How accurate are drone mapping surveys?

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Drones are being used more and more to provide maps to support construction, development, mining, agricultural and other commercial activities. Queensland Drones has been providing drone mapping surveys for over two years now, so it seems like a good time to discuss their sources, accuracy and reliability.

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Aerial drone maps are being used to support all kinds of planning, design and assessment

First, let’s be clear about what drone maps are not … they are not a substitute for a survey by a registered surveyor. Drone mapping can be amazingly accurate and there are many situations where drone mapping is all you need to inform your project or design, but before you rely on drone-based maps to plan earthworks, building foundations or other structural and engineering uses where accuracy is critical, they need to be ground-truthed or otherwise assessed by a qualified surveyor. Well-constructed drone maps can be certified by a registered surveyor and some of our drone mapping work is commissioned by surveyors for their clients.

When we create aerial survey maps, we use industry-standard GIS-grade survey equipment to level and align the photogrammetric data for maximum accuracy. Some drone operators don’t use GCPs at all, but rely on a 3rd party processing platform like DroneDeploy to provide the accuracy. These platforms provide pretty outputs, but not accuracy.

But to understand that fully, we need to talk about what accuracy really means.

What does “accuracy” mean for drone surveys?

There are two types of accuracy when it comes to drone mapping surveys (or any other kind of survey) – Relative Accuracy (also known as local accuracy) and Absolute Accuracy (also known as global accuracy).

Relative Accuracy is the extent to which you can accurately measure one point in a survey map relative to another point in the same map. For example, you may want to measure the elevation difference between two points in the map, or the length of a line drawn between two points in the map. So if two objects in the real world are a meter apart, they should also measure a meter apart on your drone survey map. If they measure 1.1 metres apart on the drone map, it only has a relative accuracy of 10 cm.

Absolute Accuracy is the extent to which the survey map can be accurately positioned within and overlaid on top of real world mapping data. For example, you may need to measure the height of a point in your map relative to other points in the real world, like sea level or a local flood level, or you may need to be able to measure the distance between a point in the survey map and a point outside the survey map, like a sewerage connection. The latitude, longitude and elevation of any point in a survey map should correspond accurately to its actual GPS coordinates as measured with a GPS survey device. The difference between the GPS data in the map and the GPS data for the same point in the real world is the X/Y/Z accuracy of the survey map (X being Longitude, Y being Latitude and Z being elevation).

Off-the-shelf drones rely on commercial satellite technology to know where they are and to navigate between locations – technologies like GPS and GLONASS, which use satellites positioned 20-40,000 km up in space. While satellites can provide fairly accurate positioning, communication between the satellites and the drone can be affected by all kinds of things including solar radiation (sunspot flares), heavy cloud, tall buildings, trees (where the satellites are low on the horizon) and more. Despite all this, the GPS/IMU in a modern drone can figure out where the drone is in space down to 1-2 metres.

Sounds OK until you consider that 1-2 metres may mean the difference between something being on the ground, or being above your head!

Drone maps are created by “stitching” together hundreds of overlapping photos taken at a specific altitude. Every time two images are stitched together, there is a potential for some distortion to force them to fit together. While this may only be a millimetre or two, by the time it has accumulated across hundreds of photos it can become a metre or more. So while the positional accuracy of each individual photo may be 1-2 metres, the positional accuracy of the finished map may be 2-5 metres. And because drone images tend to distort a little more toward the edges, stitching them without correction (known as “levelling”) can create a “bowl” effect in the final map which distorts measurement even further.

Does this mean drone mapping is inherently inaccurate? No, it means that drone mapping without well-developed GPS correction methodology can be inaccurate, but serious commercial UAV mapping operators should have well-developed methodologies for accurately positioning their drone in space and for correcting the distortions inherent in drone images.

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Drone surveys convert overlapping aerial photos into a 3d point cloud

Relative Accuracy is typically used to measure distance and volume, so you might want to know how long a section of road is, or how much material is left in a stockpile, or how much will need to be removed to dig a hole this big. You can make a lot of local or relative measurements without the map necessarily being accurate about its position in space, because the measurements are relative to points within the generated map itself. So as long as the map is not distorted, relative accuracy can be useful.

The larger the mapped area, the less the error in relative accuracy is critical to the outcome. For example, if we assume that relative accuracy is one metre for a given aerial mapping technology and we’re measuring 25km of fence line, an error of 1m or so is not terribly important. But if we’re measuring a retaining wall 10m long, an error of 1m is pretty significant.

We maintain a high level of relative accuracy in our UAV survey maps by using ground control points to tie the images to known and measured positions on the ground, by maintaining a known “ground sampling distance”, and by using survey-grade levelling to ensure all image positions correspond accurately to their actual position on the ground.

Absolute Accuracy refers to how well a given point on a photogrammetric map corresponds to the actual position of that point if it were measured, for example, by a surveyor.

If we use the example of a road, if the drone imagery says the starting point of the road is Latitude -52.34567, Longitude 267.4321 and is 120m above mean sea level, and a surveyor’s measurement says something different, then we have an absolute accuracy error. Absolute accuracy is the more critical measure of how useful a drone mapping survey will be when used for design, construction or establishing property boundaries. It’s less critical when surveying for agriculture, for example.

We maintain a high level of absolute accuracy by establishing our ground control points using Trimble survey-grade GPS equipment. We are often able to establish ground control point accuracy to less than 1 cm horizontally and 1.5-2 cm vertically.

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Basic ground control points can be as simple as white crosses marked on the ground at points measured with accurate GPS equipmentThe number and distribution of the ground control points in a drone mapping survey will significantly contribute to the absolute accuracy of the mapping that results. Aligning the ground points to known survey points will also improve their reliability and their acceptance for survey purposes.

There are many ways that small accuracy errors can be introduced into a drone mapping survey. For example, if the centre of a GCP is a line 2cm wide (so it can be easily identified in the photos), then positioning that point in the imagery will have a potential error of 1-2 cm just because of the thickness of the line. Add this to the positional accuracy of PPK or RTK (typically 1-2cm at best) and you have a total potential error of 4-6 cm already.

Drone maps created using good quality drone equipment, careful flight planning, commercial grade GPS ground control points and commercial grade processing software can potentially be accurate to around 2-3cm horizontally and around 5-6cm vertically.  Because we prefer to err on the side of caution, we claim our aerial mapping accuracy as <10cm horizontal and <15 cm vertical, which is generally sufficient for most purposes. Anyone claiming accuracy higher than this probably does not understand what they are doing.

Note always that drone operators are not surveyors. While our mapping can be created at very high levels of accuracy, it should be certified by your surveyor before being used for engineering purposes.

If you would like to enquire about drone mapping for your projects, please contact us using the links below.