  We describe a geometric triangulation technique, based on time-elongation
maps constructed from imaging observations, to track coronal mass ejections
(CMEs) continuously in the heliosphere and predict their impact on the Earth.
Taking advantage of stereoscopic imaging observations from STEREO, this
technique can determine the propagation direction and radial distance of CMEs
from their birth in the corona all the way to 1 AU. The efficacy of the method
is demonstrated by its application to the 2008 December 12 CME, which manifests
as a magnetic cloud (MC) from in situ measurements at the Earth. The predicted
arrival time and radial velocity at the Earth are well confirmed by the in situ
observations around the MC. Our method reveals non-radial motions and velocity
changes of the CME over large distances in the heliosphere. It also associates
the flux-rope structure measured in situ with the dark cavity of the CME in
imaging observations. Implementation of the technique, which is expected to be
a routine possibility in the future, may indicate a substantial advance in CME
studies as well as space weather forecasting.
