  Weak gravitational lensing provides a direct way to study the mass
distribution of clusters of galaxies at large radii. Unfortunately, large scale
structure along the line of sight also contributes to the lensing signal, and
consequently affects the measurements. We quantify the effect of distant
uncorrelated large scale structure on the inferred mass profile of clusters as
measured from weak lensing. We consider NFW profiles, and find that large scale
structure is a major source of uncertainty for most practical situations, when
a model, with the mass M_200 and the concentration parameter c as free
parameters, is fit to the observations. We find that the best constraints are
found for clusters at intermediate redshifts (z~0.3). For a cluster at z=0.3,
optimal results are obtained when the lensing signal is measured out to 10-15
arcminutes. Measurements at larger radii do not improve the accuracy with which
the profile can be determined, contrary to what is expected when the
contribution from large scale structure is ignored. The true uncertainties in
M_200 and the concentration parameter c are ~2 times larger than when distant
large scale structure is not included in the error budget.
