Theory question 202
Surveying(a) Describe each of the following instruments used in chain surveying. (i) Ranging poles; (ii) Arrows. (b) With the aid of a diagram, show how you would overcome an uncompleted building which poses an obstacle to chaining.
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(a)(i) Ranging poles: straight round poles of wood or light metal, about 2-3 m long; painted in alternate bands (red and white or black and white) about 50 cm wide so they are seen from far; pointed metal shoe at one end to push into the ground; used to mark stations and line up the chain line.
(ii) Arrows: thin steel pins about 38 cm long; one end sharp to stick into the ground, the other bent into a ring for carrying; often tied with red cloth to be seen; usually 10 in a set; used to mark the end of each chain length and count chains.
(b) Method of parallel lines (the building blocks both sight and chaining): choose A and B on the chain line before the building; set out equal perpendiculars AC and BD (with a cross staff or 3-4-5 triangle); range and chain the line CD past the building to points E and F; set out perpendiculars EG and FH back to the chain line, equal to AC; G and H then lie on the original chain line and the hidden length BG equals DE; continue chaining from G, checking that G, H and the line beyond are in line. Diagram showing the chain line, the building, the four equal perpendiculars and the parallel line CDEF.
Show the equal perpendiculars clearly on the diagram.