  We compute the effects induced by the use of small CMB maps on the
measurement of the $\cl{l}$ coefficients of the angular power spectrum and show
that small systematic effects have to be taken into account. We also compute
numerically the cosmic variance and covariance of the $\cl{l}$ spectrum for
various spherical cap like maps. Comparisons with simulations are presented.
The calculations are done using the standard method based on the spherical
harmonic transform or using the temperature angular correlation spectrum.
