GM_SurfaceInterpolation (Figure 20) is a list of codes that may be used to identify the interpolation mechanisms specified by an application schema. Valid values for "interpolation" include, but are not limited, to the following:<br/>a) None (none) - the interior of the surface is not specified. The assumption is that the surface follows the reference surface defined by the coordinate reference system.<br/>b) Planar (planar) - the interpolation method shall return points on a single plane. The boundary in this case shall be contained within that plane.<br/>c) Spherical (spherical), Elliptical (elliptical), Conic (conic) - the surface is a section of a spherical, elliptical or conic surface.<br/>d) TIN (tin) - the control points are organized into adjoining triangles, which form small planar segments.<br/>e) Parametric Curve (parametricCurve)  - the control points are organized into a 2-dimensional grid and each cell within the grid is spanned by a surface which shall be defined by a family of curves.<br/>f) Polynomial Spline (polynomialSpline) - the control points are organized into an irregular 2-dimensional grid and each cell within this grid is spanned by a polynomial spline function.<br/>g) Rational Spline (rationalSpline) - the control points are organized into an irregular 2-dimensional grid and each cell within this grid is spanned by a rational (quotient of polynomials) spline function.<br/>h) Triangulated Spline (triangulatedSpline) - the control points are organized into adjoining triangles, each of which is spanned by a polynomial spline function.<br/>If more than one interpolation description fits the method used, then the most restrictive one will be used. <br/>GM_SurfaceInterpolation::<br/>none <br/>planar <br/>spherical <br/>elliptical <br/>conic <br/>tin <br/>parametricCurve <br/>polynomialSpline <br/>rationalSpline <br/>triangualtedSpline<br/>