Elevation-Derived Hydrography Data Acquisition Specifications: Topology Requirements

Topology is a set of rules and behaviors defining the spatial relationships between features in a geospatial dataset. The 3DHP relies on the topology of features in the hydrographic network to maintain a continuous network and support functions such as network navigation.

Topology Rules

• Remove vertices that are less than 1.5 m apart.
  • Do not compromise correct feature placement on to the elevation surface by removing more vertices than necessary.
  • All features shall have a smooth, non-rasterized appearance while maintaining correct horizontal and vertical position relative to the elevation surface.
• Split all line features at polygon boundaries.
  • Code flowlines within a waterbody polygon as Artificial path (FCode: 55800)
  • Artificial path features must be completely within the waterbody polygon, starting and ending at the nodes that are coincident with inflowing and outflowing features.
• All line features shall be one segment, with no breaks within the feature.
• All interacting features shall have a node (a start/beginning, or end/terminating, vertex) at any intersection or confluence of more than one feature.
  • Features that change FCodes within a flowline segment shall be split at that point.
  • Intersecting line features shall be split at that intersection, unless there is evidence the features do not interact (for example, where a pipeline crosses a Stream/river feature).
  • Lines meeting polygons shall be split at the polygon intersection point. The start and end nodes shall exist on the polygon boundary at the connection. Avoid splitting the polygon feature.
  • Artificial path features within a polygon shall have an end or start node snapped to incoming linework. The start and end nodes shall 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 may share edges.
• Linear features of the dataset shall create a complete network.
  • Flow shall move from upstream to downstream.
  • Elevation values shall descend from upstream to downstream.
  • All hydrographic features within the DPA shall be collected, regardless of their outflow location. This may create isolated network features.
  • 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.
  • If a full network cannot be delineated, a comment is required in the collection report.
  • No network loops are allowed.
  • At the inlet and outlet of the FDPA
    • Lines shall be snapped to the FDPA. 
    • Polygons shall be coincident with the FDPA.
• Line features smaller than 1.5 m shall be removed or merged with a longer feature.
• All features shall have a complete set of attributes associated with them.
• Line features shall not have a valency greater than five at junctions.

Topological Relationships Between Elevation-Derived Hydrographic Features

Elevation-derived hydrographic features must be ordered to ensure correct topological relationships, and the order can be determined by the coincidence of start and end nodes, as well as polygons and lines. The following is a list of common required topological relationships but is not exhaustive of all relationships (Figure 24).  

Correct Feature Relationships:

1. The end node for a Drainageway feature must be coincident with a start node of a downstream linear feature excluding Connector: Indefinite Surface.
2. A Drainageway feature must be a headwater or connected to a headwater drainageway feature, and end where a channel is detectable on the elevation surface. Headwater Drainageway features may have an end node coincident with another Drainageway feature’s end node or start node.
3. A Connector: Culvert may split a Drainageway feature. Headwater Drainageway features may have a start or end node coincident with a culvert start node, and the Drainageway may continue downstream of the culvert, having a start node coincident with the upstream culvert end node.
4. A culvert connector is not considered a feature within a channel that would signal the end of an indefinite surface connector classification. Stream/river, Connector: Indefinite Surface, and Connector: Culvert features may have start and end nodes coincident with other Stream/river, Connector: Indefinite Surface, and Connector: Culvert features start and end nodes.
5. Waterbody polygon features must be coincident with Artificial path features if part of the stream network. Artificial path features must have a start and/or end node coincident with another linear feature. Headwater and outflow waterbodies must have Artificial path features starting in the middle of the waterbody feature. 
6. Waterbody polygon features may be isolated from the stream network and are not coincident with any other features or nodes. Isolated waterbodies shall not have an Artificial path. 
7. Waterbody features may be situated as the outlet of a network with no outflow from the waterbody. For such terminal waterbodies, the upstream Stream/river feature’s end node coincident with an Artificial path start node within the waterbody. The Artificial path end node would be situated in the middle of the water body and would be coincident with a Sink point.

Incorrect feature relationships:

8. Drainageway features may not have end nodes coincident with Connector: Indefinite Surface start nodes. A Drainageway becomes another feature type when it encounters a detectable channel on the elevation surface.
9. Stream/river features may not have an end node coincident with a Drainageway start node. A Drainageway is always a headwater feature. A Connector: Indefinite Surface should be used downstream of a channelized feature.
10. Waterbody polygon features must contain Artificial path features to connect to downstream features, unless isolated from the stream network.
11. Drainageway end nodes may not connect to the start node of a culvert that has an end node connecting to the start node of a Connector: Indefinite Surface. A culvert does not signal a change in feature type (see topological relationship 3 above).
12. If a Stream/river feature and a Drainageway have coincident end nodes, there cannot be a Drainageway downstream. Once a Drainageway encounters a channelized feature (Stream/river or Canal/ditch), any unchannelized feature downstream must be coded as a Connector: Indefinite Surface.

Media

Sources/Usage: Public Domain. View Media Details

Z-Values

• Features shall be delivered in file geodatabase, format as pointZ, polylineZ, or polygonZ feature classes.
• All features shall conform to the georeferenced information defined in the “Spatial Reference System” section of this specification.
  • A file with appropriate projection information shall accompany all hydrographic feature deliveries.
• At all intersections, regardless of feature type, the geometry of intersecting vertices shall match with 0.001 meters in x, y, and z, unless there is no evidence of interaction between the features (for example, pipelines).
• Features used to breach 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 Connector: Culvert, Connector: Terrain breach, Connector, or Pipeline shall not be used.