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Elevation-Derived Hydrography Data Acquisition Specifications: Alignment

By NGP Standards and Specifications

Elevation-Derived Hydrography Data Acquisition Specifications

Alignment

Return to EDH Data Acquisition Specifications Table of Contents

Alignment

Horizontal Alignment

Vertical Alignment

Completeness

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 both horizontally and vertically. Features shall also be spatially consistent with NHD features, where appropriate (if an existing feature is spatially correct and will remain unchanged).

Horizontal Alignment
  • New features shall align appropriately with NHD features outside of the collection area, for instance, if a stream is added to an existing channel, it shall be as close as possible to the stream network nodes. See Collection Area.
  • Features shall be aligned according to topology rules.
  • Features shall be snapped within 0.001 meters across elevation tile and project boundaries in horizontal and vertical (x, y, and z) spatial dimensions (see coordinate precision). Data shall be sufficient to pass all USGS validation tests against these specifications.
  • Lines
    • Lines shall generally be oriented from upstream to downstream, although some exceptions exist (see Special Cases).
    • No lines shall have pseudo nodes (other than headwater beginning nodes) or breaks within reaches.
    • Intersections with other features shall be within 0.001 meters of each feature’s vertices.
    • At all intersections, regardless of feature type, the geometry of all coincident start and end nodes shall match within 0.001 meters in x, y, and z (see coordinate precision).
    • Features shall align horizontally with the bare-earth DEM they were derived from, within the positional assessment limits described in the “Horizontal Positional Assessment” section (Figure 26).
  • Polygon
    • If a polygon is incomplete because it is on the boundary of the collection area, it shall be coincident with the outer extent of the FDPA.
    • Where Lake/pond and Stream/river boundaries coincide with a hydroflattened surface, the 3DEP hydroflattening breaklines can be used directly as waterbody polygons.
      • The boundary shall represent the hydroflattened surface cartographically and be smoothed to remove a rasterized appearance, while maintaining a representation of the hydroflattened surface.
      • Vertices do not need to be placed on hydroflattened cells but should closely represent the hydroflattened shoreline.
    • Horizontal discontinuities along a waterbody shoreline resulting from tidal variations during elevation data collection are retained in 3DEP final DEMs and will be reflected in the placement of Stream/river vertices.
  • Points
    • The x and y coordinates of points coded as sinks placed at the end of an isolated network shall be within 0.001 meters of the x and y vertex coordinates of the end-node of the associated isolated network.
Media
EDH Data Acquisition Specifications Report Figure 18
Sources/Usage: Public Domain. View Media Details
Figure 26. An example of poor horizontal alignment. There are many instances in which the streams (in blue) fall outside the apparent channel of the elevation-derived surface.
Vertical Alignment
  • Features shall edge match within 0.001 meters across tile and project boundaries in the vertical (z) spatial dimensions.
  • Data shall be sufficient to pass all USGS validation tests against these specifications.
  • Features shall align vertically with the bare-earth DEM they were derived from, within the positional assessment limits described in the “Vertical Positional Assessment” section.
  • At all intersections, regardless of feature type, the geometry of all intersection vertices shall match within 0.001 meters in x, y, and z, unless there is evidence of no interaction between features. For example, pipelines over a stream may not interact.
  • Lines
    • Streams must flow monotonically from upstream to downstream, with no rise in elevation. Exceptions must be identified with a valid FlowClass attribute that exempts the feature from downstream monotonicity. A comment explaining why monotonicity is not possible may be included.
      • 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.
      • Change in z-values are determined by subtracting a vertex’s z-value from the adjacent, connected upstream vertex’s z-value.
    • The z-value assigned to vertices on linear features shall be precise to 0.001 meters. (See coordinate precision)
      • Vertical Tolerance: When the vertex’s z-value is subtracted from the corresponding DEM pixel value, the absolute difference shall be:
        • Within 0.001 meters above the elevation surface, and
        • Within 1.001 meters (lidar) or 2.001 meters (IfSAR) below the elevation surface.
      • Exceptions to the vertical tolerance requirement are features that are used to breach surface terrain (Connector: Culvert, Connector: Terrain breach and Connector features), Underground Conduit feature, or overland pipelines (Pipeline features). 
    • Above-ground pipelines visible on the elevation surface shall use the elevation of that surface (See Pipelines in Appendix 1).
  • Polygons
    • 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).
    • All landward water surface edges shall be at or below the immediately surrounding terrain.
    • Differences in the location of the land/sea interface caused by discontinuities from tidal variations during elevation data collection and found in 3DEP DEMs will be reflected in placement of waterbody vertices.
    • Lake/pond and Playa
      • Where Lake/pond feature boundaries coincide with a hydroflattened surface, 3DEP hydroflattening breaklines can be used directly as waterbody polygons. The z-values of the boundary vertices shall match the elevation value of the hydroflattened surface within 0.001 meters, even if not placed on the hydroflattened surface (see Supplemental Information for Polygon Vertical Alignment section).
      • If Lake/pond boundaries do not coincide with a hydroflattened surface but are within the capture conditions required for the hydrography of an area, they shall be collected. Best judgment must be used to portray the boundary relative to the Lake/pond extent on the elevation surface.
        • The z-values of the boundary vertices shall be equal to the elevation of the downstream segment’s start node.
        • If the waterbody is isolated, an elevation value representative of the waterbody surface shall be applied to all the waterbody polygon’s vertices. EClass shall equal 13.
      • Flattened waterbodies shall have a flat and level water surface (a single elevation for every bank vertex defining the waterbody’s perimeter).
      • 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).
      • If a polygon overlaps elevation-source boundaries that have a vertical offset, an elevation value that is equal to the lowest portion of the waterbody can be used.
        • Limitation field for the waterbody shall equal 1.
        • Artificial paths passing through the waterbody shall maintain downstream monotonicity but are not required to match the waterbody elevation value.
      • Long narrow Lake/pond features, with decreasing water surface elevations downstream shall:
        • Present a gradient downhill,
        • Follow the immediately surrounding terrain,
        • Have z-values that correspondingly decrease downstream.
    • Stream/river and Canal/ditch:
      • Boundary vertex z-values shall match the corresponding DEM pixel value within 0.001 meters. 
      • Downstream monotonicity is enforced on the corresponding Artificial path feature. 
      • The entire water surface edge shall be at or below the immediately surrounding terrain.
  • Points
    • The z coordinate of points coded as sinks placed at the end of an isolated network shall be within 0.001 meters of the z-value of the end-node of the associated isolated network.  
Supplement Information for Polygon Vertical Alignment

