In the framework of 3D tomography project, we propose to build a geometrical modeling tool based on parametric representation of horizons. We decided to use the Gocad modeler because it has numerous features oriented toward the manipulation and the visualization of 3D objects. Since in the Gocad project the horizons are modelised as triangulated surfaces, we have built an interface which allows the user to map a triangulated surface into its parametric piecewise counterpart. The efficiency of the transformation of a triangulated surface into a parametric representation depends on the capabilities of the discrete smooth interpolation involved in the Gocad projet. Indeed, we show that solving the mapping problem changes the variable topology of the triangulated network into a regular one based on quadrangles.