Preliminary survey method

The preliminary survey is a detailed instrumental survey of the routes chosen after the reconnaissance, carried out to compare them and fix the best alignment. Its objectives are to collect topographic and other data, estimate the cost and select the final alignment.

Conventional method - procedure

1. Primary traverse: a closed or open traverse is run along each route using a theodolite or total station, with chainage pegs at 20-30 m (or 50 m) intervals; the traverse stations are tied to the main control points.
2. Levelling: longitudinal levelling along the traverse, and cross-sections at regular intervals (about 50-100 m in plains, 20-30 m in hills) extending across the survey strip on each side; bench marks established at about every 250-500 m.
3. Topographic detail: details such as streams, buildings, roads, fields and trees are surveyed in a strip about 100-200 m wide (contour interval 1-2 m) to prepare the contour plan.
4. Drainage and hydrology data: stream crossings, catchments, high flood level, existing structures.
5. Soil and geology survey: soil classification and CBR/subgrade tests at intervals; location of quarries and construction materials.
6. Traffic and land use data where needed.
7. Cost estimate of each alternative route, including earthwork, pavement, cross-drainage works and land.
8. Comparison of the alternatives and selection of the final route.

Result

The contour plan, longitudinal section and estimates are prepared; the chosen alignment is then marked in the field for the final location survey.

Modern practice uses total stations, GPS, drone/aerial photogrammetry and digital terrain model software (the aerial method), which saves time.