Weisbach Triangle : When transferring ground level azimuth U/G by suspensions of two wires down a shaft. It is difficult to set up the theodolite exactly in line with them. In addition one wire being nearer obscures. The wire beyond leading to an inaccurate bisection. In weisbach method the theodolite is set up slightly out of line forming a small triangle with the two wires. This triangle is known as weisbach triangle & the azimuth of the line joining the two wires is found by solution of the triangle of formed. .α is obtained form the triangle W1 TW1 by the sine rule α sin β sin α= ----------------- e = c.sinα b In a weisbach triangle W2 TW1 called the weisbach angle β & the ratio c/b must be very small for which the theodolite station (T) should be very near to the wire (W1). If the weisbach angle is less than 25 minutes. There is no necessary of measuring sides. The angle should be measured very accurately to avoid any error. Purpose of correlation survey : Th
Weisbach Method of correlation : The weisbach method of correlation is usually preferred when only one shaft is available. The method consists of suspending two plumb wises which form a plumb plane and azimuth of the same is determined at the surface and taken U/G at the shaft inset and thereafter transferred to the U/G base or reference line. Two plane wires A & B made of anticorrosive crucible steel 0.1 cm diameter having breaking strength of 150” g and to avoid king on a small drums fitted with brake or ratchet to hold them in any desired position are lowered down the shaft tin washers are passed down by the wires from the surface & they are free of obstruction. The smaller plummets are then replaced by symmetrically shaped finned plumb bobs made of lead. When the wires hang vertically each plumb bobs immersed in a barred of water to cut down oscillation due to (i) simple pendulum motion, (ii) Irregular mass in strata, (iii) air current, (iv) dropping water. The lo