This paper explores the effect of correlation on the statistics of separation of two lines when using regularly-spaced offsets to measure the distance between them. The paper describes two methods, one based on time-series modelling and the other on moving block bootstrap analysis,
to show how adjacent offsets are correlated and how this degrades the confidence with which the displacement of one line with respect to the other can be determined. In a case study the displacement between two vector representations of the same coastline is determined using offsets. In the
case study it is shown how confidence in the displacement is degraded because of correlation between adjacent offsets. It is suggested the methods have application in the field of GIS when offset models are used to determine the positional accuracy of linear vector features.