Maps can be loaded into HEC-MetVue, for visualization and basin average computations, as shapefiles. Shapefiles in and of themselves do not contain enough information to draw maps intelligently. For example, suppose a shapefile that has twenty Polygon definitions. Nothing within the shapefile specification informs us what these polygons represent. The polygons could represent lake surfaces, basin outlines, or a political district. It is useful to relate this polygon line to an attribute type so that it could be displayed as a double width green line or a dotted blue line. Point values, such as a rainfall gage location can be defined in a shapefile. But, again, nothing in the shapefile specification defines what this point represents or even a name to give it. Custom specifications must be defined that are in addition to the shapefile specifications in order to supply enough information to make use of the spatial features.