Blanking files are used to reduce the extension of a grid (obtained for instance by the spatial interpolation of xy-points) to the area of interest (e.g. catchment or administrative unit). The rainfall/runoff model uses blanking files to calculate volumes for catchments out of the grids (dimension mm) calculated for the whole nation.
The file name is *.bln. A blanking file is an ASCII-file of the format
with n the number of vertex points, 0 an indicator, that blanking should be done outside and not inside the polygon, and "abcd" an optional text attribute.Blanking files can be constructed out of ordinary GIS files (ArcView shape files for instance) by counting the numbers of vertexes and than just listing the coordinate pairs of the vertexes.