  We demonstrate insights into the three-dimensional structure of defects in
graphene, in particular grain boundaries, obtained via a new approach from two
transmission electron microscopy images recorded at different angles. The
structure is obtained through an optimization process where both the atomic
positions as well as the simulated imaging parameters are iteratively changed
until the best possible match to the experimental images is found. We first
demonstrate that this method works using an embedded defect in graphene that
allows direct comparison to the computationally predicted three-dimensional
shape. We then applied the method to a set of grain boundary structures with
misorientation angles nearly spanning the whole available range
(2.6-29.8{\deg}). The measured height variations at the boundaries reveal a
strong correlation with the misorientation angle with lower angles resulting in
stronger corrugation and larger kink angles. Our results allow for the first
time a direct comparison with theoretical predictions for the corrugation at
grain boundaries and we show that the measured kink angles are significantly
smaller than the largest predicted ones.
