Elevation-Derived Hydrography Data Acquisition Specifications: Positional Assessment Requirements
Elevation-Derived Hydrography Data Acquisition Specifications
Positional Assessment Requirements
Elevation-Derived Hydrography Positional Evaluation and Reporting
Positional Assessment and Reporting
Hydroflattening Polygon Exception
Vertical Positional Assessment of Hydrographic Features Relative to the Digital Elevation Model
Horizontal Positional Assessment of Elevation-Derived Hydrography
Positional Assessment
The goal of the positional assessment is to create a hydrography product that is vertically and horizontally integrated with the 3DEP bare-earth DEM. The elevation-derived hydrography’s positional assessment is always measured against the bare-earth DEM source.
Elevation-Derived Hydrography Positional Evaluation and Reporting
Ideally, all features, or as many features as possible, should be visually inspected to make sure that they meet the requirements described in this specification. If a complete review is not feasible, a stratified random sample may be used to select a subset of features to evaluate the accuracy of the dataset.
For a holistic review of the dataset, the features reviewed should be:
- Representative of all features in the dataset, so they should contain at least one feature for each EClass and FCode present in the dataset.
- Representative of the complete geographic area of the dataset,
- Representative of special cases that make up features, including, but not limited to:
- Headwaters
- Confluences between Stream/river reaches.
- Intersections with polygons and stream features
- Canal/ditch features
- Isolated networks
- Drainageways
- Intersections near roads
- Culverts
- Islands within polygon features
- Representative of land cover and geologic types or geo-physical regions, including but not limited to:
- Urban areas
- Low slope areas
Positional Assessment and Reporting
Positional assessment results shall be reported for vertical and horizontal geometry of the hydrographic features relative to the 3DEP bare-earth DEM (see hydroflattening exception below).
- Positional assessment indicates how accurately vector hydrographic features are positioned relative to the feature as represented on the DEM.
- This measure is always represented as meters (plus or minus).
Hydroflattening Polygon Exception
- An exception to the reporting requirement is for vertices that have been adjusted to maintain monotonicity for hydroflattening.
- Vector features used for hydroflattening must follow the requirements of the LBS.
- Vector features used for hydroflattening shall be integrated into the elevation-derived hydrography dataset and follow the Elevation-Derived Hydrography Acquisition Specifications except for positional assessment and reporting requirements.
Vertical Positional Assessment of Hydrographic Features Relative to the DEM
- All lines and water-surface edges shall be at or just below the elevation value of the immediately surrounding terrain, within the vertical tolerance (Figure 25).
- Exceptions to this requirement are features that are used to breach surface terrain (Connector: Culvert, Connector: Terrain breach and Connector features), Underground Conduits, and overland pipelines (Pipeline features).
Horizontal Positional Assessment of Elevation-Derived Hydrography
The horizontal positional assessment evaluates the placement of vector hydrographic features against the bare-earth DEM from which they were derived. Linear, polygonal and point features shall be within the specified tolerances for the feature type as detected on the elevation surface, as follows.
Linear Features
Stream and other linear channel features shall stay within the apparent channels in the elevation data and shall not leave the channel.
Polygonal Features
Lake/pond and other polygonal features shall match the apparent boundary of the feature in the elevation data and shall not vary from the boundary of the feature.
Point Features
Point features shall remain within 3 meters of the apparent location of the feature in the elevation data.
Digital Elevation Model (DEM) Limitation
Elevation data may contain limitations that make it unsuitable for deriving hydrographic data, making it difficult or impossible to accurately portray hydrography that aligns with the elevation surface and reflects true ground conditions. If portions of the DEM are unsuitable for the derivation of hydrography, a polygon surrounding the area shall be provided to the USGS. The polygon shall be accompanied by written justification explaining why hydrography could not be derived in that area.
Note, that the DEM Limitation polygon is intended to identify areas where the DEM is unsuitable for deriving hydrographic feature. It is not intended to be used for individual stream segments with small surface obstructions.
General characteristics of areas with DEM limitations include:
- High degree of roughness (degree of irregularity of the surface) rendering channels indiscernible.
- Channels that cannot be discerned over continuous areas.
- Channels visible in imagery or orthorectified radar intensity (ORI) images but cannot be modeled with geomorphic derivatives or similar analysis of the elevation surface.
- A continuous drainage network that cannot be modeled over the area.
If DEM Limitation polygons must be used, the NHD may be compared against the elevation-derived linework to help determine the extent of the DEM Limitation area and appropriate representation of flow through the area. In cases of significant discrepancies between the elevation-derived linework and NHD, the linework shall be reviewed against the source DEM and high-resolution imagery. The imagery shall be acquired within 10 years of the date of the elevation collection and its resolution must be within three times the resolution of the source elevation. Contact the USGS for further guidance if imagery with an appropriate resolution is not available. This review is intended to verify that the elevation data is of inadequate quality to derive linework and that the NHD or imagery better represents the flow path.
Delineating Features in Areas with DEM Limitations
Two methods are available for delineating hydrographic features in an area with DEM Limitations: by incorporating the NHD data into the elevation-derived hydrography network, or by using a current imagery source to delineate the stream network. The choice of method shall depend on the date and accuracy of the NHD or imagery (ORI), as well as the level of effort involved. Consult the USGS if further guidance is required.
Delineating the DEM Limitation Polygon
For both methods of hydrography delineation within a DEM limitation polygon:
- A DEM Limitation polygon shall be:
- Delineated around areas where the drainage network is difficult to derive given the quality of the elevation data source.
- Stored as a feature class in the spatial metadata geopackage deliverable.
- DEM Limitation polygons shall not be delineated around a single feature or small number of features. They are meant to report areas where alternate hydrography delineation methods are necessary due to significant issues with bare-earth classification or surface artifacts in the DEM.
Features within the DEM Limitation Polygon
- Follow all topology rules and relationships in this specification.
- Monotonicity shall be maintained to ensure the inlet and outlet features inside the DEM Limitation polygon have z-values appropriate to maintain monotonicity through the rest of the hydrographic network.
- Vertical tolerance will not be enforced within a DEM limitation area.
- End points of features flowing into and out of the DEM limitation polygon must match in x, y, and z values.
- Document the source and method in the metadata and collection report.
- Assign Limitation field a value of 1.
- Add appropriate description in Methods field.
Use of NHD to Capture Hydrographic Features in Areas with DEM Limitations
- Evaluate the NHD in areas where the DEM Limitation polygon is being used and if the NHD is better than what could be produced using standard drainage network derivation methods, use it in place of derived linework.
- Add “NHD” as the Source field.
- Apply z-values to NHD feature vertices using guidance from section above.
Use of Imagery to Capture Hydrographic Features in Areas with DEM Limitations
- Identify a suitable imagery source for delineating hydrographic features.
- Imagery resolution shall be within three times the cell size of source elevation.
- Imagery shall be collected within 10 years of the elevation capture date.
- Add “xxx Imagery” as the Source field, where xxx refers to the name of the imagery product used to define hydrographic features.
- Apply z-values to vertices using guidance from section above.
Density in Areas with DEM Limitations
If DEM Limitations affect the ability to reach the predicted density for the area, document the issue in the collection report.