The fact, that when the shape-maintained equation being used to change band of Gauss Projection there exists the best precision area as well as precision abnormal area, is discovered.

To improve the accuracy of large floating point number calculatation, an iterative approach of inverse transformation for Gauss projection with floating coordinate system is presented.

An implicit method is employed in solving diffusion term and projection term, such as Gauss-Seidel iteration method, Fast Fourier Transform.