  We propose an alternative analysis of the microwave background temperature
anisotropy maps that is based on the study of the roughness of natural
surfaces. We apply it to large angle anisotropies, such as those measured by
COBE-DMR. We show that for a large signal to noise experiment, the spectral
index can be determined independently of the normalization. We then analyze the
4 yr COBE map and find for a flat $\Omega=1$ universe, that the best-fitting
value for the spectral index is $n = 1.15^{+0.39}_{-0.34}$ and for the
amplitude $Q_{rms-PS}= 14.1^{+3.9}_{-3.5}\mu K$. For $n=1$, the best-fitting
normalization is $Q_{rms-PS}|_{n=1}= 16.2^{+1.4}_{-1.3}\mu K$.
