  The electromagnetic process $e^{+}e^{-}\to p\bar{p}$ is studied with the
initial-state-radiation technique using 7.5 fb$^{-1}$ of data collected by the
BESIII experiment at seven energy points from 3.773 to 4.600 GeV. The Born
cross section and the effective form factor of the proton are measured from the
production threshold to 3.0 GeV/$c^{2}$ using the $p\bar{p}$ invariant-mass
spectrum. The ratio of electric and magnetic form factors of the proton is
determined from the analysis of the proton-helicity angular distribution.
