Photogrammetry is the technique that allows measurement of two points on the land or any other surface using a 2D photograph resulting in a survey map that gives a detailed idea about the spatial distribution of objects on the surface. The conversion of the mapped image can be done using the scale of the photogrammetric image. Photogrammetry also allows the mapping of motions on the surface using high-speed photography using computational models giving an increasingly accurate understanding of relative motions.
The word photogrammetry comes from the word “photo” meaning “light”, “gram” meaning “drawing” and “metry” meaning “measurement”.
Colonel Aimé Laussedat is referred to as the father of Photogrammetry who used terrestrial photographs to derive topographical maps of an area in 1849. However, the first reference of photogrammetry goes as far as 1480 when Leonardo da Vinci used the art of perspective and project geometry in many of his works. In 1865 the photogoniometer was developed by Geodesist Porro which contributed largely to the modern day photogrammetry by removing lens distortion. Meydenbauer introduced the term “photogrammetry” in the year 1893. Another landmark development was the introduction of the aerial camera, by the American Brock brothers in the year 1914, which could be mounted on a plane instead of manually holding it by the side.
In 1957 the first analytical plotter was developed by Helava. The development of the analytical plotter helped in digital processing during the 70s with the development of computers. In the 2000s the better and much complex software’s were developed and used for processing photogrammetric maps.
How does photogrammetry work?
Photogrammetry is based on the concept of “perspective geometry”. It is very similar to the way that our brains convert images using the concept of intersection calculation. Photogrammetry works by using multiple images of the scene taken from different positions and angles.
Considering that the perspective center is the camera lens, whose focal length is known, and the light rays traveling through the center reflect the image of a point from the scene then the angles of the light rays from the scene can be calibrated to yield distance between the points. If at a given scene 3 points are considered whose 3D coordinates are known with reference to a coordinate system then the position of the perspective center can be determined along with the orientation of the image with reference to the coordinate system.
With the data on the position and orientation of the image that is formed the 3D vector can be determined through the perspective center in relation to each light ray. Overlapping images with known 3D vectors will help to determine the actual 3D positions of the relative points of the scene.
Types of Photogrammetry
The most common way to classify photogrammetry is based on the position of the camera that takes the photographs to be studied. There are two basic types of photogrammetry based on the camera position.
i) Aerial Photogrammetry
ii) Terrestrial and Close-Range Photogrammetry
• Aerial Photogrammetry: In this type of photogrammetry the camera is mounted on an aircraft which can take hundreds of photos of the location from various angles. For this, a precision photogrammetric camera is used to get high-quality images, which is mounted on an aircraft, UAVs or drones. Frequently satellite images are used as well. The Aerial photogrammetry mode is used for geological investigations, land and soil surveys, civil engineering for urban development, logistics, military intelligence, and others.
There are various types of photographs that can be used in Aerial Photogrammetry. They include:
- Vertical photographs: In Vertical photographs is where the axis of the camera is vertical to the location such that the lens axis is perpendicular to the surface of the earth. The resulting photograph is of a much smaller area but resembles the maps over flat terrains.
- Oblique photographs: Here, the axis of the camera is at an inclined position as compared to the vertical line. These photographs are primarily used as a supplement map. However, the area covered is relatively small and is in the shape of trapezoid. The view of the photographs is much more similar to the actual terrain unlike the vertical photographs due to the fact that axis is inclined and hence gives a fair idea of the terrain elevation.
- Convergent photographs: In convergent photographs, the photographs are taken with a pair of wide angled cameras with single lens or with a single twin lens wide angled camera. The camera or cameras are mounted such that the axis of the two lenses converges when tilted by a certain angle from the vertical axis much similar to the human eyes.
- Trimetrogon photographs: The trimetrogon photograms are an assemblage of three photographs of the location that are taken at a time consisting of one vertical and two oblique images. The direction of the vertical lens is at a right angle to the line of flight and the obliques are taken at an angle of 60 degrees from the vertical thus producing composite images from horizon to horizon.
• Terrestrial and close-range photogrammetry: In this mode of photogrammetry the camera is either hand-held or mounted on a tripod or pole. The camera is located on the ground and images taken are non-topographic in nature and is often referred to as image-based modeling. No special type of camera is required for close range photogrammetry. The usual everyday camera or phone cameras can be used to measure engineering structures, hills, rocks, stockpiles, mines, etc.
In the terrestrial and close-range photogrammetry the subject is less than a 1000ft away from the camera. There are many reasons that make CRP the preferred and often a powerful tool for geospatial professionals. The primary reason being the advent of 3D photogrammetry also called as multi-ray photogrammetry.
The heightened interest and use of unmanned aerial systems have furthered the use of CRP as these system rely solely on the close range photographs that are stereo-paired and then turned into 3D point clouds. Due to the fact that the terrestrial and close range photogrammetry can use the normal everyday cameras the cost involved for any study is reduced significantly.
Photogrammetry development and software
Photogrammetry techniques have seen significant changes over the years. More changes came about with the advent of computational photogrammetry which not only automated the calculation but also increased the accuracy and rate of conversion of 2D images to 3D geometric representations of a surface.
In the initial stages, photogrammetry was more a plane table representation which then developed to the Analog form in the early and mid-1900s. With the development of computers and software, Analytical photogrammetry came into being. Now the digital form of photogrammetry is relevant and popular like the DEM or digital elevation models, digital maps and others.
Some of the current software that is being used for photogrammetry are;
- Context Capture & Acute 3D: This software solution allows the production of high-resolution 3D models from simple photographs often referred to as Reality Modeling.
- Photo Modeler: This software helps to produce 3D models and measurements from photographs by image-based modeling and close-range stereo photogrammetry.
- Photosynth: It analyzes digital photographs and helps to generate 3D models of photos and a point cloud of photographed object.
- Pix4dMapper: This software uses computer vision algorithms and photogrammetry to transform multispectral and thermal images into 3D models and maps.
- Reality Capture: This photogrammetric software helps to create 3D models from unordered photographs or laser scans. It is commonly used in full body scanning, gaming, surveying, creating visual effects etc.
- Socet Set: This software inputs stereo display photographs and automatically generates a DEM, digital feature vector data and orthorectified images.
- Data Mapper: DataMapper written in PHP is an Object Relational Mapper which is designed to interpret and map database tables into easy to understand objects.
- Drone Deploy: This is a Drone and UAV mapping platform that captures images, creates maps and 3D models.
- DAT/EM International Summit Evolution: This software includes CAD and GIS interfaces that provide a powerful tool for capturing and interpreting 3D information from stereo data.
- Intergraph Z/I Imaging: This is a traditional package used as a mapping platform that comes with a large format digital camera with increased image capture and resolution capacity.
- KLT Associates ATLAS: This is a 3D map data collection and editing software used for digitizing map data.
- PCI Geomatica: This is a remote sensing software package for processing earth observation data into 3D images.
- GSI V-Stars: This is a specialized photogrammetric package which is aimed to provide highly accurate measurements. It is incorporated with the camera and measurement application that takes images and interprets them with accuracy.
Due to the increasing ease and accuracy of photogrammetry, numerous applications have been evolved using this mapping technology. Photogrammetry has opened the doors to the intricate mapping required in various sciences and geological studies. This is an evolving field and will definitely see much more advancement with the integration of digital technology.
There are various software and websites that allow free access to learn and apply photogrammetry. The access to images has simplified with the use of UAV, drones, and UAS making the use of photogrammetry much simpler and accessible to most.
Also Read: What is satellite mapping & imaging?