Accuracy in Drone Topographical Surveys – What You Need to Know.
With drones becoming an ever-familiar site on construction projects, the accuracy of their survey output is always a topic of discussion. Can a remote platform really deliver the results we expect? And what range of accuracy can you expect from your surveys? The answer isn’t as straightforward as you might think.
Before we delve into the merits of specific platform setups, let's consider how topographical surveys are generated from aerial imagery. The fundamental element is the use of triangulation in photogrammetry. Using stereo imagery produces lines of sight, which provide coordinates in 3D space of the target site.
It’s like your eyes finding depth in the real world and the same principles applied to measure bomb targets in WW2. We’ve moved on a bit since then, of course. With the benefit of larger camera sensors and high-resolution images, we can now stitch images together to produce very detailed and true plan-view survey results known as orthomosaics.
An orthophoto is a geometrically corrected image i.e. the scale is uniform. Unlike an uncorrected aerial photograph, when a scale is applied, an orthophoto can be used to extract dimensional information. The accuracy of that scale is what drives the overall accuracy of the survey and there are two ways we can think about accuracy:
Relative/Internal Accuracy – this is a measurement of actual dimension i.e. does a 1m distance on-site show as 1m on the survey drawing?
Absolute/Global Accuracy – the degree at which a point on the survey corresponds to a fixed coordinate system in the real world. I.e. the degree in which we show the x,y,z coordinates relative to their real-world location.
So how does this apply to the broad range of customers who can benefit from drone surveying?
For a start, you’ll need to add that crucial scale factor into the mix. There is a multitude of options but for the purposes of this blog, we will consider two. They are GPS on the drone and GPS-based Ground Control Points (GCPs).
Data Capture Method Global Accuracy Internal Accuracy
Drone GPS 700mm 70mm
Ground Control GPS 20mm 20mm
Simply put, the accuracy of the control network will directly influence the accuracy of the final result. A common error in drone surveying is to fly without any control in place and rely solely upon the accuracy of the onboard GPS. Whilst some platforms are equipped with survey-grade GPS that can produce accuracies in the region of +/-60mm. A ground control network will always be required for final scaling and QA/QC monitoring. By deploying accurate GCP targets we would typically see internal accuracy in the range of +/-35mm and 55mm for horizontal and relative accuracies respectively.
With an average of 100 million individually measured points on any survey, this technology presents a far more efficient way of collecting data than traditional ground-based methods. But it is still worth noting that with an increase in accuracy comes at an increased cost. To determine the extent of the accuracy needed in each situation will depend on:
How and where the final survey results will be used
The key decisions that stem from the survey results
It takes a lot of skill to get survey-grade accuracy using a drone. There can be many sources of error that can impact accuracy. Strict procedures for land survey using drones are necessary for collecting data in the field consistently, repeatedly and accurately.
When instructing your own survey, it is important to make sure that the drone provider has qualified surveyors as well as drone pilots along with extensive insurance including professional indemnity cover. Innovair is regulated by the RICS (Royal Institute of Chartered Surveyors). This means that we adhere to their code of conduct and are regulated and monitored to ensure that we meet the highest standards of our survey service.
Survey projects are getting more complex and more costly to complete. So, the desire among surveyors to use drones to achieve these projects will only keep rising. The data generated in the final survey output is powerful and thus, valuable to clients. At Innovair, we provide the topographic and visual information that you need to enhance decision-making, reduce costs, save time and improve safety.
Find out more information about our Drone Survey Service here