Elevation-Derived Hydrography Data Acquisition Specifications: Hydrologically Conditioned and Enforced DEM

The Digital Elevation Models (DEMs) used to derive hydrography for the U. S. Geological Survey’s (USGS) 3D Hydrography Program (3DHP) is typically constructed from multiple lidar data projects. There are hydrologically conditioned and enforced DEMs.

Hydrologically Conditioned DEM

Hydrologic conditioning enables continuous water flow across an elevation surface and includes the removal of isolated sinks or pits, retaining only real sinks. Hydrologic enforcement, in contrast, adjusts the elevation surface coincident with the hydrological network. While hydrologic conditioning is relevant to the entire land surface and is done so that water flow is continuous across the surface, whether the flow is in a stream channel or not.

A hydroconditioned elevation surface, for the purposes of the 3D National Topography Model, 3DHP data delivery, shall have the following characteristics:

• A Z-value corresponding to every vertex in a linear feature from the elevation–derived hydrography data shall replace a z-value of the corresponding cell in the source elevation DEM.
  • Exceptions are:
    • Any feature that is not used for elevation derivatives (EClass = 0),
      • Any feature that has uninitialized (undefined or indeterminate) flow associated with it,
      • Area of complex channels, dam/weir, ice mass, playa
  • Any feature that crosses above other hydrographic features (for example, pipelines)
• Any raster cell that is overlain by an elevation-derived linear feature, but does not have a vertex associated with it, must have a z-value that is at or below the upstream vertex value and at or above the downstream vertex value.
  • Linear interpolation between vertices is required for cells that represent the linear features to avoid stairstep appearance.
• Z-values corresponding to the linear feature from the elevation–derived hydrography data shall replace the z-values in the surface elevation DEM using the exact z-value or the z-value minus a constant value throughout the DPA.
  • The altered surface must minimally affect slopes along linear features and relative vertical relationship between linear hydrographic features.
• Downstream monotonicity shall be maintained for cells that correspond to the elevation-derived hydrographic features.
  • All Connector: Culvert, Connector, and Underground Conduit vertex z-values must be placed so that terrain that blocks flow has been replaced.
• Polygon features that were not hydroflattened in the source DEM are not required to be hydroflattened.
• All elevation values for cells that correspond to the elevation-derived hydrography linear features shall be connected and flow to an outlet point in the DPA, unless a sink is identified as the terminus of a network or as a true sink on the landscape.
  • Any isolated network flowline features shall end at a sink point feature.
    • The Sink point feature z-value shall be equal to the end point of the isolated network z-value and the z-value within the hydro-conditioned DEM.
    • Raster cells that correspond to isolated portions of the network shall maintain downstream monotonicity and other rules defined here.
• All elevation values on the source elevation surface, other than those that correspond to the elevation-derived hydrography linear features, shall flow towards an elevation-derived hydrography cell, or the outlet point in the DPA, or an identified sink.
  • Sink areas that do not serve as the end point of an isolated network, true Sink feature, or outlet point shall be filled such that flow shall not end in the sink areas.
    • Sinks shall be filled using algorithms that cause minimum alteration to the elevation surface.
    • Large, filled sink areas within the hydroconditioned surface may be verified to ensure there are not missing breach features. The following criteria may be used to review filled sinks in the terrain.
      • A filled sink area in which the contiguous cells create a surface area greater than a quarter acre (1,011 square meters) for lidar-derived data, or greater than 1 acre (4050 square meters) for IfSAR-derived data, and
      • With depth that is ≥ 0.2 meters below the non-filled elevation surface for lidar-derived data, or a depth ≥1 meter for IfSAR-derived data.
• A flow direction raster generated from the hydro-conditioned surface must reflect and maintain the elevation-derived hydrography network and allow continuous flow across the surface except where true sinks exist.
  • Expected Outcomes
  • Obstructions are removed from the elevation surface where manmade or natural anomalies impede the flow paths of water on the DEM surface.
  • Downstream monotonicity is enforced within cells that correspond to the stream network.
  • All cells on the DEM surface shall flow towards an outlet point or true sink in the elevation surface.
  • The hydro-conditioned DEM can be used to create accurate derivatives that reflect watershed boundaries, flow direction, or flow accumulation models.

Hydrologically Enforced DEM

Hydro-enforcement enables hydrologic and hydraulic models to depict water flowing under structures, rather than appearing to be dammed by these structures in the computer model because of road deck elevations higher than the water levels. Hydro-enforcement also ensures downstream monotonicity at the cells that correspond to a stream network.

A hydro-enforced elevation surface, for the purposes of the 3D National Topography Model, 3DHP data delivery, shall have the following characteristics:

• The hydro-enforced elevation surface shall match the source DEM used to derive the elevation-derived hydrography data in z-values except where z-values have been replaced by the z-values assigned to the elevation-derived hydrographic features for downstream monotonicity.
  • Z-values for every vertex in every linear feature from the elevation–derived hydrography data shall replace the z-values of the corresponding cell in the source elevation DEM.
  • Exceptions are:
    • Any feature that is not used for elevation derivatives (EClass = 0),
      • Any feature that has uninitialized (undefined or indeterminate) flow associated with it,
      • Areas of complex channels, dam/weir, ice mass, playa
  • Any feature that crosses above other hydrographic features (for example, pipelines)
• Any raster cell that is overlain by an elevation-derived linear feature, but does not have a vertex associated with it, must have a z-value that is at or below the upstream vertex value and at or above the downstream vertex value.
  • Linear interpolation between vertices is required for cells that represent the linear features to avoid stairstep appearance.
• Downstream monotonicity shall be maintained for cells that correspond to the elevation-derived hydrographic features.
  • All Connector: Culvert, Connector, and Underground Conduit vertex z-values must be placed so that terrain that blocks flow is replaced.
• Polygon features that were not hydroflattened in the source DEM are not required to be hydroflattened.
  • Artificial path features that flow through polygon features must be enforced in the elevation surface following the rules defined above.
• In cases where polygon features were hydroflattened during abnormal flow conditions and create a condition where z-values for a stream network cannot both maintain downstream monotonicity, and match the elevation of the hydroflattened surface of a waterbody polygon:
  • The z-values of the stream network flowing into and out of the polygon, and the Artificial path features through the polygon, shall maintain downstream monotonicity independent of the hydroflattened polygon boundaries.
  • Features that do not match the z-values of a hydroflattened surface shall be included in the Collection Report. Comments shall also be included on individual features within the file geodatabase of delivered features.
• All elevation values for cells that correspond to the elevation-derived hydrography linear features shall be connected and flow to an outlet point in the DPA, unless a sink is identified as the terminus of a network or as a true sink on the landscape.
  • Any isolated network flowline features shall end at a Sink point feature.
    • The Sink point feature z-value shall be equal to the end point of the isolated network z-value and the z-value within the hydro-conditioned DEM.
    • Raster cells that correspond to isolated portions of the network shall maintain downstream monotonicity and other rules defined here.
• Expected Outcomes
  • Obstructions are removed from the elevation surface where manmade or natural anomalies impede the flow paths of water on the DEM surface.
  • Elevation values reflect the z-values of the vertices of the elevation-derived hydrography linear features used for hydro-enforcement.
  • Downstream monotonicity is enforced within cells that correspond to the stream network.
  • Cells along the network shall flow towards an outlet point or true sink in the elevation surface.