  Model-independent measurements for the cosmic spatial curvature, which is
related to the nature of cosmic space-time geometry, plays an important role in
cosmology. On the basis of the Distance Sum Rule in the
Friedmann-Lema{\^i}tre-Robertson-Walker metric, (distance ratio) measurements
of strong gravitational lensing (SGL) systems together with distances from type
Ia supernovae observations have been proposed to directly estimate the spatial
curvature without any assumptions for the theories of gravity and contents of
the universe. However, previous studies indicated that a spatially closed
universe was strongly preferred. In this paper, we re-estimate the cosmic
curvature with the latest SGL data which includes 163 well-measured systems. In
addition, possible factors, e.g. combination of SGL data from different surveys
and stellar mass of the lens galaxy, which might affect estimations for the
spatial curvature, are considered in our analysis. We find that, except the
case where only SGL systems from the Sloan Lens ACS Survey are considered, a
spatially flat universe is consistently favored at very high confidence level
by the latest observations. It is suggested that the increasing number of
well-measured strong lensing events might significantly reduce the bias of
estimation for the cosmic curvature.
