Correlation length of image

Hi

I am working of a project where I have to determine the correlation length of some model parameters. This can be related to determining the correlation length in an image. Such as one of the images below:

This is three realizations of spherical Gaussian covariance model with:

Mean = 0.863

Variance = 8e-6

correlation length in horizontal direction = 6 pixels

correlation length in vertical direction = 6*0.3 = 1.8 pixels

I have to approximate the value above from the images (The 3 matrices are attached, p1, p2 and p3). The mean and variance I have determined as follows:

meanValue = mean(mean([p1; p2; p3]));
variance = mean(var([p1; p2; p3]));

But I cannot find a way to determine the correlation length.

Any suggestions will be very much appreciated!!

Thanks in advance.