Photogrammetry, the method of getting useful information from photographs.
Photogrammetry may be the technique of estimating the geometric properties of objects from photographs.
An image is a two dimensional representation of the object. Though photographs let us visualize the actual scenes and objects that are depicted in it, there is an important component that goes into making that visualization close to reality. That input is our brain which either has prior knowledge of the scene or makes assumptions about this. This visualization is good enough for most everyday purposes, but is insufficient for many applications. As an example we cannot tell just by looking at a photograph whether the car that's depicted in it is definitely an actual car or a small model unless there is some other real life object beside it to check it with.
Photographs also are afflicted by the lack of the third dimension. All real life objects have three dimensions. A photograph does not show the third dimension, that is the depth. The main reason we are able to interpret the depth of a scene is mainly due to effects of light and shadows and the relation of objects to other objects that we know about rather than just from information captured within the photograph.
Techniques of photogrammetry:
Photogrammetry uses methods from many fields including geometry and optics. It has four main aspects that are inputs and outputs. The main reason these aspects are both inputs and outputs happens because if we know several of these aspects it is possible to find the other unknown aspects. Let us take a look at these four aspects:
a) The 3D co-ordinates of objects in a photograph: This is the depth from the objects in the photograph as well as their relative position to one another in the third dimension.
b) The image co-ordinates on the photograph: These are the actual co-ordinates from the objects on the 2D photograph.
c) The exterior orientation or perspective: This is the point of view or angle from the camera.
d) The geometric parameters from the imaging process: This is also called the inner orientation. This is basically the parameters of the camera such as the focus, the depth of field, the errors and also the distortions.
There is one additional ingredient that photogrammetry uses and that is the additional information that we know concerning the photograph such as the actual size of objects or the scale of the photograph.
Applications of photogrammetry:
Photogrammetry has numerous applications. The most common application is to locate the third dimension of the scene by photographing the scene from two different camera locations. The main difference between the co-ordinates of the objects in the two different perspectives enables algorithms to calculate the 3rd dimension.
This method has far reaching applications ranging from areas like preparing topographical maps, architecture, engineering, manufacturing, quality control, geology, archeology, meteorology, police investigations and computer graphics for starters. Archeologists use it to produce plans of large complex sites. In the absence of other objective data it's used by meteorologists to gauge the wind speeds of a storm. Accident investigators who sometimes have to conduct investigations a long time after the event with the help of photographs use it to reconstruct the accident from positions and damages to vehicles.
The principles involved with Photogrammetry are not just the invention of individual. This is precisely the technique our brain uses to gauge the third dimension of the objects that people see. We have two eyes because both versions gives a slightly different view of the scene. This subtle difference will be interpreted by our brain allowing us to perceive the third dimension.