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Photogrammetry & Orthophoto

Photogrammetry

Photogrammetry is a set of techniques to determine the shape, size, position in space of an object from photographs. Widely used for topographic surveys, photogrammetry has many other applications: archeology, architecture, public works ...

The general principle is based on the human perception of relief by stereoscopic viewing. In the case of aerial photogrammetry, an aircraft equipped with a room shooting flying over an area, so that a portion of the land is within two pictures corresponding to two different positions of the plane.

If we observe simultaneously a picture with one eye and the second with the other eye with an optical appropriate tool (mirror stereoscope, computer with special glasses, etc..), we see the raised area of ​​land view of the two images. Human vision makes it possible to see relief in a wide range of provisions of these two images. But if we place them in a relative position exactly similar to what they had at the time of the shooting, then observed the stereoscopic image is an exact scaling of the real terrain photographed, provided that the room shooting is perfect (that is to say, brings no distortion in the image) or image has been corrected for distortion. To exploit this scene while stereoscopic rendering the device superimposed on each image point (the "balloon"), which include human vision as a small object whose position is adjustable to control the height above the image ground through the appropriate commands. The operator will have to walk the balloon in the image of all objects to be measured, while the camera archive all digital information produced.

Photogrammetry :: image 1 Photogrammetry :: image 2 Photogrammetry :: image 3 Photogrammetry :: image 4

 Orthophoto

We call orthophoto, image obtained by treating an aerial view digital or film whose geometry has been adjusted so that each point is superimposed on a flat map that corresponds to it. In other words, an orthophoto seems to be taken vertically from all points contained these points are located on a perfectly flat ground.

Three operations of corrections has to be done  when calculating the orthophotography  from a stereoscopic aerial view :

  1.  Correcting the inclination of the shooting.
  2.  Leveling the ground, that is to say the inclination of the objects rectify due to the slopes (relief).
  3.  Correct the parallax error committed in aiming, and the optical distortion of the lens and camera used.

 

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