- natgrid -- a two-dimensional random data interpolation package based on Dave Watson's package nngridr.
- dsgrid -- a three-dimensional random data interpolator based on a simple inverse distance weighting algorithm.
- fitgrid -- an interpolation package for one-dimensional and two-dimensional gridded data based on Alan Cline's package Fitpack, an early version of which is available from netlib. Fitpack uses splines under tension to interpolate in one and two dimensions.
- csagrid -- an approximation package for one-dimensional, two-dimensional, and three-dimensional random data based on David Fulker's package Splpack. Csagrid uses cubic splines to calculate its approximation function.

The fitted function methods fit an algebraic surface to the known data and then pick the interpolated values from the fitted surface. The weighted average methods calculate interpolated values as weighted averages of known values.

Among the weighted average methods, the simplest are known as the inverse distance weighted average methods. These methods are implemented by calculating weights for all input coordinates such that the weights are inversely proportional to the distance an input coordinate is from the point where an interpolated value is desired. The sum of the weights is normalized to unity and an interpolated value is the sum of the products of the functional values and the calculated weights.

Since Csagrid uses an approximation algorithm, the calculated surface will not necessarily pass through the original data points. Also, the Csagrid algorithm can become unstable in data sparse regions. The weighted average algorithm used in Dsgrid is stable in all cases and the resulting surface will pass through the original data values.

In most all cases natgrid will be the package of choice. It implements a very stable algorithm and has parameters for adjusting the smoothness of the output surface.

The two-dimensional surface entries in Fitgrid offer user-settable parameters for specifying derivatives along the boundary of the output grid that are not available in Natgrid.

Csagrid will produce a two-dimensional surface that is an approximation and may produce results that are smoother in appearance than those produced by Fitgrid or Natgrid.

Dsgrid is not recommended for two-dimensional surfaces - Natgrid is superior in all respects.

If you have NCAR Graphics installed at your site, then the ngmath library will be located in $NCARG_ROOT/lib/libngmath.a . If you want to load the library directly, check your environment variable settings to see if you have $NCARG_ROOT set and, if not, consult your local system administrator to see what you should set it to. If you are using NCAR Graphics, then, for Fortran and C codes, by adding a "-l ngmath" flag to "ncargf77" or "ncargcc" the ngmath library will be loaded.