The concept of digital city is playing an important role in urban planning. 3D city models are the basic components for such ideas. Manual 3D model reconstruction is time consuming and expensive. But Lidar provides a more efficient way to capture 3D data in millimeters, leading to better reconstruction performance. Traditionally, city models are built manually based on city blueprints and aerial images, but now city models can be generated from Lidar data automatically. Lidar technology provides us a more efficient solution to build digital cities.