This subclass addresses the general case of a surface elevation defined parametrically and by its lateral extent. The equation used is described as a String. All coefficients and constants are given under coefficients, in the order of occurrence in the equation from left to right. The syntax of the equation follows that of the C programming language, with x, y, and z considered as reserved variables. The general form of the equation is defined as 0 = ... For example, 0 = a*x + b*y + c*x*y + d*z + e.
In cases where the arealExtent divides the surface into two or more surfaces occupying the same lateral extent, such as the top and bottom of a sphere, a point is used to indicate which one of the resulting surfaces are of interest. The point must reside on the surface section of interest.
If it is intended to describe a closed surface, such as a sphere, the arealExtent is left unspecified. If the equation defines a cylinder, the domain of the arealExtent is PolygonComplex with an outer and inner boundary; these boundaries are used to indicate the ends of the cylinder.