Having established baselines on site, the survey area was covered in line with PAS128 M4P 0.5m spacings. Connections were made to gas and water surface features (where possible), while electricity cables were traced from access points such as cable risers, cable pits and lamp posts. Telecoms inspection covers were also lifted and duct routes traced where possible. GPR surveys in the roads utilised a vehicle-based mobile mapping system to minimise traffic disruption.

Surveying the sewers and drains was a much more difficult proposition, mainly because of the large amounts of silt deposits and blockages, the heavy effluent flow rates and large distances between manholes which made it impossible to successfully survey certain locations.

Using a combination of sonde tracing, CCTV surveys and gyroscopic mapping our teams were able to survey the majority of the area and once jet cleaning of the most heavily-silted areas has taken place we will be able to complete the full assignment.

The course of the pipe is mapped by a gyroscopic probe that records its exact path in three dimensions between the accurately determined entry and exit points, using inbuilt accelerometers and gyroscopes. The unit is pushed back and forth through each sewer run four times (twice in each direction) using rods attached to the device and takes multiple positional recordings along its path. The bespoke software then calculates the angular change compared to the previous reading and is accurate to 15cm over a 500m distance.

It is not necessary to know the exact position of the probe as it travels from entry point to exit point as the software establishes where it has been after it is retrieved from the pipe. The software converts the autonomously logged data to an accurate 3-dimensional line in the same coordinate system as the given coordinates at the entry and exit points.