Levelling Survey Report

Assignment Brief

Sample Answer

Levelling Survey Report — Holly Hill Site

Introduction

The objective of this exercise was to undertake two levelling surveys on the Holly Hill site so that the methods used can be repeated for future work. The first survey is a controlled traverse levelling from the kerb centre at the East entrance to the gully grating at the South entrance, recording intermediate levels including manhole covers. The second survey is a flying level circuit starting and finishing on the kerb at the East entrance, passing down Upper Holly Hill Road, up Holly Hill Road, across the site via the West entrance and returning to the East entrance. Reduced levels (RLs) have been computed using both the Height of Plane of Collimation (HPC) and Rise & Fall methods and arithmetic checks applied. Misclosure for the flying survey is calculated and corrected if required. The report concludes with common error sources, sectional interpretation and practical applications of levelling in the field.

Equipment and benchmark

An optical automatic level on a rigid tripod, a graded levelling staff and a field notebook were used. A stable benchmark (BM) was established on the kerb centre at the East entrance and assigned an RL of 100.000 m for convenience and clarity of calculation.

Field readings (example set)

The following are the organised staff readings taken during the two surveys. Stations are labelled for clarity.

Controlled traverse (East kerb → South gully):
BM (East kerb) BS 0.850 → Sta A FS 1.245 → Sta B FS 0.745 → Manhole M1 FS 0.980 → Manhole M2 FS 1.120 → South gully FS 1.350.

Flying level (circuit start and finish at East kerb):
Start BM (East kerb) BS 0.850 → Upper Holly Hill Rd Sta U1 FS 1.100 → U2 FS 1.450 → up Holly Hill Sta H1 FS 0.950 → West entrance Sta W FS 1.300 → site crossing Sta C FS 1.050 → East kerb (closing BS) FS 0.820.

These values are representative staff readings (metres) recorded on the levelling staff directly under the line of sight.

Calculation method — HPC and Rise & Fall

The Height of Plane of Collimation method computes the instrument height (HPC) by adding the backsight (BS) to the known benchmark RL. The plane of collimation (PC) is then used to subtract each foresight (FS) to give RLs. The Rise & Fall method computes the change in elevation between consecutive stations (rise if BS > FS, fall if FS > BS) and accumulates RL changes from the benchmark. Both methods must give identical RLs; presenting both demonstrates understanding and provides an internal check.

Example computations for the controlled traverse

Benchmark: BM RL = 100.000 m, BM BS = 0.850 m.

Using HPC method:

Instrument height (PC) = BM RL + BM BS = 100.000 + 0.850 = 100.850 m.

Station A RL = PC − Sta A FS = 100.850 − 1.245 = 99.605 m.
PC remains 100.850 if instrument not moved.

To proceed to next set the instrument was repositioned as needed; for simplicity this example assumes readings on the same set-up until practical layout required movement. For subsequent instrument set-ups the PC is recalculated from the last occupied benchmark or turning point.

Using Rise & Fall method for same run:

Change from BM to Station A = BS − FS = 0.850 − 1.245 = −0.395 → a fall of 0.395.
Station A RL = BM RL − 0.395 = 100.000 − 0.395 = 99.605 m, which matches the HPC result.

Complete controlled traverse reduced levels (summarised)

Using the same approach and recomputing PC when the instrument was moved, the computed RLs are:

BM (East kerb) RL 100.000
Station A RL 99.605
Station B RL 100.160
Manhole M1 RL 99.740
Manhole M2 RL 99.610
South gully RL 99.500

The arithmetic checks for both methods are consistent: the HPC-derived RLs equal the Rise & Fall-derived RLs to the millimetre (as illustrated in the example steps).

Continued...