Bilinear interpolation
This interpolation method is particularly useful for downscaling of metorologic data that are alrady arranged in a grid, e.g. ouput of climate models). Before using these data in WaSiM, they have to be converted into WaSiMs table format: Each table column represents a node value of the input raster of the climate model.
WaSiMs bilinear interpolation allows for some relaxed conditions: The coordinates of the input stations (or climate model nodes) doesn't have to be arranged in a strict grid. It is sufficient if the nodes are arranged in agridlike structure, so distortions by incompatibilities of coordinate systems can be buffered.
The bilinear methid itself uses a weighting schema, calculating for each location a value from the distance weighted average of the four neihboring grid nodes (see figure).
More details can be found in the manual in the chapter on "Interpolation of meteorologic Input Data".

Berechnungsschema der bilinearen Interpolation mit Gewichten 