  We assess the accuracy with which future galaxy surveys can measure
cosmological parameters by deriving a handy approximation that we validate
numerically. We find that galaxy surveys are quite complementary to future
Cosmic Microwave Background (CMB) experiments. By breaking parameter
degeneracies of the Planck CMB satellite in a cold dark matter cosmology, the
Sloan Digital Sky Survey might be able to reduce the Planck error bars by about
an order of magnitude on the large-scale power normalization and the
reionization optical depth, down to percent levels. However, pinpointing
attainable accuracies to within better than a factor of a few depends crucially
on whether it will be possible to extract useful information from the mildly
nonlinear regime.
