Case Study

A comparison of manual surveying vs photogrammetry solutions.

For this comparison project, we worked with a client operating a 900 Hectare facility in the East of England. The facility comprises 56 different stockpiles of different materials and varying sizes/volumes spread out across the area. The different stockpiles need to be volumetrically assessed on a monthly basis.

Overview of the facility with numbered stockpiles.

Methodology 

We used a UAV system to provide analytics for the entire site and focused on one specific stockpile (No. 6) for comparative analysis with land surveying techniques.  

We followed a 4-step approach to complete the UAV survey: 

  • We completed a detailed pre-survey check-list to capture all details relevant to the project 
  • A GNSS site engineer created a complete boundary ground control lay-out. This took approximately 60 minutes to complete. 
  • The GNSS Aerial team programmed and flew a UAV to capture aerial imagery for the whole site. This took approximately 65 minutes and 3 flights. 
  • The data was then transferred to the GNSS data centre for analysis. This post-production phase took approximately two working days. 

For the manual survey, we manually surveyed the stockpile with a GNSS GPS rover. 200 single points were taken to create the best possible analysis; the total survey took about 35 minutes. 

Results 

Both sets of data were modelled into a DTM for further comparison in LSS. We then calculated a cut and fill balance between the two DTMs. In summary: 

  • The UAV survey DTM was 156m³ less than the manual survey.
  • This represented about 1% of the stockpile volume.
Close up of comparative survey profiles.

The UAV survey collects points at 3cm spacing with minimum triangulation, a total of more than 1.6million 3D points. In comparison, the manual survey captured around 200 3D points. The manual survey DTM therefore “smooths out” the curves that are mapped in detail by the UAV survey, particularly where the surface is concave. This has the result of systematically overestimating the total volume in the stockpile. 

Conclusions 

As an engineering company we continue to embrace new technology methods alongside our traditional techniques. We have found that UAVs are a highly accurate and reliable alternative to manual working for some projects. Other important advantages of UAV systems are:

  • Safer working, enabling us to capture data without putting people on the ground in active sites.
  • Speed, enabling us to survey much larger areas on a regular basis. 

Nevertheless, we recognise that UAVs are not a universal panacea, and that manual alternatives will still be relevant in some cases – or, indeed, when weather conditions make it impossible to use UAVs. As a matter of policy we therefore always fully assess each project individually to be sure that we deploy the right tools for the job.

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