What Is Photogrammetry?

What Is Photogrammetry?

For centuries, photogrammetry has played a critical role in our understanding of faraway objects and the Earth’s surface. Its uses have expanded over the years and have led to a powerful range of game-changing technologies in industries like construction, engineering, medicine and much more.

Photogrammetry gathers measurements and data about an object by analyzing the change in position from two different images. It uses things like perspective, advanced processing software and photo analysis to get the job done, but it can happen on the ground or from the air. In this guide, we’ll explain the different types of photogrammetry and how it can be used.

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.

Types of Photos

Since aerial photogrammetry is one of the most common methods, let’s take a look at how those photos get classified.

Typically, aerial photos will fall under one of two categories:

  • Vertical photographs: These images occur when the camera axis is vertical. So, if you put the camera in an airplane, its lens would point down to the ground for a birds-eye view.

  • Tilted photographs: Though the axis may be nearly vertical, tilts in the aircraft can cause an image to be unintentionally tilted in one direction. Within the category of tilted photographs, we have oblique photos, in which you can see the horizon line, and low-oblique photos, in which there is no apparent horizon. The classification depends on the level of tilt of the camera off of its vertical axis.

The lens of the camera can also offer a range of coverage. For instance, an ultra-wide-angle lens captures a larger field of view than a normal-angle lens. It would gather more of an image in its sights but could create distortion at its edges, depending on the lens and camera design.

When collecting aerial photos, operators capture many images in succession. These images need to overlap with each other, so the image processing software can identify the changes and understand where specific objects are placed. When it can capture those common items, it can more effectively stitch the photos together or gather data about their positions.

What Are the Principles of Photogrammetry?

This process can get complex, but it all comes down to the concept of triangulation. Triangulation involves taking pictures from a minimum of two different locations. These pictures create lines of sight that lead from each camera to specific points on the object being photographed. The intersection of these lines plays into mathematical calculations that help produce 3D coordinates of the specified points.

Triangulation is used in a wide variety of fields, from agriculture to military intelligence, but it is commonly associated with land surveying. Surveyors use theodolites and triangulation to gather the location of a point with the help of angle measurements. Triangulation networks can also help with a surveying system by maximizing accuracy.

It’s actually similar to the way our eyes work and create depth. Depth perception occurs when we see an object from slightly different angles, those angles coming from each of our eyes. Our brains process the two images and make them into a single image that we can comprehend in a process called stereopsis. This whole process is similar to triangulation.

Necessary Features

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.

Types of Photogrammetry

While we can classify photogrammetry based on the location of the camera, we can also break things down by the type of photogrammetry being conducted. These types vary based on the kind of data being gathered.

Two forms of photogrammetry that you’re likely to encounter are:

  • Interpretive: Interpretive photogrammetry is all about identifying objects and gathering significant factors from an image with careful and systematic analysis. Photo interpreters gather information about their subjects, such as characteristics and features, by analyzing and evaluating the photos carefully. The job may involve remote sensing technologies. Remote sensing combines photo interpretation with data from remote sensing instruments, like cameras on satellites or aircraft and sonar systems on ships.
  • Metric: In metric photogrammetry, the goal is to find measurements. A researcher may pull specific data and measurements from a photo with the help of other information about the scene.

Metric photogrammetry also covers planimetric and topographical mapping:

  • Planimetric mapping focuses on planes and includes elements outside of elevation, like roads, rivers and lakes. It ignores these topographic features, only focusing on geographic objects.
  • Topographical mapping does the opposite, revealing the shape of the land and its elevations and contours. It shows the Earth’s surface in comparison to a specific reference point, like sea level, and can be used for underwater surfaces, too.

Uses of Photogrammetry

The ways that photogrammetry comes to life can vary widely by collection method, data gathered, industry use and compatible technologies.

