FFT2 - having trouble interpreting output

Hi there,

I have been using fft2 to identify wavelengths in both x and y directions in bathymetric data. After the code ran perfectly for schematised cases, picking out the correct wavelengths etc, I began using it on the 'real' data with some good results.

However - I recently noticed that the output matrix for fft2 is not only mirrored (as a 1d fft vector would be) but also contains another spectrum which I had failed to take into account before IMAGE

I had been using only one of the spectra in my analysis (top left) but it seems there is a second (bottom left). We have scratched our heads around here as to what this second spectrum means (one for frequencies one for amplitudes?) but have drawn plenty of blanks.

What exactly is fft2 calculating here, and how would one combine bother spectra?

Thanks for your help in advance! David