Time series data is usually stored and processed in the form of discrete trajectories of multidimensional measurement points. In order to compare the measurements of a query trajectory to a set of stored trajectories, one needs to calculate similarity between two trajectories. In this paper an efficient algorithm for calculating the similarity is presented for a set of trajectories containing one increasing measurement dimension, for example time series data. An even more efficient version of the algorithm suitable for situations where all dimensions are increasing, such as many spatio-temporal data sets, is also presented. Furthermore, the similarity measurement technique nearly fulfills the requirements of a metric space, which is a clear improvement to the currently used procedure. The performance of the algorithm is validated first by using data measured from a hot strip mill and then by using synthetically generated trajectories. The new algorithm outperforms the currently used p...