One of the most relevant and successful uses of drone technology within the Aggregates and Construction industries is for conducting volumetric surveys of stockpiled materials. But how exactly does the technology work in this use case and why is it such a valuable tool?
It’s these questions we aim to answer in this post.
Why is drone technology a good fit for stockpile surveys?
To put it simply, drone technology offers a faster, cheaper alternative to traditional surveying methods, without sacrificing data accuracy.
The equipment is also much more convenient to use – instead of hauling around RTK receivers, Total Stations, targets, poles and tripods, a basic drone survey can be conducted with as little as one camera-equipped drone and a few spare batteries.
Faster stockpile surveys
Anyone who has conducted a stockpile survey with the aforementioned equipment will know that the data acquisition stage is time-consuming and arduous! With drones on the other hand, data acquisition can usually start within 15 minutes of arriving on site and once in the air, data can be collected at a rate of less than 5 minutes per hectare.
Data processing is fast and easy, too. Simply upload your drone images into a photogrammetry software tool and in just a few hours you’ll be provided with an orthorectified 3D reconstruction of the site. Stockpile volume calculation is automated by the software – all you have to do is use the native volume tool to tell the software which stockpiles you want to know the volumes of.
How accurate are stockpile volumetric surveys with drones?
This is a VERY common question, so we’ll answer it in a way that makes it easy to understand.
The main point to note is that for the purpose of calculating a stockpile’s volume, the most critical component is “Relative (AKA: Local) Accuracy” – meaning the accuracy of point-to-point distances within the digital reconstruction. As a “rule of thumb”, you can typically expect relative accuracy of a photogrammetric product to be up to 3 times its ground resolution (AKA: Ground Sample Distance). For example, if your digital reconstruction had an average ground resolution of 10mm per pixel, you could expect the relative accuracy to be in the region of 10 to 30mm.
Another important consideration when discussing volumetric accuracy, particularly when comparing against GPS surveys, is the shape and surface of the stockpiles that you’re measuring. Seldom are stockpile surfaces ever regular in shape, and that’s where a drone-based survey really has the edge over a GPS survey. Where the latter will tend to generate ‘X’ number of square metres per data-point, a drone survey will generate hundreds of data-points per square metre. This means a much denser point cloud, which factors in all of the stockpile surface undulations and irregularities.
Creating a similarly dense point cloud from a GPS survey is simply not feasible, so the net result is a volume calculation that is based on a rough approximation of the stockpile’s surface, whereas drones and photogrammetry produce calculations based on what is essentially a digital twin of the stockpile.
Another technology used for calculating stockpile volumes is a 3D Laser Scanner. It’s without dispute that this technique is recognised as the benchmark to which the accuracy of others should be compared. With that in mind, numerous tests show that correctly executed drone-based volumetric surveys are accurate to within just 1 or 2% of ground-based laser scanning.
How does drone software calculate volume?
Once you’ve captured the drone images, uploaded them to your photogrammetry software of choice, and received the orthorectified 3D reconstruction, you now have just one manual process to do: to define the ‘toe’ of the stockpile(s) – the points where the stockpile’s slope meets the ground. This enables the software to calculate the stockpile base area. Once this is set, the software will extract the height information from the thousands of data-points above the stockpile base layer(s) you have defined, and provides you with the following calculations:
- Cut: The volume of material you’d need to extract from a pile to level the area with the surrounding ground
- Fill: The volume of material you’d need to add to a hole, trench etc. to level the area with the surrounding ground
- Volume = Cut – Fill
What you’ll need to start using drones for stockpile surveys
Before buying any equipment or software, you’ll need to consider the purpose of what you’re going to be doing – do you only need to report the volume of your stockpiles at any given point in time; or do you need to accurately measure and track volume changes of permanent stockpiles over time? If it’s the former, all you need is:
- a good quality, general purpose drone and camera (ideally with mechanical shutter)
- a photogrammetry software subscription with volumetric capability – it’s also a good idea to look for a solution that has good, in-built reporting features
- training to certify your staff for commercial drone operations
An ideal solution for this use case is the Get2Work Stockpile Pro Package.
Alternatively, if you need to accurately and reliably track volume changes of permanent stockpiles, you can either implement some basic ground control (eg. Ground Control Points + Checkpoints) with the above solution; or look to an RTK or PPK solution – the Get2Work Stockpile Elite Package is a great option here.