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