How accurate are drone surveys?

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.