In the DoSurface(Depth on Surface), the depth information of 3D world is represented by a set of segments based on a particular surface. Each segment belongs to a location on the surface. The DoSurface is quite similar to the well-known run-length representation of binary images. As the geometric features of 2D object can be computed very easily by using the run-length representation, those of 3D object can be computed in a very similar way. This paper presents an efficient method for calculating the geometric properties of 3D objects represented by DoSurface.