How to calculate spatial frequencies in fft2 of an image

I apply fft2 on an image and get amplitudes for each spatial frequency. But what are these frequencies? How can I calculate them, knowing the size of the image in pixels and the dimension of the area in km shown on the image? I need to get these frequencies as cycles per km. Thank you.

[Aymane]

Comment estimer la fréquence spatiale prépondérante (module u et angle teta ) d'une image

Answer 1

[Rick Rosson]

Similar question with accepted answer