Hydroflattening Breaklines – IfSAR

Hydroflattening breaklines from the Alaska 3DEP IfSAR collection can be used directly as waterbody boundaries but must be smoothed to remove the rasterized appearance. An initial check should be performed to ensure the breaklines are spatially and temporally representative of the hydroflattened surface, particularly on borders between tiles that were merged to create the elevation surface. The polygon boundary must be an accurate representation of the hydroflattened surface. If IfSAR breaklines do not match the hydroflattened area within the elevation surface, adjustments to the breakline to better represent the lake surface are permissible. Add a comment within the ‘Comments’ field to note that the boundary was adjusted beyond smoothing any rasterized appearance. Z-values assigned to vertices shall match the hydroflattened surface elevation. EClass shall equal12.

In this example (Figure 27) the IfSAR breakline polygon (shown in red) was smoothed but remains an accurate representation of the hydroflattened surface. The hydroflattened surface (shown in blue) has an elevation value of 735.599 meters. The vertices of the smoothed polygon (shown in black) should also equal 735.599 meters.

Media
Overhead image of an elevation surface showing the difference between the jagged IfSAR breaklines and smoothed breaklines.
Sources/Usage: Public Domain. View Media Details
Figure 27. Smoothed IfSAR breaklines in black with the original breaklines shown in red.

Hydroflattening Breaklines– Lidar Collections

Hydroflattening breaklines from a lidar collection can be used directly as waterbody boundaries. No editing or horizontal adjustments are needed if the hydroflattening breakline polygon was collected with the source 3DEP DEM and was used to hydroflatten the DEM surface. EClass shall equal 11. An initial check shall be performed to ensure the breaklines are spatially and temporally representative of the hydroflattened surface, particularly on borders between lidar projects that were merged to create the elevation surface (Figure 28).

Media
An overhead image showing the difference between polygon outlines using the different requirements.
Sources/Usage: Public Domain. View Media Details
Figure 28. Difference between current and previous requirements to delineate polygons.

If the lidar hydroflattening breaklines do not match the hydroflattened area within the elevation surface, adjustments to the outline to better represent the lake surface are permissible. EClass shall equal 12. Z-values assigned to vertices shall match the elevation of the hydroflattened surface (Figure 28).

Waterbody is Not Hydroflattened – Lidar and IfSAR

All polygon outlines for waterbodies not on the hydroflattened surface must approximate the waterbody outline based on the elevation surface. Z-values assigned to vertices shall match the start node of the downstream segment exiting the waterbody. EClass shall equal 13.

In the example below (Figure 29), the vertices are assigned the z-value of the downstream segment’s start node, 187.1 meters. The Artificial path feature must be downstream monotonic and may slope downwards or be flat.

Media
An overhead image showing a polygon feature with an artificial path running through it monotonically.
Sources/Usage: Public Domain. View Media Details
Figure 29. An example of an Artificial path feature maintaining downstream monotonicity. Z-values assigned to waterbody vertices are equal to the start node of the Connector feature at the outlet of the waterbody.
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.
  • Features shall be coded with the appropriate UniqueID, FClass, EClass, FCode, Desc, source, method, FlowClass and Limitation.
  • UserCodes shall be used where applicable.
  • Domains shall match those specified in the Tables section and Appendix 1.
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