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Level Six Service Avoidance Survey


Midland Survey Ltd were asked to carry out a TSA (The Survey Association) level six Utility survey on a very large college site in Oxfordshire. A level six survey is a high specification survey that requires manhole cards that detail information at each drainage access point on the site and a desktop record search.

Historical records of the site indicated that air raid shelters were present. Before the client could begin their construction project, it was important to identify the location of the air raid shelters as this is best safety practice. Leaving these objects underground can lead to costly construction mistakes and risks the safety of employees.

After consulting the client, we recommended drill works on the site and work to establish if/where the air raid shelters were present.

For this project, the budget and timescale to complete the surveying work were limited. The client had tight deadlines to confirm design work following the survey results. These factors were considered and led to our recommendation of a Midland Survey Ltd level six service avoidance survey.

A level six service avoidance survey would allow us to cut costs dramatically and allow for a quick turnaround. We specifically targeted known historical areas of interest on the site, where the air raid shelters were shown on records. The team cleared drill locations individually, whilst still providing them with the level six detail that was required.

Firstly, we conducted an initial utility survey on the site using radio detection techniques of each area. After that, the following steps were taken.

Equipment used

  • Cobra dual frequency ground penetrating radar. The cobra has the benefit of collating information onsite so it can be downloaded as a reference for each scan taken.
  • RD1000 ground penetrating radar.
  • Leica global positioning system.
  • RD8000 and RD7000.
  • Line trace and drainage rod.

Process followed

  1. A grid was set out using the Leica GPS. We were able to input coordinates to the GPS before site works began. We did this by drawing the grid onto the provided third party topographical survey and then extracted the coordinates. This allowed us to reduce our time on the site, leading to lowered client costs whilst improving the accuracy of the GPR grid.
  2. Using the Cobra GPR unit, the grid was scanned at 1m intervals. Each anomaly or service found is clearly marked on the ground using spray paint and recorded on site using the Leica GPS.
  3. We also used on site analysis of the RD1000 GPR unit. This allowed us to cross-reference the data collected by the Cobra GPR unit.
  4. The air raid shelters were located with both GPR units and the extents clearly marked up on site using spray paint. After discussing the results with the engineer, the locations of the trial pits were agreed upon. This would maximise the amount of information that could be gained from each pit, prove all locations are clear of services and ready to dig.
  5. All of this site information is collected using Leica GPS system.
  6. The Digging works commence.
  7. The air raid shelters have been found.
  8. Our work now moves to the office. The information from the site has now been downloaded and imported into the provided base drawing.


The pictures attached clearly show the air raid shelters and also the trial pit locations. The grid is a reference to the GPR scans which are also annotated and provided to the client.

To summarise, the project was a success. We were able to complete the surveying work to meet the clients restricted budget and time. Identifying the precise location of the air raid shelters allowed the client to arrange removal/ plan their construction project around the obstacles. Utility mapping surveys are an essential stage in the planning process. If you are in need of a survey for an upcoming project, Midland Survey Ltd have the skills and experience to help.

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