A surface created from algebraic functions, such as polynomials, cubic splines, and so forth. A process for generating estimated functional values at arbitrary locations beginning with a set of known functional values. At the known values, the approximated values need not equal the known values. This distinguishes an approximation from an interpolation . At a given point on an interpolated surface , the aspect at that point is the direction of steepest descent.
Aspects are angles measured clockwise from north.
In the context of cubic splines, a basis is a set of cubic polynomials each of which is non-zero on a finite domain and such that any cubic spline can be calculated as a linear combination of cubics in the basis. Any cubic polynomial in the basis is referred to as a basis function. The bounding polygon of a set of points in the plane is the polygon of smallest area that contains all of the given points. A circumcircle is any one of the circles in a set of circles satisfying the empty circumcircle criterion . A parameter that controls the behavior of how Natgrid does its gridding, such as whether negative values should be allowed.