Important Terminologies of Photogrammetry

 

1. Vertical photograph: A photograph exposed with the optical axis of the camera coinciding with the direction of gravity or optical axis of the camera held vertical.

2. Tilted Photograph: A photograph taken with the optical axis unintentionally tilted from the vertical by a small amount, usually less than 30.

3. Focal length: The distance from the front nodal point of the lens to the plane of photograph, as the distance Lo in the Figure 1.

 

vertical photograph

4. Exposure Station: The space position of the front nodal point (perspective centre) at the time of exposure, as the point L in the       Figure 1.

5. Flying Height: The elevation of aircraft/front nodal point/exposure station at the instant of exposure above mean sea level or any reference datum.

6. Principal point: The point where the perpendicular dropped from the front nodal point meets/strikes the plane of photograph, as the point O in the Figure 1.

 7. Nadir Point: The point, where the plumb line dropped from the front nodal point meets the plane of photograph, as the point n, in the Figure 1. It is the point on the photograph vertically beneath the exposure station.

8. Tilt: The angle formed between the optical axis of the camera and the plumb line, as the angle oLn in the Figure 1. It is also the angle which the plane of tilted photograph makes with the plane of vertical photograph.

9. Isocentre: The point where the bisector of angle of tilt meets the plane of photograph, as the point i in the Figure 1.

10. Principal plane: The vertical plane containing the optical axis, as the plane Lno in the Figure 1.

11. Principal Line: The intersection of principal plane with the plane of photograph, as the line ‘no’ in the Figure 1. 

12. Axis of symmetry (Isometric parallel): The line in the plane of photograph perpendicular to the principal line and passing through the Isocentre. The axis of tilt is a horizontal line as the line perpendicular to the principal line. The other perpendicular lines are called plate paralles.

13. Crab: The condition caused by the failure to orient the camera with respect to the track of aircraft. In vertical photograph, crab is indicated by the edged of photograph not being parallel to the air base line.

14. Drift: The lateral shift or displacement of the aircraft from its course, due to the action of wind or other causes.

 15. Floating mark: A mark seen as occupying a position in the 3 dimensional space formed by the stereoscopic fusion of a pair of vertical photograph and used as a reference mark for examining or measuring the stereoscopic model.

floating_mark

16. Fiducial Mark: Marks located in the middle of the sides of the focal plane opening. These are exposed on the negative when the picture was made

17. Fiducial centre: The point of intersection of straight line of joining opposite Fiducial marks.

 

Figure 3: Fiducial Marks and Fiducial Centre

Figure 3: Fiducial Marks and Fiducial Centre

 

18.  Air base: The length of line joining two exposure stations.

 19. Photo base: It is the distance between the principal points of two adjoining vertical photographs.

20. Dia-positive: A positive print (photograph) on a transparent medium (glass/film).

 21. Displacement: Any shift in the position of images on a photograph which does not alter the perspective characteristics of the photograph is called displacement (shift due to tilt of the camera and relief of the objects).

22. Emulsion: A suspension of light sensitive silver salt (especially silver chloride or silver bromide in a colloidal medium usually in gelatin) which is used for containing photographic films, plates and paper.

 23. Equivalent vertical photograph: Theoretically, truly vertical photograph taken at same camera station with a camera whose focal length equal to that of camera taking a corresponding tilted photograph.

24. Exposure interval: The time interval between the exposures of successive aerial photographs.

25. Model scale: The ratio that exists between the distances measured in a stereoscopic model and the corresponding ground distances. 

26. Neat model: The portion of the overlap of a stereo pair of photograph that is actually utilized in photographic procedures. Generally, the neat model approximates a rectangular area whose width is equal to the air base and length equal to the width between the flights.

 27.  Stereogram: A set of photograph correctly oriented and mounted for stereoscopic viewing.

28.  Eye base: The distance and orientation of the line between centers of rotation of eye ball of an individual.

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