Two dimensional modelling is a far more complex operation than one dimensional modelling. Drawing devices in 2D is often a challenge and the noding process is far more difficult. Finally, 2D modelling needs computers with a larger amount of memory space. The requirements for 2D modelling is now being met by personal comuputers as memory costs have fallen and memory sizes approaching a 1 GByte or more are possible. Historically 2D and 3D programs have been on Unix but they may move to PCs.