Second, while we generally seek to infer the motion of large crustal units, what we actually measure is the motion of a mark or monument on or just below the ground surface. Spurious motion of the mark (monument noise) -unrelated to motion of the larger crustal units of interest has been identified as an important noise source for many geodetic measurements [Johnson and Agnew, 1995; Langbein et al., 1995]. Analysis of long (decade or more) time series of high-precision two-color electronic dis- tance measurement (EDM) data from sites in California suggests that monument noise can be modeled as a random walk [Langbein and Johnson, 1997].