Hydrographic features will be captured as either three- dimensional (3D) point, line, or polygon geometry (table 9). Some features may be collected either as 3D lines or 3D polygons, determined by minimum area or length of shortest access (Archuleta and Terziotti, 2020). EClass indicates how the features are used in elevation surface treatments. FClass indicates how the features are used in a hydrography product.

3D Elevation Program Light Detection and Ranging Base Specification Required Hydroflattening Collection

The features required to meet the hydroflattening requirement may be collected using the specifications outlined within this document (EClass=1).
The subset of features identified in table 11 can be used to fulfill the hydroflattening requirement.
Either all EClass=1 features, as defined within this document, can be used to create a hydroflattened surface, or a subset that meets the minimum size for hydroflattening requirements can be used to create a hydroflattened digital elevation model (DEM) to meet the Lidar Base Specification (LBS) requirements.

Elevation-Derived Hydrography Feature Collection

The correct geometry shall be used to capture each feature type.
At a minimum, a hydrographic feature collection shall do the following (see figure 3):
- capture all features from table 4, table 5, and table 9 that are present in the high-resolution NHD,
- capture any additional features that meet the capture conditions described in Archuleta and Terziotti (2020), and
- remove features that are not visible in the lidar data, or appropriate imagery, even if they were in the original NHD.
Additional features shall be collected for the following reasons (figure 4):
- if there is clear evidence of the feature in the lidar data source,
- if there is clear evidence of the feature using an appropriate ancillary data source (see table 12 for examples),
- if a method has given good results for delineation of stream channels or other features and it is quality assured using the lidar data and other high-quality ancillary datasets,
- if it is necessary to connect a hydrographic network.
All criteria described in the following special cases shall be met.

Special Cases

12-Digit Hydrologic Unit Consistency

New features collected within 12-digit hydrologic unit boundaries will evaluate inconsistencies (figure 5) in line density. New features shall be collected to be consistent with the most densely collected part of the 12-digit hydrologic unit (figure 6).
Where geomorphology, geology, or other terrain features create actual differences in stream density, the natural representation of features that depicts the disparity in density will be captured.
If a geographic area has an extremely dense stream network collected to meet local needs, a less detailed depiction of adjacent areas is allowed, but connectivity with major networks shall be maintained (figure 7).

Culvert Connector

The removal of surface points at bridges is done within the bare earth elevation surface to ensure that water features beneath bridges are continuous and shorelines are followed (figure 8). If a bridge was not removed, a hydrography feature below a transportation feature shall be regarded as a culvert.
A culvert must be a separate feature, nodematched (snapped) at each end to the up and downstream hydrographic features.
Elevation attribution (EClass=3) will identify the culvert separately (figure 9).
If a polygon feature contains a culvert within it, the polygon will not be split. The culvert feature shall be delineated on the artificial path of the polygon feature.

Headwaters at Roads

Identify any stream whose initiation point (headwater point) is within 100 feet (ft) or 30 m of a road (figure 10).
If a stream channel is visible (in imagery or a lidar surface) upstream from the road, extend the Stream/river through the road using the rules described for delineation of culverts, and extend the Stream/river at least 100 ft or 30 m upstream from the road intersection.
If a stream channel is not visible upstream from the road, but there are other indications that a culvert is at the intersection of the road that allows flow to continue into the Stream/river, extend the Stream/river to the road and add a culvert through the road feature. Extending the Stream/river upstream from the culvert is not necessary if a channel is not visible.
If a stream channel is not visible upstream from the road, and no other indications of a connection between the upstream area and the headwater exist, then no action is required.

Drainageway

Over mapping features to include some features that are not actual streams may be necessary and cost effective.
When a high degree of uncertainty exists for headwater features, a feature code (FCode 46800, drainageway) will be used. This code indicates that further investigation is necessary to determine if a hydrographic feature exists on the ground.
Drainageway features are flowlines delineated where terrain modelling indicates potential headwater drainage, but no channel is detectable (figure 11).
The drainageway code must only be applied at the initiation of flowlines or confluence of other drainageway features. The drainageway code must not be applied downstream of other non-drainageway flowlines or waterbody features).

