Stages of engineering survey for alignment

To fix the best alignment, the engineering survey is done in stages, moving from broad study to detailed work. Each stage narrows the options.

1. Map study

• Using topographic maps (1:25,000 or 1:50,000), satellite images or aerial photographs, the possible routes are marked between the terminals through the obligatory points. Ridge lines, valleys, river crossings, passes and obstructions are studied.
• Gives alternative alignments at low cost.

2. Reconnaissance survey

• A quick field examination of the routes picked on the map, on foot or by vehicle, with simple instruments (abney level, compass, tape, altimeter).
• Collects: topography, geology, drainage and stream crossing, soil, materials, land use, climate/rainfall, settlements, forests, cost of land.
• Rough estimates of cost; best routes (one to three) are selected.

3. Preliminary survey

• A detailed survey of the selected route(s) to compare and fix the final one: a traverse with a theodolite/total station, levelling along it, contouring of a strip (a 100-200 m belt), cross-sections, stream data, soil and material survey, drainage and traffic data.
• Output: contour plan, preliminary estimate and comparison of alternatives.

4. Final location and detailed survey

• The chosen alignment is fixed on the ground using the plan from the preliminary survey: centre line pegging, curves set out, bench marks, profile and cross-sections, soil and material tests, hydrological data.
• Detailed drawings, bill of quantities and cost estimate for tender.

Map study -> Reconnaissance -> Preliminary -> Final location
 (paper)       (field, quick)     (detail)      (set-out, design)
 many routes      2-3 routes     1-2 routes      1 alignment