  We explore the complementarity of weak lensing and galaxy peculiar velocity
measurements to better constrain modifications to General Relativity. We find
no evidence for deviations from GR on cosmological scales from a combination of
peculiar velocity measurements (for Luminous Red Galaxies in the Sloan Digital
Sky Survey) with weak lensing measurements (from the CFHT Legacy Survey). We
provide a Fisher error forecast for a Euclid-like space-based survey including
both lensing and peculiar velocity measurements, and show that the expected
constraints on modified gravity will be at least an order of magnitude better
than with present data, i.e. we will obtain 5% errors on the modified gravity
parametrization described here. We also present a model--independent method for
constraining modified gravity parameters using tomographic peculiar velocity
information, and apply this methodology to the present dataset.
