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1. General

A photogrammetric camera is considerably larger and more complex than a hand-held or cell phone camera. Film as the image capture medium has been largely replaced by a digital detector array, but the general camera characteristics and terminology are the same.

Figure B-1 is stylized version of an airborne mapping camera.

 
Figure B-1
Aerial Camera

The film or sensor array is located on the focal plane, where the image comes to focus. At typical flying heights, the image distance and focal length are the same.

The camera uses a fixed-focus lens system, Figure B-2, due to the flying speed and and height.

Figure B-2
Lens System


The lens system's front and read nodal points, N and N', serve as the reference points for measuring object, image, and focal distances. Each light ray enters the front nodal point and emerges from the rear nodal point parallel with its original direction.

Most traditional mapping cameras used a 6.00 inch nominal focal length lens, with some as short as 3.5 inches, others up to 9 inches.


2. Image Coordinate System

Metric photogrammetric cameras use a 9 inch square film format. The frame surrounding the negative at the focal plane contains fiducial marks located at the middle of each side; some cameras include corner fiducials, Figure B-3.

Figure B-3
Fiducial Marks

Connecting opposite fiducial marks defines the image's coordinate system. The axes should intersect at the principal point, o. The principal point is at the perpendicular intersection of the optical axis and focal plane, Figure B-4. 

Figure B-4
Principal Point, o

3. Calibration

As with any measuring instrument, a camera is subject to instrumental errors. Aberrations and distortions are one group of errors. The fiducial marks are another: do the axes intersect at right angles? Does their intersection, Figure B-5, coincide with the principal point?  Does the optical axis intersect the focal plane at right angles?

Figure B-5
Principal Point Calibration


A camera calibration includes information to compensate systematic errors. These include geometric camera attributes (focal length, fiducial coordinates, principal point coordinates, etc) as well as lens distortion behavior.  For example, the following is calibration data for a Wis DOT mapping camera.

Camera Serial No. 266611B
Focal length 152.2650 mm
Principal point x -0.0020 mm
Principal point y -0.0040 mm

Fiducial coordinates

Side x, mm y, mm   Corner x, mm y, mm
A -112.0120 0.0030   E -110.0080 110.000
B -0.050 111.9850   F 109.9950 109.9920
C 111.9940 -0.0030   G 109.9930 -110.000
D -0.0040 -111.9950   H -110.0080 -110.000

Although basic ground information can be determined using an image and general camera data, more accurate results are possible with calibration data.

Performing a camera calibration is beyond the scope of the material presented herein. Any photogrammetry text will have an extensive discussion of calibration processes as well as use of the data.

 

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