  The divergence of the correlation length $\xi$ at criticality is an important
phenomenon of percolation in two-dimensional systems. Substantial speed-ups to
the calculation of the percolation threshold and component distribution have
been achieved by utilizing disjoint sets, but existing algorithms of this sort
cannot measure the correlation length. Here, we utilize the parallel axis
theorem to track the correlation length as nodes are added to the system,
allowing us to utilize disjoint sets to measure $\xi$ for the entire
percolation process with arbitrary precision in a single sweep. This algorithm
enables direct measurement of the correlation length in lattices as well as
spatial network topologies, and provides an important tool for understanding
critical phenomena in spatial systems.
