Abstract--This paper presents a similarity measure that combines low-level trajectory information with geographical domain knowledge to compare vessel trajectories. The similarity measure is largely based on alignment techniques. In a clustering experiment we show how the measure can be used to discover behavior concepts in vessel trajectory data that are dependent both on the low-level trajectories and the domain knowledge. We also apply this measure in a classification task to predict the type of vessel. In this task the combined measure performs better than similarities based on domain knowledge or low-level information alone.