Photogrammetry is the art and science of extracting 3D information from photographs. The process involves taking overlapping photographs of an object, structure, or space, and converting them into 2D or 3D digital models.
Photogrammetry is often used by surveyors, architects, engineers, and contractors to create topographic maps, meshes, point clouds, or drawings based on the real-world.
Types of photogrammetry
There are two primary types of photogrammetry: aerial and close-range.
1. Aerial photogrammetry
The process of utilizing aircrafts to produce aerial photography that can be turned into a 3D model or mapped digitally. Now, it is possible to do the same work with a drone. Drones have made it easier to safely capture hard-to-access or inaccessible areas where traditional surveying could be dangerous or impractical.
2. Close-range photogrammetry
Close-range photogrammetry is when images are captured using a handheld camera or with a camera mounted to a tripod. The output of this method is not to create topographic maps, but rather to make 3D models of a smaller object.
The Basics of Photogrammetry
The process of photogrammetry can vary, but the general idea revolves around gathering information about an object from photos of it. The photos are taken from different locations and angles to allow for precise calculations that help analysts gather the data they’re looking for. Typically, they use things like photo interpretation and geometric relationships to gather measurements. With the data gathered from photogrammetry, we can create maps and 3D models of real-world scenes.
The technology has been around for a long time and has been an important part of a variety of research in the last century. Its principles date back to Leonardo da Vinci’s research on perspective in 1480 — and many theories say it goes back even further. After the invention of flight and World War II, photogrammetric technology really increased, with powerful camera designs and new aircrafts built specifically for aerial photography and better camera positioning. All of the new inventions even put photogrammetry on the moon to map its surface during the Apollo missions.
If we break down the word, we can clearly see all of the parts that make up photogrammetry in play. “Photo” refers to light, “gram” means drawing and “-metry” refers to measurements. Photogrammetry uses photos to gather measurements with which we can create drawings and models.
What is Aerial Photogrammetry?
Taking aerial photos is one of the most common approaches to mapping out an area. In this process, a camera is mounted on an aircraft and pointed toward the ground with a vertical or near-vertical axis. As the plane follows its flight path, the camera takes multiple overlapping photos, which are then processed in something called a stereo plotter.
The stereo plotter is an instrument that helps determine elevations by comparing two different photos and conducting the necessary calculations. With the help of photogrammetry software, we can process this information and create digital models out of it.
What Is Terrestrial Photogrammetry?
These images are taken from a fixed position on the ground with a camera’s axis parallel to the Earth. Data about the camera’s position, such as its coordinates, are collected at the time the photo is taken. The instruments used for terrestrial photography are often theodolites, though regular cameras are sometimes used as well. Terrestrial photogrammetry for surveying typically requires fewer resources and skilled technicians to accomplish, but it may take longer to cover a large portion of land.
What Is Space Photogrammetry?
Moving out to a larger scale, space-based photogrammetry occurs with cameras either fixed on Earth, in an artificial satellite or positioned on the moon or another planet. In fact, photogrammetry was touted as a key part of space exploration even in the ’60s, and technological advancements have made it even more relevant. It can tell us about cloud patterns, create accurate maps of Earth and gather data about faraway planets.
Some aspects are necessary for any photogrammetric model. These features include:
Tie points: Tie points are coordinates that can be linked across multiple overlapping images. Typically, these are features present in both or all of your photos. The tie points help the photo adjust with shared coordinates.
Ground control points (GCP): GCPs help to orient the image in relation to the Earth’s surface. They use known coordinates to position the image within the real world.
Bundle adjustment: The adjustment helps to remove any distortion within a set of images. It reduces errors from real and predicted image points.