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<br />Data Structure <br /> <br />The structure of the DLG data will be described by considering the following <br />subject areas: levels of structuring, topology, and topological elements. <br /> <br />Levels of Structuring <br /> <br />The term Digital Line Graph (DLG) is used to describe a digital map data set in <br />vector form. DLG-3'data published by FEMA have a full range of attribute codes, <br />have full topological structuring, and have passed certain quality-control <br />checks. These three properties are required by users whose work includes both <br />graphic and analytic applications. <br /> <br />TouolO2:V <br /> <br />Current data collection is directed toward producing fully topologically <br />structured DLG data, referred to as DLG-3. The DLG-3 concept is based on graph <br />theory in which a two-dimensional diagram is expressed as a directed graph <br />composed of a set of ,nodes (topologically significant points), lines, and areas <br />in a manner that explicitly expresses logical relationships. Applied to a map, <br />this concept is used to encode the digital data with the spatial relationships <br />between map elements that are obvious when the map is examined visually. The <br />spatial relationships include such concepts as adjacency and connectivity between <br />features on the map. The abstraction of the map data according to the rules of <br />graph theory preserves the spatial relationships inherent in the map graphic and <br />creates a logical and consistent data file structure for computer processing. <br /> <br />A digital file of cartographic or geographic data that maintains the spatial <br />relationships inherent in the map is called a topologically structured data file. <br />A topologically structured data file can support simple graphic applications, <br />such as plotting streams and floodplain boundaries, as well as more advanced <br />applications, such as computations and analysis involving areas and lines and <br />their spatial relationships. <br /> <br />Topolo~ical Elements <br /> <br />A DLG-3 file is composed of three separate, but related, elements: nodes, lines, <br />and areas. Nodes define the location of the endpoints of every line, and a <br />single node may mark the start or end of one or more lines, Thus, nodes occur <br />at intersections of linear features and at other places on linear features where <br />the feature is subdivided into separate line segments. <br /> <br />A line is an ordered set of points that describes the position and shape of a <br />linear feature on the map. Each line starts at a node and ends at a riode, has <br />an area to the left of its direction of travel, and has an area to the right of <br />its direction of travel. The direction of travel is arbitrarily determined at <br />the time of data capture. Lines connect to each other at nodes, and a line does <br />not cross itself or any other line, A line may describe the boundary between <br />two areal map features, such as counties, or may define a map feature by itself, <br />such as a stream, A special line, called a degenerate line, is used to define <br />features symbolized as isolated (non-connecting) point features on a map, A <br /> <br />10/93 <br /> <br />3 <br />