In addition to using shapefiles for mapping, shapefiles can also be used for computing basin averages, totals, and other statistics based on the intersection of various polygon shapes and TIN/measurement data.
Under each Map Window node, the Map Files Node contains a list of all shapefiles. Each of these shapefiles can be used as a Background or Basin Average Map. Note that both Background and Basin Average Maps will be drawn when the "Show Maps" layer is enabled (see Map Layers). If designated as a Basin Average Map, and the shapefile contains polygons, the map will also be displayed in the "Show Polygon/Basin Averages" layer.
Shapefiles used for basin average computations can also take advantage of the TYPE field in the DBF file if it is present. The Properties->Layer Settings-> Polygon/Basin Averages Settings portion of the Properties Window will enumerate the various polygon attributes available for the selected attribute schema and allow them to be turned off or on in the map window. Polygon shapes that do not have a recognizable TYPE field will also be accounted for. By using these extensions, it is possible to turn off a basin average polygon rendering while leaving on a subbasin polygons contained within the same shapefile. This is particularly convenient if the basin polygon is defined later in the file than the subbasin polygons, obscuring the smaller subbasins with the basin polygon.
In addition to allowing display in the "Show Polygon/Basin Averages" layer, selecting a shapefile as a Basin Average Map also allows the shapefile to be used in Hyetograph/Time Series Creation. Although they are excluded from display in the "Show Polygon/Basin Averages" layer, point shapefiles that are designated as Basin Average Maps are available for Hyetograph/Time Series Creation.