  We present a compressed data structure to store free trajectories of moving
objects (ships over the sea, for example) allowing spatio-temporal queries. Our
method, GraCT, uses a $k^2$-tree to store the absolute positions of all objects
at regular time intervals (snapshots), whereas the positions between snapshots
are represented as logs of relative movements compressed with Re-Pair. Our
experimental evaluation shows important savings in space and time with respect
to a fair baseline.
