Spherekit is an integrated toolkit for spatial interpolation and comparison of spatial interpolation algorithms. It is UNIX-based and includes a complete graphical user interface (GUI). It uses Generic Mapping Tools (GMT) for display of interpolated fields. The package features several unique capabilities:
Spherekit permits interpolation over continental or global scales because its computations are based upon spherical distances and orientations. Conventional interpolations are based upon planar projections of the earth, which produce distortions of some kind over large distances. In Spherekit, projections are applied only for display purposes, after the interpolation has been carried out in spherical geometry. The user can select from a wide range of interpolation algorithms and can experiment with any associated parameter settings.
Spherekit permits the user to incorporate knowledge or information about the processes that produced the spatial variations. This strategy, also known as "smart interpolation," is implemented through the interpolation of user-defined, derived variables. A simple equation editor is available to produce combinations of observation variables, and several nonlinear transforms are built-in. A digital elevation model (DEM) is included so that elevation can be treated consistently with other variables.
Error analysis is an integrated component of Spherekit. The performance of an interpolation method and its associated set of parameters is evaluated using cross-validation. The error at each observation point is defined as the difference between its actual value and its estimated value using the remaining n-1 points. The resulting error field can be displayed either at the data points or interpolated to a regular grid, to reduce any spatial biases. Error difference fields, comparing a pair of methods, can be easily displayed.