Indefinite Surface Connector

Indefinite Surface Connector features shall be used where evidence of channelization is not present in the digital elevation model surface but connectivity between an upstream and downstream channel is indicated by terrain modelling (figure 12).
- An Indefinite Surface Connector feature will not have more than 20 percent of the length within a channel.
- If a section within an Indefinite Surface Connector segment is channelized for more than 100 meters CONUS (200 meters Alaska), it will be split and the section that is channelized will be coded as a Stream/river, Canal/ditch, or other channelized hydrography feature.
Situations where Indefinite Surface Connector features may be used include low confidence areas in the DEM or heavy vegetative cover in which the channel cannot be resolved.
Indefinite Surface Connector features may also be used to connect through areas having conservation treatments such as grassed waterways, which are designed to prevent soil erosion and the formation of channels.
If a feature does not have evidence of channelization, and is a headwater, the Drainageway FCode shall be used.
The Indefinite Surface Connector FCode is recommended for use in situations where streams sink into the ground under low or normal flow conditions but would flow over the surface during high flow or flood conditions and connect to downslope hydrographic features.
- Avoid use of the Indefinite Surface Connector FCode if karst terrain indicates underground flow is predominant and year-round (See READ Rules for “Underground Conduit”).

Terrain Breach Connector

Used to breach terrain (or elevation) features that block the flow in a drainage network, such as a small rise in elevation, landslides, moraines, glacial till, or berms. This connector is used to breach flow blockages on the elevation surface; with no known manmade feature such as a pipeline or culvert connecting upstream and downstream flow (figure 13).
Do not use the Terrain Breach to represent underground flowpaths in known karst, permafrost or thermokarst terrain (see Underground Conduit in READ Rules).
A Terrain Breach Connector feature is not necessary:
- If the rise in elevation requires the z-values along the linear feature to be less than 1 meter below the surface for lidar, or 2 meters below the surface for IfSAR source elevation, or
- if the rise in elevation extends in length for three pixels or less than the source resolution (3 meter for lidar, 15 meters for IfSAR) within the terrain.

Non-NHD Feature Connector

Used to provide network connectivity to or through a polygon feature that is represented in an external dataset maintained by another agency such as the National Wetlands Inventory, the Randolph Glacier Inventory, or other datasets related to hydrography.
This connector shall be used with a dataset recognized by the USGS for these purposes.
- Currently there are no Non-NHD datasets approved for use with this connector.

Canals and Ditches

If a Canal/ditch feature is named, or if a Canal/ditch feature is greater than or equal to 984 ft (300 m) along the longest axis, then it will be captured.
If a Canal/ditch feature is needed to provide network connectivity, it will be captured.
If a Canal/ditch feature is within agricultural fields, is less than 300 m, and drains to another Canal/ditch or other hydrologic feature, it will not be captured (figure 14).
Isolated Canal/ditch features or networks will not be collected unless they are of particular significance to the regional hydrology.
If a project has a special need for Canal/ditch features not covered by these capture conditions, a separate User- Code attribute should be added to allow those features to be removed from the 3DHP ingestion process.
- Coding should follow the rules for additional user- defined features: FClass, EClass, and FCode should be coded as 2, 0, and 0, respectively. The Desc field may be used to provide a text description. A unique user-defined code should be added to the UserCode field. Codes should not duplicate other defined features or coding needed for 3DHP ingestion.

Stream/river

Areas of complex interlacing channels will be captured as individual features visible within the elevation surface (figure 15 and figure 16).
Areas with dynamic interlaced stream channels will be captured as a minimum set of hydrographic features.
At least five Stream/river features will be captured to represent a complex set of channels.
Stream/river features will represent the width of the complex features by adding channels distributed throughout the braid plain area that contains interlaced features.
Complex flows will be represented if more than five channels are present, regardless of the permanence of the channels within them.
The resultant braided channel linework should maintain the characteristics of the section of river being depicted.

Elevation-Derived Hydrography Data Acquisition Specifications: Delineation of Hydrographic Features Requirements

Delineation of Hydrographic Features