This master thesis report presents the development of a geographical visualization system using taxi data. The system uses a large data base from a taxi company that have previously never used the data for visualization purposes. The taxi company requested a system that processes the data on a server on demand and visualizes it on a web client using heat map visualization as a primary visualization technique. The web client was supposed to be easy to use, provide deeper knowledge about the business of a taxi company and at the same time kept interactive with low latency for data requests. A big part of the thesis focuses on techniques for decimating an original data set to a smaller representational data set to be used for heat map visualization and sent to a web client from a server. The project continues by optimizing the system to keep latency to a minimum and finally developing a web client to explore the data. The result is a system with promising latency that is easy to use for exploring data and gaining a deeper knowledge about a taxi business.