  We analyse the covariance of the one-dimensional mass power spectrum along
lines of sight. The covariance reveals the correlation between different modes
of fluctuations in the cosmic density field and gives the sample variance error
for measurements of the mass power spectrum. For Gaussian random fields, the
covariance matrix is diagonal. As expected, the variance of the measured
one-dimensional mass power spectrum is inversely proportional to the number of
lines of sight that are sampled from each random field. The correlation between
lines of sight in a single field may alter the covariance. However, lines of
sight that are sampled far apart are only weakly correlated, so that they can
be treated as independent samples. Using N-body simulations, we find that the
covariance matrix of the one-dimensional mass power spectrum is not diagonal
for the cosmic density field due to the non-Gaussianity and that the variance
is much higher than that of Gaussian random fields. From the covariance, one
will be able to determine the cosmic variance in the measured one-dimensional
mass power spectrum as well as to estimate how many lines of sight are needed
to achieve a target precision.
