Survey Instruments and Their Use
Survey Instruments and Their Use
This domain covers the instruments an Amin actually handles: tape and chain, ranging rods, cross-staff and optical square, compass, level, theodolite, total station and GPS. Questions test selection, least count, adjustments and bearing conversion.
Core rules
- A ranging rod marks a station; a cross-staff or optical square sets out a perpendicular offset. A prism square is a compact optical square.
- Surveyor's compass: least count 15 minutes; prismatic compass: 30 minutes. Both give magnetic bearings. A level must have its bubble centred before reading.
- Whole-circle bearing (WCB) is measured clockwise from magnetic north, 0° to 360°. Reduced bearing (RB) is measured from the nearer north or south end, 0° to 90°.
- Conversion: 0-90 WCB = NE, RB = WCB; 90-180 = SE, RB = 180 - WCB; 180-270 = SW, RB = WCB - 180; 270-360 = NW, RB = 360 - WCB.
- Local attraction is detected by comparing fore and back bearings of a line; the difference should be 180°. Affected stations are corrected from an unaffected line.
Worked example
Convert WCB 230° to RB. It lies between 180° and 270°, so the quadrant is SW and RB = 230° - 180° = 50°. The bearing is S 50° W. Check: S 50° W means 50° west of south, i.e. 180° + 50° = 230° clockwise from north.
Traps
Do not label 230° as NW; that quadrant begins at 270°. Do not average fore and back bearings without first correcting local attraction. Do not read a prismatic compass without allowing the prism to be held level and the card to settle. Least-count errors (15 minutes versus 30 minutes) are common.