9.4 Camera model of compact digital photo-cameras

The above data are mostly used while using professional aerial photographing instruments. They work with focal length and photo-negative size of several decimeters, with constant geometrical settings. However, we can ortho-rectify the images taken from aircrafts by compact hobby cameras (Figs. 57 & 58). Of course, the focal length of the camera can be altered by the zoom function and can be different from image to image. The actual focal length is stored in the meta-data of the digital image (practically in the EXIF tag of the image; Fig. 59). The focal length can be altered step by step, therefore we should define several camera model for a single camera, one model to one focal length value.

Oblique airphoto of a village taken by hobby camera

Fig. 57. Aerial photo taken by a compact digital camera (by the courtesy of Z. Barcza).

Rectified airphoto on topographic map

Fig. 58. Rectified version of the above photograph. The optical axis is far from the nadir direction, that’s why the strange shape – however the fitting is good even in the far corner.

There are no fiducial in the hobby cameras, so they should be substituted by other positions. Practically, the corner points of the images can be used as fiducial points. This solution can be quite inaccurate at traditional negatives or dia-positives but provides surprisingly good results with digital cameras. The problem with the traditional film is the not exact planar position of the film material in the camera, there are small undulation remained. Therefore the frames are not exactly in the same position, with respect to the camera mechanics. A further error source is the film development: usually not the original frame is processed, which means the lost of the internal orientation. These problems do not occur at digital cameras. The film frame is represented by the CCD sensor. Its size is a characteristic constant for the camera. Therefore, the position of the sensor corners can be defined as frame points. The exact internal orientation can be obtained by defining the image coordinates of the four corners of the images (Table 6).

Sensor type

Width (mm)

Height (mm)

1/10”

1.28

0.96

1/8”

1.6

1.2

1/6”

2.4

1.8

1/4”

3.2

2.4

1/3.6”

4

3

1/3.2”

4.54

3.42

1/3”

4.8

3.6

1/2.7”

5.37

4.04

1/2.5”

5.76

4.29

1/2.3”

6.17

4.55

1/2"

6.4

4.8

1/1.8”

7.18

5.32

1/1.7”

7.6

5.7

1/1.6”

8.08

6.01

2/3”

8.8

6.6

1”

12.8

9.6

1.5”

18.7

14

Table 6. Physical size of different CCD sensors of digital cameras, for defining the camera models.

Window of an image processing software

Fig. 59. The EXIF tag of the picture shown in Fig. 57. The make and the type of the camera makes the CCD-size (Table 6) searchable. The focal length is also needed for the camera model.