Photogrammetry | TarmacView

Photogrammetry is a method that measures the position, size and shape of objects from overlapping photographs.

How it works

The camera takes many photographs of the same area from different positions. Software finds the same points in several images. It then calculates the camera positions and the 3D coordinates of those points. The FAA calls the first step aerial triangulation. Stereo analysis is a measurement from an image pair.

Structure from motion (SfM) is the common way to do this with drone images. It estimates the camera positions and the 3D structure from the overlapping images, without a prior list of control points. Multi-view stereo then adds more points, and each point appears in at least three images.

The first result is in an arbitrary 3D coordinate system. To place it in a real-world coordinate system, you add ground control points (GCPs) or camera positions from a GNSS receiver.

What it produces

A dense 3D point cloud.
A digital surface model (DSM).
An orthomosaic, which is a corrected image map of the area.
A textured 3D mesh.

What sets the quality

Two image settings matter. Ground sample distance (GSD) is the ground area that one pixel covers. Overlap is the share of an image that the next image repeats. Forward overlap is along a flight line. Side overlap is between flight lines. Higher overlap makes processing more likely to succeed, but it needs longer flights and more processing time.

The accuracy of the result also depends on the camera and lens, the software and the site. Dense vegetation reduces it.

Use at airports

A 2023 FAA report describes tests of small drones for obstacle data at five airports, reviewed with the NOAA National Geodetic Survey. It found that drone images with 3D stereo analysis can meet the accuracy of AC 150/5300-17 and AC 150/5300-18. For obstacles that penetrate the surfaces of a vertically guided approach, AC 150/5300-18 asks for 20 ft horizontal and 3 ft vertical accuracy.

The study calls drones most practical for small surveys. It suggests the obstacle clearance surfaces of PAPI and visual glide slope indicators as one use.

Frequently Asked Questions

What is ground sample distance (GSD)?

Ground sample distance (GSD) is the area on the ground that one image pixel covers. A lower GSD shows more detail. GSD depends on the camera and on its height above the ground. The same camera gives a lower GSD at a lower altitude.

Does photogrammetry need ground control points?

Usually, yes, or an equivalent. The raw result is in an arbitrary 3D system. To place it in a real-world system, use ground control points (GCPs) or camera positions from a GNSS receiver.

Can drone photogrammetry meet airport obstacle survey accuracy?

It can. A study by the FAA and the NOAA National Geodetic Survey found that UAS imagery with 3D stereo analysis can meet AC 150/5300-17 and -18. The result depends on the camera, the GSD, the overlap and the software. The study advises users to test their own combination.

This is a reference entry. TarmacView provides drone inspection services and doesn't sell the equipment or materials described in this glossary.