Low altitude aerial photograph for use in photogrammetry. Location: Three Arch Bay, Laguna Beach, CA.
Photogrammetry is the science of making measurements from photographs, especially for recovering the exact positions of surface points. It may also be used to recover the motion pathways of designated reference points on any moving object, on its components, and in the immediately adjacent environment. Photogrammetric analysis may be applied to one photograph, or may use
high-speed photography and
remote sensing to detect, measure and record complex 2-D and 3-D motion fields (see also
sonar,
radar,
lidar, etc.). Photogrammetry feeds measurements from remote sensing and the results of
imagery analysisinto
computational models in an attempt to successively estimate, with increasing accuracy, the actual, 3-D relative motions within the researched field.
Its applications include satellite tracking of the relative positioning alterations in all Earth environments (e.g. tectonic motions etc.), research on the swimming of fish, of bird or insect flight, and other relative motion processes (
International Society for Photogrammetry and Remote Sensing). The quantitative results of photogrammetry are then used to guide and match the results of computational models of the natural systems. They help to invalidate or confirm new theories, to design novel vehicles or new methods for predicting or/and controlling the consequences of earthquakes, tsunamis or other events, or to understand the flow of fluids next to solid structures and many other processes.
In the simplest example, the distance between two points that lie on a plane parallel to the photographic
image plane, can be determined by measuring their distance on the image, if the
scale (
s) of the image is known. This is done by multiplying the measured distance by 1/
s.
Photogrammetric methods[edit]
Photogrammetry has been defined by the
American Society for Photogrammetry and Remote Sensing (ASPRS) as the art, science, and technology of obtaining reliable information about physical objects and the environment through processes of recording, measuring and interpreting photographic images and patterns of recorded radiant electromagnetic energy and other phenomena.
[2]
Photogrammetry uses methods from many disciplines, including
optics and
projective geometry. Digital image capturing and photogrammetric processing includes several well defined stages, which allow to generate 2D or 3D digital models of the object as an end product.
[3] The data model on the right shows what type of information can go into and come out of photogrammetric methods.
The
3-D co-ordinates define the locations of object points in the
3-D space. The
image co-ordinates define the locations of the object points' images on the film or an electronic imaging device. The
exterior orientation[4] of a camera defines its location in space and its view direction. The
inner orientation defines the geometric parameters of the imaging process. This is primarily the focal length of the lens, but can also include the description of lens distortions. Further
additional observations play an important role: With
scale bars, basically a known distance of two points in space, or known
fix points, the connection to the basic measuring units is created.
Each of the four main variables can be an input or an output of a photogrammetric method.
Stereophotogrammetry[edit]
The stereophotogrammetry technology
Rapid 3D Mapping applied on the Royal Castle of Sweden.
A special case, called
stereophotogrammetry, involves estimating the three-dimensional
coordinates of points on an object employing measurements made in two or more photographic images taken from different positions (see
stereoscopy). Common points are identified on each image. A line of sight (or ray) can be constructed from the camera location to the point on the object. It is the intersection of these rays (
triangulation) that determines the three-dimensional location of the point. More sophisticated
algorithms can exploit other information about the scene that is known
a priori, for example
symmetries, in some cases allowing reconstructions of 3-D coordinates from only one camera position. Stereophotogrammetry is emerging as a robust non-contacting measurement technique to determine dynamic characteristics and mode shapes of non-rotating
[5][6] and rotating structures.
[7][8]
Integration[edit]
Photogrammetric data with a dense range data in which scanners complement each other. Photogrammetry is more accurate in the x and y direction while range data are generally more accurate in the z direction. This range data can be supplied by techniques like
LiDAR, laser scanners (using time of flight, triangulation or interferometry), white-light digitizers and any other technique that scans an area and returns x, y, z coordinates for multiple discrete points (commonly called "
point clouds"). Photos can clearly define the edges of buildings when the point cloud footprint can not. It is beneficial to incorporate the advantages of both systems and integrate them to create a better product.
A 3-D visualization can be created by georeferencing the aerial photos
[9][10] and LiDAR data in the same reference frame, orthorectifying the aerial photos, and then draping the orthorectified images on top of the LiDAR grid. It is also possible to create digital terrain models and thus 3-D visualisations using pairs (or multiples) of aerial photographs or satellite (e.g.
SPOT satellite imagery). Techniques such as adaptive least squares stereo matching are then used to produce a dense array of correspondences which are transformed through a camera model to produce a dense array of x, y, z data which can be used to produce
digital terrain model and orthoimage products. Systems which use these techniques, e.g. the
ITG system, were developed in the 1980s and 1990s but have since been supplanted by LiDAR and
radar-based approaches, although these techniques may still be useful in deriving elevation models from old aerial photographs or satellite images.
Applications[edit]
A somewhat similar application is the scanning of objects to automatically make 3D models of them. Some programs like
RealityCapture, Acute3D's
Smart3DCapture, now part of Bentley Systems and renamed
ContextCapture,
Pix4Dmapper,
Photoscan,
123D Catch,
Bundler toolkit,
[12][13] PIXDIM, and
Photosketch[14] have been made to allow people to quickly make 3D models using this photogrammetry method. It should be noted though that the produced model often still contains gaps, so additional cleanup with software like
MeshLab,
netfabb or
MeshMixer is often still necessary.
[15]
Photogrammetry is also commonly employed in collision engineering, especially with automobiles. When litigation for accidents occurs and engineers need to determine the exact deformation present in the vehicle, it is common for several years to have passed and the only evidence that remains is accident scene photographs taken by the police. Photogrammetry is used to determine how much the car in question was deformed, which relates to the amount of energy required to produce that deformation. The energy can then be used to determine important information about the crash (such as the velocity at time of impact).
Software[edit]
