1. A system implemented in a computer, comprising a processor and a memory, for defining and representing three-dimensional civil engineering design components, comprising civil engineering objects and a library of predefined relative parametric models, wherein types of said civil engineering objects comprise spot object, linear object and surface object, and wherein each civil engineering object comprises: a) positioning geometry data for defining three-dimensional geometry and spatial positioning of said civil engineering object in a Cartesian coordinates system, wherein said positioning geometry data of said spot object is geometric definition data of a three-dimensional point, said positioning geometry data of said linear object is geometric definition data of a three-dimensional alignment, and said positioning geometry data of said surface object is geometric definition data of two three-dimensional alignments, and b) relative parametric model name data for specifying the name of said relative parametric model that applies to said civil engineering object, wherein said relative parametric model further comprises a relative parametric model drawing and relative parametric model means, and c) relative parametric perimeters data for specifying values of the relative parametric perimeters, and d) geometric calculation means for calculating three-dimensional geometric features and spatial coordinates of said civil engineering object from said positioning geometry data and said relative parametric perimeters data, according to said relative parametric model, whereby the civil engineering objects define and represent three-dimensional geometry and spatial positioning of the civil engineering design components through their positioning geometry data and relative parametric perimeters data, supported by a library of predefined relative parametric models.