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.
In the Project Explorer, under every mapping node is a basin average node. Shapefiles that are placed here are not used in the map layer of the program. Instead they are used to perform basin average computations. In the Map Windows, shapefiles designated for basin average computations have their own distinct draw layer. All the above discussion dealing with how to select and manage these shapefiles applies to the basin average shapefiles as well. In fact, the same shapefile can be used for both purposes and often is.
Shapefiles used for basin 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.
The basin average layer associates the polygons with a particular display characteristic, similar to the contouring layer, using both the currently selected contour palette and scale to make the rendering depending on the computed value for the basin polygon.