1. A method of producing BREP data from electronic polygonal data, the method comprising: accessing the polygonal data with a computer, the polygonal data defining a mesh of polygonal data points, each having a valence corresponding with the number of polygons defined thereby, wherein the data points comprise: a plurality of valence four data points having a valence of four, and a plurality of non-valence four data points having a valence other than four; selecting one or more of the non-valence four data points, wherein the selected non-valence four data points are selected at least partly on the basis of having a valence other than four; generating a plurality of continuous curves based on the polygonal data; identifying a plurality of first data points corresponding to non-valence 4 vertices in the polygonal data; identifying a plurality of second data points corresponding to polygonal data having a third derivative greater than a threshold; based at least in part on the continuous curves, the first data points, and the second data points, generating a plurality of BREP surface boundaries defining one or more continuous BREP surfaces, wherein each of the continuous BREP surface boundaries terminates on one of the selected non-valence four data points and passes through one or more particular valence four data points; generating the BREP data based at least in part on the continuous BREP surfaces; generating one or more additional continuous surfaces, each additional surface being generated based on points corresponding to vertices in the polygonal data which are near one of the non-valence 4 vertices corresponding to an identified non-valence 4 data point, wherein at least one of the additional continuous surfaces terminates at at least one of the second data points; improving the continuity of the BREP data using the continuous additional surfaces; and storing the improved BREP data in a computer readable data storage.