  We extract the individual contributions from u and d quarks to the Dirac and
Pauli form factors of the proton, after a critical examination of the available
measurements of electromagnetic nucleon form factors. From this data we
determine generalized parton distributions for valence quarks, assuming a
particular form for their functional dependence. The result allows us to study
various aspects of nucleon structure in the valence region. In particular, we
evaluate Ji's sum rule and estimate the total angular momentum carried by
valence quarks at the scale mu = 2 GeV to be J_v^u = 0.230 +0.009 -0.024 and
J_v^d = -0.004 +0.010 -0.016.
