Photogrammetry1

Photogrammetry, the method of getting useful information from photographs.



Photogrammetry is 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 which are depicted in it, it comes with an important component that goes into making that visualization close to reality. That input is our brain which either has knowledge of the scene or makes assumptions about it. This visualization is good enough for most everyday purposes, but is insufficient for many applications. As an example we can't 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's some other real life object beside it to compare it with.



Photographs also are afflicted by the lack of the third dimension. All real life objects have 3d. A photograph does not show the 3rd dimension, that is the depth. The main reason we are able to interpret the depth of a scene is mainly because of effects of light and shadows and also the relation of objects with other objects that we know about rather than just from information captured in the photograph.



Techniques of photogrammetry:



Photogrammetry uses methods from many fields including geometry and optics. It's four main aspects that are inputs and outputs. The reason these aspects are both inputs and outputs is because if we know two or more 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 of the objects in the photograph and their relative position to one another in the third dimension.



b)    The image co-ordinates about the photograph: These are the actual co-ordinates of 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 the distortions.



There is one additional component that photogrammetry uses and that is the extra information that we know about the photograph such as the actual dimensions of objects or the scale of the photograph.



Applications of photogrammetry:



Photogrammetry has numerous applications. The most common application is to find 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 from the two different points of view 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 to name a few. Archeologists use it to produce plans of large complex sites. In the absence of other objective data it is 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 by using photographs use it to reconstruct the accident from positions and damages to vehicles.



The principles involved in 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 we see. We have two eyes each of which gives a slightly different view of the scene. This subtle difference will be interpreted by our brain allowing us to perceive the 3rd dimension.