Surveying and Leveling
Surveying and Leveling examines horizontal control (chain, compass, plane table, theodolite, total station) and vertical control (levelling). Expect numericals on reduction of levels, bearing conversions and local attraction, plus definitions of contour, benchmark, base line and ranging.
Principles
Work from the whole to the part, and fix a new station by at least two independent measurements. A chain survey uses a framework of triangles; the base line is the longest and most carefully measured line, and offsets are taken at right angles with a cross staff or optical square.
Bearings
Whole circle bearing (WCB) is measured clockwise from magnetic north, from 0 to 360 degrees. Reduced bearing (RB) is the acute angle from the nearer meridian with a quadrant prefix. Conversions: NE, RB = WCB; SE, RB = 180 - WCB; SW, RB = WCB - 180; NW, RB = 360 - WCB. Fore bearing plus or minus 180 degrees gives the back bearing.
Worked example: height of instrument method
Station A is a benchmark of RL 100.000 m with a backsight of 1.500 m. HI = 100.000 + 1.500 = 101.500 m. At station B the foresight is 2.100 m, so RL = 101.500 - 2.100 = 99.400 m. In the rise-and-fall method the same reading gives a fall of 0.600 m, because a foresight greater than the backsight means the ground has fallen.
Local attraction
If fore bearing minus back bearing is not 180 degrees, local attraction is present. Identify the line free from attraction, correct that bearing first, then apply the correction to every other bearing taken at that station.
Traps
- Adding the foresight instead of subtracting it.
- Mixing metres and feet in the level book (1 ft = 0.3048 m).
- Giving the RB of a SW bearing as WCB + 180 instead of WCB - 180.
- Omitting curvature (0.0785 D squared) and refraction (one-seventh of curvature, opposite sign) corrections.