The goal of the positional assessment is to create a hydrography product that is vertically and horizontally integrated with the 3DEP bare-earth DEM. In general, existing NHD specifications and 3DEP LBS will be followed to ensure completeness and accuracy. Positional assessment of the elevation-derived hydrography 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 accuracy standards described in this specification. If a complete review is impossible, a stratified random sample may be used to select a subset of features to determine the accuracy of the dataset.

To do 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, so they should have features distributed within each 12-digit hydrologic unit that intersects the DPA;
Representative of the 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 is meant to indicate how accurately the vector hydrographic feature is positioned relative to the feature as represented on the DEM.
This measure is always represented as (plus or minus) meters.

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 purposes shall be integrated into the elevation-derived hydrography dataset and shall follow the Elevation-Derived Hydrography Acquisition Specifications other than the requirement for positional assessment and reporting.

Vertical Positional Assessment of Hydrographic Features Relative to the Digital Elevation Model

All lines and water surface edges shall be at or just below the elevation value of the immediately surrounding terrain, within 1 m of the location on the bare-earth DEM (figure 17).
- Exceptions to this requirement are features that are used to traverse surface terrain (culverts, connectors), are underground conduits (culverts, connectors, pipelines), or are overland pipelines (pipelines).

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 visible channels or other hydrographic features visible on the elevation surface.

Linear Features

Stream and other linear channel features shall stay within the apparent channels in the elevation data and shall not leave the channel.

Hydrographic feature positional assessment.—Whereby streams and other linear channel features will stay within the apparent channels in the elevation data and will 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.

Hydrographic feature positional assessment.— Whereby lake/ pond and other polygonal features will match the apparent boundary of the feature in the elevation data and will not vary from the boundary of the feature.

Point Features

Point features shall remain within 3 m of the apparent location of the feature in the elevation data.

Hydrographic feature positional assessment.— Whereby point features will remain within 3 m of the apparent location of the feature in the elevation data.

Alignment

Alignment specifications describe the geometry and placement of features. It is important that all features collected shall be logically and spatially consistent with the elevation data horizontally and vertically. Features shall also be spatially consistent with existing NHD features, where appropriate (if an existing feature is spatially correct and will remain unchanged).

Horizontal Alignment

New features shall align appropriately with existing NHD features outside of the collection area. For instance, if a stream is added to an existing stream channel, it should “snap” to the stream network nodes.
Features shall be aligned in accordance with topology rules.
Features shall edge match exactly across tile and project boundaries in the horizontal (x, y) spatial dimensions.
Delivered data shall be sufficient for the USGS to effectively use for ingestion into the 3DHP or for hydroflattening DEMs as needed.
Lines shall be oriented from upstream to downstream.
No lines shall have pseudonodes (other than headwater beginning nodes) or breaks within reaches.
Polygonal water features shall begin and end at the upstream end of that polygon’s centerline (artificial path).
Where any other features intersect, the intersection shall be coincident with vertices of each feature.
At all intersections, regardless of feature type, the geometry of all intersection vertices shall match exactly in x, y, and z, unless there is no evidence for interaction between the features (pipelines for instance).
Features shall align horizontally with the lidar-derived bare-earth DEM they were derived from, within the positional assessment limits described in the “Accuracy” section (figure 18).
Horizontal discontinuities along the shoreline of a waterbody resulting from tidal variations during the collection are considered normal and shall be retained in the final DEM.

Vertical Alignment

Features shall edge match exactly across tile and project boundaries in the vertical (z) spatial dimensions.
Delivered data shall be sufficient for the USGS to effectively use for ingestion into the 3DHP or for hydroflattening DEMs as needed.
Each vertex in a line shall be at the same or a lower elevation value than the preceding vertex in the direction of flow from upstream to downstream.
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 of no interaction between features (pipelines for instance).
Features shall align vertically with the lidar-derived bare-earth DEM they were derived from, within the positional assessment limits described in the “Accuracy” section.
All lines shall be at or just below the elevation of the immediately surrounding terrain, as defined by the lidar points classified as bare earth.
Flattened waterbodies shall present a flat and level water surface (a single elevation for every bank vertex defining the waterbody’s perimeter).
Long impoundments that are considered lake/pond, whose water surface elevations decrease with downstream travel, shall present a gradient downhill water surface, following the immediately surrounding terrain.
The entire water surface edge shall be at or below the immediately surrounding terrain (the presence of floating waterbodies will be cause for rejection of the deliverable).
Flattened streams and rivers shall present a flat and level water surface bank to bank (perpendicular to the apparent flow centerline).
Flattened streams and rivers shall present a gradient downhill water surface, following 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 will be represented as it exists while maintaining an aesthetic cartographic appearance.
The entire water surface edge shall be at or below the immediately surrounding terrain.
If a polygon is incomplete because it is on the boundary of the collection area, the water surface shall be flat and level, as appropriate for the type of waterbody (level for lakes, gradient for rivers, and so forth).
All landward water surface edges shall be at or below the immediately surrounding terrain.
Unusual changes in the water surface elevation that may take place over the course of the collection (for example, different river stages because of increased or decreased discharge from an upstream dam) shall be documented in the project metadata.
Vertical discontinuities within a waterbody resulting from tidal variations during the collection are considered normal and shall be retained in the final DEM.

Completeness

All features shall be collected to form a complete stream network without breaks, unless there is evidence a break should occur (for example, isolated waterbodies or subterranean streamflow). All topology rules shall be followed.
All features shall be coded with the appropriate FClass, EClass, FCode, Desc, source, and method.
UserCodes shall be used where applicable.
Domains shall match those specified within the “Elevation-Derived Hydrography—Representation, Extraction, Attribution, and Delineation Rules” (Archuleta and Terziotti, 2020).

Elevation-Derived Hydrography Data Acquisition Specifications: Positional Assessment Requirements

Positional Assessment