The ellipsoid used is GRS1980. The semi-major axis of the ellipsoid used is 6378137. There are many methods of interpolating precipitation from monitoring stations to grid points. Some provide estimates of acceptable accuracy in flat terrain, but few have been able to adequately explain the extreme, complex variations in precipitation that occur in mountainous regions. The primary effect of orography on a given mountain slope face is to cause precipitation to vary strongly with elevation.
Orographic effects operate at a hierarchy of spatial scales.
For example, a large mountain barrier can enhance precipitation over a broad area windward of the crest through forced uplift of moist air flow. Imbedded within this major effect are minor, smaller-scale precipitation perturbations caused by, say, vertical air motion associated with river valleys carved into the barrier. In addition, the density of most routine precipitation networks is sufficient to resolve only the larger-scale orographic effects at best. The relationship between precipitation and orographic elevation varies from one slope face to another, depending on location and orientation. Each topographic facet is a contiguous area over which the slope orientation is reasonably constant.