“How Drone Photos Turn into 3D Surveys?”

• We get this question all the time. It’s a good one, and definitely something you should care about
• Producing accurate, useful 3D drone surveys requires three ingredients: aerial drone imagery, good ground control, and the science of photogrammetry.

The gist of the science

• Essentially, if you see the same feature from three or more known positions, you can triangulate its location in space (a.k.a nail down those exact X, Y, and Z coordinates). A feature is any visually distinct point in an image.
• If you took an average image from your survey, you’d easily be able to pick out many “features” between images. The more features you match, the better you can relate images to each other and reconstruct objects within them. This is exactly what photogrammetry software does for one feature, and the next, and the next, and so on, until it’s covered your entire site.
• Once you have a lot of these features—think millions—you can create a “cloud” of points. Each point has a matched feature describing your surveyed area in that location. You can then turn your point cloud into any regular outputs used in geospatial software, like a 3D mesh or digital elevation model (DEM).

“How Drone Photos Turn into 3D Surveys?”

• The best way to visualize this is to use your eyes—literally. Your eyeballs are using photogrammetry all the time.
• You have two eyes (two cameras), processing a live feed of your surroundings. Because your eyes are slightly apart, you’re getting two different inputs at slightly different angles. (Test this yourself by holding up a finger in front of your face. Look at it with one eye closed, then the other. You’ll notice your finger jumps relative to background objects.) Your brain knows how far apart your eyes are, which allows it to process this info into a sense of distance by merging both feeds into a single perspective. (If you’ve ever tried to catch a ball with one eye closed, you know it’s difficult, as you have no depth perception.) Your mind is rendering a live depth map of the 3D world from two 2D inputs—just like how Propeller renders a 3D survey from many 2D photos.
• Once you know the shape of something in 3D (from the features and points above), you match that up with its visual appearance. This is where image stitching and “ortho-rectification” come in. Stitching is like putting together a jigsaw puzzle. When you assemble one, you look for common features—corners, bright shapes, etc.—to fit together everything more easily.

Why use Photogrammetry for Surveying and Mapping?

• Photogrammetry has a long history of use in the surveying and mapping field. Photogrammetry can be used for point measurements or for generating point clouds. Compared to its chief rival, LiDAR (Laser Imaging, Detection and Ranging), point-cloud generation from photogrammetry is cost-effective, portable, and versatile.

The Benefits of Photogrammetry in Ground-Based Surveying

• Photogrammetry is the process of generating a 3D model from a set of 2D photographs. In surveying, this is done by taking two or more images of the same point from different angles. These images are then loaded into a photogrammetry software
• Notwithstanding these technical differences, photogrammetry has always presented a very compelling “business proposition”, for several reasons:
• It’s cost-effective. LiDAR equipment is expensive and requires specialized expertise to operate properly. Photogrammetry requires only a DSLR camera and software that can be run on a standard computer.
• It’s easily available. Due to the need for specialized equipment and operators, LiDAR resources are often hard to secure on short notice. Photogrammetry equipment can fit in the back of any surveyor’s truck, and you can operate it yourself.
• It’s versatile. Photogrammetry technology has evolved to the point where it’s a great general-purpose imaging tool. LiDAR has more specific use cases due to technical limitations, such as the bulk of the equipment, and difficulty in obtaining high resolution images.