1. A method for digitally coding an image consisting of pixels regularly distributed in the image and each having N particular values of previously measured radiometric properties C.sub.i in N spectral channels, N being an integer, comprising the steps of: (a) dividing the image into windows having a uniform predetermined geometrical shape and having a same predetermined number of pixels; (b) grouping the pixels in each of said windows into sets each in one of a plurality of classes (R.sub.1, R.sub.2,. .. )based on a criterion of proximity in N-dimensional space, between ends of vectors which each represent the N particular values of a respective pixel and ends of vectors which each represent respective N predetermined typical values of a respective class; (c) re-defining each of the predetermined typical values of each of said classes from the respective particular values of the pixels of the respective set; (d) saving said redefined typical values for each window for reconstruction of said image; and (e) defining each pixel in each of said windows by a code identifying the class (R.sub.1, R.sub.2,. .. ) to which it belongs; further comprising the additional step of: (f) processing high spectral frequencies of the image in each of said windows by a unit transformation having a plurality of coefficients, one of said coefficients representing a mean value corresponding to a lower spatial frequency of the image and being associated with said redefined typical values.