How accurate are drone surveys?

Key takeaways

Two drone land-survey methods: photogrammetry and LiDAR

Accuracy varies by surface: hardscape vs softscape

Define the accuracy you truly need before choosing the method

Drone survey accuracy - why the answer is always "it depends"

Ask an engineer how accurate a drone survey is and the honest answer is: it depends. It depends on the method, the surface, and - critically - on what level of accuracy you actually need for the job in hand. "As always, when we're talking about surveys, there's two things you want to understand," says Bob Foley of Engineers With Drones. "Are you surveying land? Are you mapping land? Or are you surveying some form of asset? Are you inspecting an asset?" For this article we are focused on land mapping, where the accuracy question is most nuanced.

Two methods, two different accuracy profiles

Land mapping by drone comes down to two technologies: photogrammetry and LiDAR. Photogrammetry stitches together a large number of overlapping images with high precision to generate a 3D model. LiDAR fires laser pulses from the drone and measures where they return, building up a point cloud. Both produce accurate spatial data - but their accuracy characteristics differ significantly depending on what the drone is flying over.

Hardscape vs softscape - the distinction that changes everything

"When it comes to accuracy, you want to be thinking about are you shooting at hardscape or softscape?" Bob explains. Hardscape is buildings, roads, footpaths, gravel, compacted ground - anything solid and defined. Softscape is tall grass, vegetation, ditches, forestry - anything where the actual ground surface is hidden beneath organic material.

The distinction matters because photogrammetry maps what it can see. If a field is full of corn, the drone sees the top of the corn - not the ground. "To the eye, the topography looks like it's nearly a foot and a half, two feet above what it actually is," Bob notes. That same field cut bare is an entirely different proposition. For hardscape, photogrammetry is the preferred technology and delivers strong results. "We're probably talking, depending on the technology, down to about two centimeters," Bob says of hardscape photogrammetry accuracy.

It is worth being clear about what two centimeters means in context. A terrestrial surveyor with the right equipment can achieve millimetre-level accuracy. Drones cannot match that - a drone is a flying machine and has inherent limitations. But for the overwhelming majority of land mapping applications, two centimetre accuracy is more than adequate.

When the vegetation is the problem - use LiDAR

For softscape, photogrammetry accuracy becomes unreliable because the sensor cannot see through the vegetation to the ground beneath. "We always want to be careful when we talk about accuracy that we're being very clear," Bob says. LiDAR changes this because it finds gaps in vegetation canopies and returns measurements from the ground surface below, producing what is called a bare earth topography - an accurate picture of the ground regardless of what is growing on it.

Define what you actually need first

One of the most common conversations Engineers With Drones has with new clients is about accuracy requirements that do not match the task. "What we see with a lot of our clients is they'll say, I want to map this forest and I want two centimetres of accuracy. And we say, no, you don't." Demanding sub-centimetre accuracy for mapping a vegetated area is not just unnecessary - it is technically meaningless if the sensor cannot reach the ground surface in the first place.

"Nobody needs that level of accuracy to map vegetation. It's not a thing. Now, if you really want to know what's under the vegetation, yes, you do." The right approach is to define the actual engineering or planning requirement, then select the method that delivers what is genuinely needed.

Full transcript

Show transcript

Interviewer:

[00:00:02] How accurate are drone surveys in real-world conditions?

Bob Foley - Engineers With Drones:

[00:00:07] As always, when we're talking about surveys, there's two things you want to understand. First of all, are you surveying land? Are you mapping land? Or are you surveying some form of asset? Are you inspecting an asset? So we're going to assume that what you're talking about here is that we are surveying land, we are mapping land and topography and things like that. That could be down to an individual facility like a factory and all of the buildings and hardscape within that. Or it could be 300 hectares of farmland, forestry, whatever.

But the mapping out of wide areas of land is what we're discussing.

Two ways of doing that, photogrammetry and LiDAR. Photogrammetry being take lots of pictures and stitch them together in a very precise way to generate a 3D model of whatever you're mapping or LiDAR, shoot lots of laser beams out of the bottom of a drone and pick up all of the points where they hit and generate a point cloud. Okay.

[00:01:07] When it comes to accuracy, you want to be thinking about are you shooting at hardscape or softscape?

Hardscape is things like buildings, roads, footpaths, very small grass, gravel, things like that. Softscape is tall grass, ferns, vegetation, ditches, forestry, plants and all that. A plowed field could be considered hardscape, but that same field 2 weeks before when it was full of corn was very much softscape.

Right. To the eye, the topography looks like it's nearly a foot and a half, 2 feet above what it actually is.

[00:01:49] Whereas once that field is cut, it's very clear where land is and vegetation is not. So understanding that is very important because you could map out an entire area photogrammetrically, which looks at what it can see and say, that's the topography of the land, and the accuracy of that is XXX. But actually, it's completely different because there's maybe 2 foot of vegetation under what you're perceiving the land to be. So it's very important to differentiate hardscape and softscape when we're mapping. As always, we need to penetrate softscape. If we need to understand what's below softscape, so vegetation primarily, LiDAR is our best tool because LiDAR, for the most part, will go through the vegetation.

It finds all the little gaps in it and sees the ground underneath. And so you get a much more accurate picture of what's called the bare earth topography if you use LiDAR. So the question you asked is what sort of accuracies can you expect from a survey? If we are doing hardscape, if we're doing hardscape and we're using the preferred technology for that, which is photogrammetry, then we're probably talking, depending on the technology, down to about two centimeters. Okay, down to about two centimeters. That's not accurate relative to terrestrial scanning. So terrestrial scanning would be a proper surveyor walking around with all of the tools at their disposal. They will get down to millimeter level accuracy, no problem.

[00:03:37] Two or three millimeters. We do not operate at that level because we've got a bouncing flying machine in the sky and you just don't get that level of accuracy. Down to about 2 centimeters of accuracy is what we can get with photogrammetry on hardscape. Photogrammetry on softscape, who knows? Because we could have 4 inches of grass, we could have, which is 10 centimeters of grass.

[00:04:05] So that's, five times what our accuracy could be anyway. So we could say, well, the top of a blade of grass is at whatever height, but we don't know what's underneath it. Or we got a ball of trees or we got a ditch and so on and so on. So we always want to be careful when we talk about accuracy that we're being very clear. And what we see with a lot of our clients is they'll say, I want to map this forest and I want 2 centimeters of accuracy. And we say, no, you don't.

Nobody needs that level of accuracy to map vegetation. It's not a thing. Now, if you really want to know what's under the vegetation, yes, you do.

Drone survey FAQs

How accurate are drone surveys in real-world conditions?

Our expert at Engineers With Drones, Bob Foley, explains that accuracy depends on the method for drone survey land mapping, and whether the area is considered hardscape or softscape.You can expect about 2 centimeters of accuracy with photogrammetry on hardscape. As to softscape accuracy, this depends on whether you need to know what's under the vegetation. If so, LiDAR is the best option for that level of accuracy to achieve a detailed bare earth topography.