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Elevation-Derived Hydrography Acquisition Specifications: Topology Requirements

Elevation-Derived Hydrography Acquisition Specifications

Topology Requirements

Topology

Topology Rules

Z-Values

Topology

Topology is a set of rules and behaviors that define the spatial relation between the features in the hydrographic network. To prepare for the conflation of hydrographic features into the NHD, topology rules must be followed. The NHD relies on the topology of the features to maintain a continuous network and to support functions such as network navigation.

Topology Rules
• Remove vertices that are closer than 1.5 m apart.

• Caution should be exercised to not compromise positional accuracy by removing more vertices than necessary.

• All features should have a smooth, nonrasterized appearance and maintain horizontal and vertical accuracy.

• Split all line features at polygon boundaries.

• Code flowlines within a waterbody polygon as “artificial paths.”

• Any artificial path must be completely within the waterbody polygon, starting and ending at the nodes that are coincident with inflowing features and out- flowing features.

• All line features should be one segment, with no breaks within the feature.

• The exception to this rule is for “culvert” features. Culverts may split another feature but must use the same FCode as the feature that flows into or out of the culvert. The EClass will be equal to the “culvert—used for hydroenforcement” code of 3.

• All intersections of features shall have a node (a start/beginning, or end/terminating, vertex) at that intersection.

• Features that change FCodes within a flowline segment shall be split at that point.

• Line features that intersect shall be split at that inter- section, unless there is evidence that the features do not interact (pipelines, for instance).

• Lines that meet polygons shall be split at the point where they intersect the polygon. The vertex will exist on the polygon boundary at the connection. Avoid splitting the polygon feature.

• Artificial paths within a polygon shall have an end or start node that is snapped to incoming linework. The vertex will exist on the polygon boundary at the connection. Avoid splitting the polygon feature.

• No lines shall have self-intersections or cutbacks.

• Polygon features shall not overlap, but they may share edges.

• The linear features of the dataset shall create a complete network.

• Flow shall be from upstream to downstream.

• Elevation values shall descend from upstream to downstream.

• Isolated pieces of the network may be present where a DPA does not match a hydrologic unit.

• Isolated pieces of the network may be present if a sink or other known break in the hydrologic network exists.

• A sink point shall be used to identify these locations.

• Features smaller than 1.5 m shall be removed or merged in with a longer feature.

• All features shall have a complete set of attributes associated with them.

Z-Values
• Remove Features shall be delivered in shapefile, file geodatabase, or Open Geospatial Consortium GeoPackage formats, as pointZ, polylineZ, or polygonZ feature classes.

• All features shall conform to the defined georeference information defined in the “Spatial Reference System” section of this report.

• A file with appropriate projection information shall accompany all hydrographic feature deliveries.

• Lakes/ponds and reservoir waterbody polygons shall be at an elevation at or below the immediately surrounding terrain.

• Lakes/ponds and reservoir waterbody polygons shall be flat and level with a single elevation value for every shoreline vertex.

• Stream/river polygons or downstream flowing lakes/ ponds and reservoirs shall present a flat and level water surface bank to bank.

• Stream/river polygons or downstream flowing lakes/ ponds and reservoirs shall have a downstream gradient water surface, at or below the immediately surrounding terrain.

• In cases of sharp turns of rapidly moving water, where the natural water surface is notably not level bank to bank, the water surface shall be represented as it exists while maintaining an aesthetic carto- graphic appearance.

• Stream/river lines shall have a downstream gradient, at or just below the immediately surrounding terrain.

• Each vertex in a line shall be at the same or a lower elevation value than the preceding vertex.

• Nontidal boundary waterbodies shall be flat and level for lakes and reservoirs, or maintain a downhill gradient for wide rivers and lakes, with the elevation at or just below the immediately surrounding terrain.

• Tidal waterbodies will be flat and level with the elevation at or just below the immediately surrounding terrain.

• Some vertical discontinuities are acceptable if caused by tidal variations during the collection process.

• At all intersections, regardless of feature type, the geometry of all intersection vertices shall match exactly in x, y, and z, unless there is evidence that there is no interaction between the features (pipelines, for example).

• Features that are used to traverse surface terrain features (EClass = 3) shall have the elevation values of the connecting features at the end points. The elevation values of the surface above the culvert, connector, or pipeline will not be used.

• Pipelines that are above ground and visible on the elevation surface shall use the elevation of the lidar- derived surface.