Stockpile Surveying: When Is It Better Not To Use Drones?
Western Cape, South Africa
Surveying, Stockpiles Volumes and Drones
As a licensed Drone Service Provider, most of our surveying is done by collecting aerial data. For stockpile surveys we collect high resolution RGB (visual) imagery which is processed together with Ground Control Points (GCP’s) in order to achieve centimetre-level accuracy within the model.
Integrated Aerial Systems was recently approached by one of its regular clients to conduct a stockpile survey. Companies need to know how much material they have on hand at the end of the year which is typically used by auditors to compile their year-end financial statements. For this particular survey, the client needed to know the tonnage of two magnesium stockpiles which were due to be loaded onto a ship the following day. Therefore, they relied on a highly accurate survey to correctly determine the space required on the ship as well as the cost of shipping.
What made this an unusual request was that the stockpile that needed to be measured was inside a warehouse. Indoor aerial stockpile surveys are indeed possible with photogrammetry or Lidar scanning but in this case the following factors did not favour the use of aerial surveying:
- As a licensed Drone Service Provider, most of our surveying is done by collecting aerial data. For stockpile surveys we collect high resolution RGB (visual) imagery which is processed together with Ground Control Points (GCP’s) in order to achieve centimetre-level occur
- The drone would not have a GPS lock indoors which would require piloting the aircraft in ATTI (fully manual control) as opposed to our precise GPS guided grid missions in the open air. This poses a safety risk and decreases the quality of the data being collected. The use of a system such as the Elios 2 (https://www.flyability.com/elios-2) would have ensured a safe operation indoors due to the durable cage surrounding the aircraft but without a GPS lock the images are challenging to capture and process into an accurate point cloud.
- Poor lighting which has a further impact on the data quality if photogrammetry is being used.
- The stockpile was right up against the warehouse walls which would not have allowed for sufficient overlap on the edges of the stockpile, which decreases the accuracy of the model.
- IAS also operates the Hovermap Lidar (https://www.emesent.io/hovermap/) which takes over from the drone’s GPS for indoor navigation and is suitable for capturing data to generate an accurate point clouds indoors and in low light. However, it was determined that for this particular survey, it would be challenging to get accurate results without combining the Lidar data with ground control points as well, which would have put the total cost above the client’s budgetary requirement
Therefore, for this survey we decided to revert to traditional surveying methods. Our registered Professional Land Surveyor, Takalani Makhavhu, used the following methodology to conduct the survey:
- A local system was used for this survey since these calculations are only relative to this site.
- A Spectra Precision Pro SP60 GPS was used to bring in surveying control/benchmarks closer to the warehouse.
- The Spectra Focus 2 (5”) Total Station was used to transverse in survey control into the warehouse and to capture all the stockpile details
Firstly, the stockpile base outline was captured, then spot shots were taken where there were changes in elevation and finally the top profile of the stockpiles were captured. A combined total of 189 points were captured for both stockpiles and 8 surveying control points were set out within the warehouse. Microstation Software was used to draft the stockpile topographical points and to perform the volumetric calculations. The concrete surface in the warehouse was used as the base level and triangles were generated from the base level to the top level, taking into consideration the changes in elevation.
The report with the volume calculations for each stockpile was delivered to the client in under 24 hours, well ahead of the already tight deadline we had to work with. Integrated Aerial Systems has extensive experience in developing bespoke survey solutions to our clients across a range of industries and despite the survey methodology being slightly outside the scope of our core service offering, Integrated Aerial Systems delivered accurate results to the client without delay.