photogrammetry1's Journal

Photogrammetry

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 an object. Though photographs allow us to 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 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 is depicted in it is definitely an actual car or perhaps a small model unless there is 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 due to effects of light and shadows and the relation of objects to other objects that we know about instead of 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 reason these aspects are both inputs and outputs happens because if we know two or more of these aspects you'll be able to find the other unknown aspects. Let's 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 as well as their relative position to each other 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 point of view: This is the point of view or angle from the camera.

d) The geometric parameters from the imaging process: This is called the inner orientation. This is basically the parameters from the camera such as the focus, the depth of field, the errors and also the distortions.

There is one additional component that photogrammetry uses and that is the additional information that we know concerning the photograph such as the actual dimensions of objects or the scale from 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 difference between the co-ordinates of the objects in the two different perspectives enables algorithms to calculate the third dimension.

This method has wide ranging applications ranging from areas like preparing topographical maps, architecture, engineering, manufacturing, qc, geology, archeology, meteorology, police investigations and computer graphics for starters. Archeologists use it to produce plans of huge complex sites. Even without the 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 man. This is precisely the technique our brain uses to gauge the third dimension of the objects that people see. We have two eyes each of which gives a slightly different view of the scene. This subtle difference is then interpreted by our brain allowing us to perceive the third dimension.

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