Chain surveying Chain surveying can be used when
Chain surveying • Chain surveying can be used when the area to be surveyed is comparatively small and is fairly flat. but when the area is large undulated and crowded with many details triangulation is not possible. In such area method of traversing is adopted.
Principles of Chain Surveying • The principle of chain surveying is to divide the area into a number of triangles. • As a triangle is the only simple plane geometrical figure which can be plotted from the length of the three sides even if the angles are not known. A network of triangles is preferred to chain surveying. • Triangulation is the principle of chain surveying. If the area to be surveyed is triangle in shape and if the lengths and sequence of its three sides are recorded, the plan of the area can be easily drawn.
Principles of Chain Surveying
Principles of Chain Surveying
Well conditioned triangle, ill conditioned triangle and ideal triangle. • A triangle is said to be well conditioned triangle when no angle in it is neither less than 30 nor greater than 120. • If in a triangle an angle is less than 30 or greater than 120 is called ill conditioned triangle. • An equilateral triangle having each angle of 60 is an ideal triangle.
Operation in Chain Surveying • The following operations are involved in chain surveying. • Chaining • Ranging • Offsetting • These three operations are done simultaneously during chain Surveying.
Offsets • Lateral measurements to chain lines for locating ground features are known as offsets. For this purpose perpendicular or oblique offsets may be taken. If the object to be located (say road) is curved more number of offsets should be taken. For measuring offsets tapes are commonly used. • Offset which can be judged by naked eye or offset less than 15 m is called short offset and offset greater than 15 m is called long offset. • Most commonly short offsets are perferred.
Offsets • Offset may also be classified as Perpendicular Offsets and oblique offset. Perpendicular Offsets • The offsets which are taken perpendicular to the chain line are termed as perpendicular offsets. These offsets are taken
Perpendicular Offsets
Offsets Oblique Offset • Oblique distance is always greater than perpendicular distance. All the offsets which are not taken at the right angle to chain line are known as oblique offsets.
Oblique Offset
Offsets
DEFINITIONS
A Survey Stations • Survey stations are the points at the beginning and the end of a chain line. They may also occur at any convenient points on the chain line. Such stations may be: 1. Main stations 2. Subsidiary stations and 3. Tie stations
1. Main stations Stations taken along the boundary of an area as controlling points are known as ‘main survey lines’. The main survey lines should cover the whole area to be surveyed. The main stations are denoted by ‘ ’ with letters A, B, C, D, etc. The chain lines are denoted by “__ … __. . . __”. 2. Subsidiary stations Stations which are on the main survey lines or any other survey lines are known as “Subsidiary stations”. These stations are taken to run subsidiary lines for dividing the area into triangles, for checking the accuracy of triangles and for locating interior details. These stations are denoted by ‘’ with letters S 1, S 2, S 3, etc. 3. Tie stations These are also subsidiary stations taken on the main survey lines. Lines joining the tie stations are known as tie lines. Tie lines are mainly taken to fix the directions of adjacent sides of the chain survey map. These are also taken to form ‘chain angles’ in chain traversing, when triangulation is not possible. Sometimes tie lines are taken to locate interior details. Tie stations are denoted by ‘’ with letters T 1, T 2, T 3. Etc.
Procedure of field work • • • Reconnaissance Index sketch Marking the stations on ground Reference sketches Taking measurement of survey lines and noting them.
1. Reconnaissance survey • The preliminary inspection of the area to be surveyed is called reconnaissance. The surveyor inspects the area to be surveyed, survey or prepares index sketch or key plan. Walk the whole area and thoroughly examine the ground, note the position of boundaries, road, and river etc. , various difficulties to chain lines, select stations, and prepare neat sketches called index sketches or key plan
2. Index sketch • After preliminary inspection of the area, the surveyor should prepare a neat hand sketch showing the arrangement of the framework and approximate position of the objects. He should note the names of the stations on the sketch maintaining some order (clockwise or anticlockwise). The field work should be executed according to this index sketch. The names and sequence of chain lines should be followed as directed in the index sketch. The ‘base line’ should be clearly indicated in the index sketch.
3. Marking stations • Stations are marked with ranging rod, or wooden peg, driving a nail or spikes if hard surface, or embedding stone with a cross mark. 4. Reference sketches • After marking the station should be referenced i. e. located by measurement called ties taken from 3 permanent points which are easily identified such as corner of building And in the final required measurements are taken and located on field book.
• EQUIPMENTS FOR CHAIN SURVEY • The following equipment's are required for conducting chain survey: 1. Metric chain (20 m) 2. Arrows 3. Metallic tape (15 m) 4. Ranging rods 5. Offset rod 6. Clinometer 7. Plumb bob with thread 8. Cross staff or optical square
9. Prismatic compass with stand 10. Wooden pegs 11. Field book 12. pencil 13. Eraser (rubber)
• Chain surveying is recommended when: 1. The ground surface is more or less level 2. A small area is to be surveyed 3. A small-scale map is to be prepared and 4. The formation of well-conditioned triangles is easy
Chain surveying is unsuitable when: 1. The area is crowded with many details 2. The area consists of too many undulations 3. The area is very large and 4. The formation of well-conditioned triangles becomes difficult due to obstacles
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