for other points.

Definition: Levelling is the process of determining the relative heights or elevations of points on the Earth's surface with respect to a datum (usually mean sea level). Purpose: - To determine the elevation of points for engineering projects (roads, canals, buildings). - To calculate cut and fill in earthwork. - To establish contour maps. - To design drainage and irrigation systems. Types of Levelling: 1. Simple levelling: When two points are close and only one set-up of the level is needed. 2. Differential (compound) levelling: When points are far apart and multiple set-ups are required. 3. Reciprocal levelling: Used when the level cannot be set up between two points (e.g., across a wide river). 4. Profile levelling: Finding elevations along a line (e.g., along a road centre line). 5. Cross-section levelling: Finding elevations at right angles to a centre line. Key Terms: - Datum: Reference surface (usually mean sea level). - Bench Mark (BM): Fixed reference point of known elevation. - Back Sight (BS): First staff reading taken after setting up the level (on a known point). - Fore Sight (FS): Last staff reading at a set-up (on an unknown point). - Intermediate Sight (IS): Staff readings between BS and FS. - Height of Instrument (HI): HI = BM elevation + BS reading. - Reduced Level (RL): RL = HI – FS (or HI – IS). Procedure: 1. Set up the dumpy level on a tripod at a suitable location. 2. Take a back sight reading on the levelling staff placed on a bench mark. 3. Calculate HI = Known RL of BM + BS. 4. Take fore sight (and intermediate sights if needed) on staff at other points. 5. Calculate RL of each point = HI – FS (or HI – IS). 6. Move the instrument and repeat for the next section. 7. Apply checks: Sum of BS – Sum of FS = Last RL – First RL.

The Great Trigonometrical Survey (GTS) of India was one of the most ambitious scientific projects of the 19th century, laying the foundation for modern surveying and mapping of the Indian subcontinent. Origin: Started in 1802 by Colonel William Lambton in Madras (now Chennai), initially as a small arc measurement along the coast. Leadership: - Colonel William Lambton (1802–1823) — Founder - Colonel George Everest (1823–1843) — Expanded the survey across India - Later continued by Andrew Waugh and others Methodology: The GTS used triangulation — a series of interconnected triangles covering the entire subcontinent. One side of the first triangle (the baseline) was measured with extreme precision using chains and rods. All other sides were calculated using trigonometry. From each hilltop station, angles were measured using large theodolites. Key Achievements: 1. Measured the arc of the meridian from Cape Comorin (Kanyakumari) to the Himalayas. 2. Established a network of triangulation stations across India. 3. Determined the height of Mount Everest (originally called Peak XV) at 8,848 m in 1852. 4. Created the datum and coordinate system still used in Indian mapping. 5. Measured the figure of the Earth and contributed to geodesy. Challenges: - Malaria, wild animals, and extreme weather killed many surveyors. - The large theodolite (over 500 kg) had to be carried up mountains. - Dense jungles and haze obstructed visibility. - Local rulers sometimes opposed the survey. Significance: - Provided the framework for all subsequent mapping of India. - Established the Survey of India as a permanent institution. - The baseline at St. Thomas Mount, Chennai, was one of the earliest survey baselines. - Contributed to the scientific understanding of the Earth's shape and size. - Remains the foundation of India's topographic map series.