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Existence of fiducials marks in a digital camera file

I have created a generic digital camera file using Erdas 2015 with the following data in it:

LPS Camera File Version: 2.0
Camera Name: "generic digital camera"
Camera Description: "generic digital camera"
Focal Length [mm]: 35
Principal Point:
    x0 [mm]: 0
    y0 [mm]: 0
Fiducial Coordinates:
    Number of Fiducials: 4
      ID       X[mm]         Y[mm]
       1           30            0
       2          -30            0
       3            0           30
       4            0          -30
Radial Distortion Observations:
    Type: "Radial Distance"
    Number of Observations: 2
      ID   Distance[mm] Distortion[microns]
       1            0                1
       2            1                2
Radial Lens Distortion Coefficients:
    k0: 0
    k1: 0
    k2: 0
Extended Parameters:
    Type: "NONE"

enter image description here enter image description here enter image description here

All included data makes sense except the existence of the fiducials marks , As I now the fiducials are existed for farm cameras not for digital ones. And when I click on the fiducials tap in the edrads it says that no fiducials are required, but when is save the file I found the following Fiducial Coordinates

Fiducial Coordinates:
Number of Fiducials: 4
ID       X[mm]         Y[mm]
1           30            0
2          -30            0
3            0           30
4            0          -30

Any Explanations for their existence please.

Edit:

“Digital sensors are fixed, so you don't need any mark on the image to define the coordinate system” Yes all references says the same thing that the Fiducials are not needed in case of digital cameras, But the issue here is Why the Erdas includes the above fiducials in case we saved a generic camera file and sometimes I receive camera files with the same fiducials (they doesn’t change even if the camera and focal length was changed. I couldn’t assume that these are the coordinates of the image corners simply because they doesn’t change even if the image size (based on the Sensor size :6.8 μ, 5.20 μ, …) was changed

This is weird and I could not figure out the purpose of include them in the digital camera file.

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  • Erdas is expensive commercial software so contact them and ask why but I suspect it is just for printing and you select something something like "print fiducials" but I certainly would not worry about it. Jan 10 '16 at 15:19
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+50

fiducial mark are used to define the coordinate system of the photograph. With film photograph, the paper moves under the objective and can get further distorted during storage and development, so you need to localise the image on each frame. On the other hand, digital sensors are fixed, so you don't need any mark on the image to define the coordinate system: you can use the corners to define the coordinate system. Once you have a coordinate system, you can locate any point on the focal plane.

In other words, you do not need to find the "fiducial marks" on the digital camera images, but you still need to define a coordinate system by providing information about the sensor size. Hence the use of "pseudo" fiducials which are at the middle of each side (the XY axis goes through the center of the image).

If you know the pixel size, you can check that the coordinates make sense if this condition is fullfilled: number of row * size of pixels = height of sensor and number of columns * size of pixels = width of sensor. If you provide the pixel size and the offset of the principal point to the software, it can read the numbers of rows/columns from the image and compute the position of the axis intersection without the need for you to provide any other input.

Note that the fiducial values do not change if you change the focal length of a camero, because it is the same sensor.

To sum up, in the case of digital camera, you define the coordinate system using the four key point coordinates on the digital image (you only need 3 points, but the fourth point allows you to detect errors). If you work with "professional" sensors, the images are usually already calibrated so that principal point is (0,0) (the center of the optics = the center of the digital image) and distortion are removed.

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