Visualizing the implicit information of trajectory data is very important for analyzing human activities and is of great value in the decision making process. The objective of our research is to provide a semantic visualization method for trajectory data and the result of analysis. To format the trajectory data, we propose an object-oriented abstract data type (ADT). An event, which represents the human activities in our ADT, is a critical element for analysis enhanced with two special attributes: a semantic-based relationship to extract implicit information; and semantic-based levels of details (LOD) as a standard for the processing of raw data. The proposed visualization concept has been implemented in a prototype system with a space-time cube. Further information is achieved by internal queries making use of semantic attributes and is shown in an advanced user defined space-time cube, which displays richer information. The results of the experimental prototype system suggest that the proposed visualization method is feasible and flexible.