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t r- t r t I , . - DLG-3 Standard for DI"IRMs The digital FIRM data are collected in logical categories that comprise four unique th~mes: (1) Political Areas - political subdivisions, e.g. city, county, etc., (2) Hap Panel 'Areas - portions of communities covered by a specific map sheet, (3) FEMA Hydrography - both water control structures and surface water, and (4) Flood Hazard Zones - inundated areas for given flood events. All thematic data related to flood risks portrayed on the hardcopy FIRMs, as well as features such as FIRM Panel Boundaries and political units, are described by this topology. Base map information, such as roads networks, address information, and similar data are not provided. There exist two ways to implement the line graph concept in DLG files: The area case and the network case. These cases are differentiated by the nature of the map category information. In the area case, the DLG is used to represent areal features such as political entities or flood hazard areas in digital form. Area line graphs correspond directly to the general line case in that each closed area on the map is represented by a distinct ~rea elemen~ or polygon. In the network case, the OLG is used to represent linear features ~uch as streams or shorelines in digital form. The network case differs from the area case in that, irrespective of the number of closed areas (or polygons) forming the map, only two area elements are encoded: (I) The area outside the map, termed the outside areaj and (2) the area within the map, termed the background area. All lines except the map boundary are considered to be contained within the background area. FEHA has determined that the majority of flood hazard data are better represented as a OLG area case as opposed to a network case. Therefore, the Political Areas, Map Panel Areas, and Flood Hazard Zones DLGs are area case DLGa. Under this concept, the map is subdivided into areas or zones. Each area contains an area centroid located somewhere within its boundaries. Each area centroid carries attribute codes indicating the type of area. The DLG was further simplified by creating four separate files specific to the aforementioned four unique themes. The FEMA Hydrography DLG is a network case DLG. In it, the map contains only two areas, the areas inside and outside the map boundary. All of the linear features within the map are defined by their attribute codes. 10/93 vii