Some of the products that come from the process include orthomosaics, digital surface models and digital terrain models. An orthomosaic is essentially a birds-eye view of a terrain that adjusts for distortion and can span wide landscapes. Digital surface models and digital terrain models represent surface levels and elevation. Surface models include buildings and trees, while the terrain model gets rid of all of these features, showing the height of the bare earth.

The most common use for photogrammetry is creating maps out of aerial photos. It is cost-effective and accurate, allowing planning entities like architects, local governments and construction workers to make clear, informed decisions about their projects without spending months scouring the landscape. It is also very detailed and can provide an exceptional level of information about an area.

Photogrammetry makes its mark in an array of industries, from medical research to film and entertainment. Here are some of the places you can find it:

1. Land Surveying

We’ve already discussed the applications of photogrammetry in civil surveying, the results of which are used by many entities, including construction crews, governments, building planners and architects. All of the data gathered from photogrammetry inform them about everything from necessary safety measures to potential project results.

2. Engineering

In the world of engineering, drone photography helps to evaluate sites for construction, as well as  create perspective images and 3D renderings. Engineers can produce images of project results or previews, as well as analyze their current progress.

3. Real Estate

In the digital age, where 80% to 81% of millennials find their homes on mobile devices, creating attractive, accurate listings can significantly improve the buying experience and their understanding of the purchase. Viewers can see the home from all angles and get a clear idea of what they’re looking at.

4. Military Intelligence

Photogrammetry also plays a role in data gathering for military programs. Accurate geo-locational models with low processing times are necessary for understanding a landscape. Aerial imagery and photogrammetric technology can work together to create accurate 3D maps quickly without any human input.

5. Medicine

While you might not think to put the medical field in the same category as land surveying, the 3D models that come from photogrammetric technology come in handy for a variety of health-related uses. It can also work alongside remote sensing technology to help develop diagnoses without invasive procedures.

6. Film and Entertainment

Photogrammetry can play a big role in set design and world-building for a variety of films and video games. 3D modeling can bring unique objects to fruition in a virtual world, like cityscapes for action sequences and accurate historical elements, such as statues and buildings. One popular franchise that uses photogrammetry is the “Battlefield” games, which have an art style that works well with these 3D renderings and recreations.

In addition to world-building, photogrammetry can also assist with designing special effects and real sets.

7. Forensics

Photogrammetry also plays a part in crime investigation. It can help to document and measure precise data about a crime scene and determine what was physically possible. There are also many photogrammetric experts that can assist in the courtroom.

8. Construction and Mining

Project engineers and contractors can use accurate 3D models to monitor and plan their worksites. The information from a photogrammetric model can help create a smart worksite with sensors and safety features that improve the environment. These models work in tandem with connected vehicles.

9. Sports

Analyzing athlete movements can help coaches and researchers understand more about their activities. They can develop virtual training systems and learn about the physical effort that players expend by tracking their body movements. Topographical maps also come in handy for outdoor athletes, like hikers, mountain climbers, skiers and snowboarders. Mapping remote areas is often easier with the help of photogrammetric technology.

10. Agriculture and Forestry

In agriculture, aerial photos can offer insights into soil quality, irrigation scheduling, nutrition and pests. Farmers can adjust their planting schedules or adjust irrigation and fertilizers with this information. They can also use photogrammetry when assessing growth and crop damage after storms or floods.

Researching and managing forests becomes significantly easier with the help of photogrammetry. It can produce models to analyze various aspects of a forest, like timber volume and height, to better understand the development of a forest.

Work With the Data Prep Experts at TOPS

If you work in an industry that could benefit from photogrammetry or have another need for 3D modeling and photogrammetric data, Take-Off Professionals can help you get it. Here at TOPS, we create detailed and accurate surface models to help improve your work. Aerial photogrammetry is one of the fastest methods we use, which makes it especially helpful if you find yourself in a time crunch.

Data are our specialty, and we can assist with several tasks, including drone data point surface modeling, topographic file generation and custom photogrammetry services that meet your specific needs. Reach out today to learn more about how TOPS can take your project to the next level.

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