Delineation and attribution errors are described in detail accompanied by visual representations with solutions provided.
Roadside Ditches and Culverts Through Driveways
Ancillary Data Feature Capture
Lake/Pond Boundary Misplacement
EDH Features Placed Through Structures
Artificial Paths Crossing Islands
Waterbody Boundary Does Not Match Surface Topography
* See below for more information about the errors. Click the image for a larger view
Error: Incorrect outflow from a dam.
Issue: Outflow of water from a dam should cross the dam. Not the emergency / overflow spillway.
Solution: Where a dam is located, a “connector” (EDH FCode 33400) should be added across the dam, and the stream should continue downstream through the obvious stream channel.
Error: Hanging artificial paths.
Issue: Artificial paths with no connection to a stream/river outside the waterbody.
Solution: If a larger scale dataset is used to generate a smaller scale dataset, care should be taken to remove features that are no longer pertinent. For instance, if artificial path connections were included in a 1:5,000 scale dataset, but some of the connecting stream segments were pruned for the 1:24,000-scale dataset, the connecting artificial paths should be removed as well.
Error: Roadside ditches and culverts through driveways.
Issue: Automated processes may delineate roadside ditches and culverts through driveways as part of the stream network that are outside of the capture requirements.
Solution: QA linework and remove extraneous features that are automatically generated. See Elevation-derived Hydrography Acquisition Specifications for more information on canal density.
Error: Ancillary data feature capture.
Issue: When DEM is inconclusive but, in order to maintain network connectivity, ancillary data sources may be used to determine feature placement.
Solution: Features captured based on information gathered from ancillary data must have ancillary data usage noted in the attribute table for the feature class. The data source must be noted for reproducibility.
Error: Dam connector length.
Issue: Connectors through dams or berms that extend too far in one or both directions across the obstruction.
Solution: If a connector feature is required through a dam or berm, it should only extend the distance across the dam structure, and no further (but be sure to connect low points on both sides).
Error: User defined code.
Issue: Features were delineated for a special project and have no EDH FCode available in Elevation-derived Hydrography Acquisition Specifications.
Solution: All NHD FTypes may be used as codes, as well as other user defined codes. These must be placed in the “UserCode” field for the feature class.
Error: Missing Z values.
Issue: Features do not contain Z value for vertices.
Solution: Assign z-values to vertices as data is created.
Error: XYZ Value does not match DEM.
Issue: X, Y, and Z coordinates of the delineated stream do not match the lowest point of the channel in profile, thus having X, Y, and Z coordinates that do not match the coordinates of the correct location on the DEM.
Solution: Checks should be done to ensure drainage features are at the lowest point of the stream channel in profile.
Error: Lake/pond boundary misplacement.
Issue: Waterbody boundary does not match shoreline.
Solution: If boundary is derived from a previously hydroflattened feature, it should match the hydroflattened boundary.
Error: NHD lake/pond not on DEM.
Issue: If a previously collected lake/pond feature no longer has a 2-acre water surface area, as of date of lidar collection, it should not be collected. In the example, Pond B has a reduced amount of water as compared to the apparent boundary on the imagery (image1) and DEM (image 2). Although the banks have a 2-acre area, the water surface is less than 2-acres. Pond A was full to the 2-acre boundary at time of lidar collection and was delineated.
Solution: Collect waterbody features at the extent of water surface area.
Error: Poor delineation quality.
Issue: Lines should be delineated to match the bottom of the channel in profile, but must maintain a smooth, natural appears.
Solution: Delineate features so they fit the DEM surface in X,Y, and Z dimensions, and so they maintain a smooth, natural appearance.
Error: Culvert placement.
Issue: Culvert does not match elevation surface.
Solution: If there is evidence for the location of a culvert under a road (i.e. concrete structures on either side of a road directing water to the pipe under the roadway), the culvert end nodes should match these locations.
Error: Waterbody outlet point.
Issue: Water should flow out of stream at the outlet as determined on the DEM. In this example, the outlet for the top waterbody is shown flowing from the northeastern portion of the lake, but the outlet should be from the southeast, highlighted by the red box.
Solution: Check to ensure automated drainage network methods find the appropriate outflow path from waterbody features.
Error: EDH features placed through structures.
Issue: Care should be taken not to place EDH features through structures on the landscape.
Solution: If the channel is not apparent on the elevation surface, check to ensure there is no structure before continuing to delineate the stream, placing a drainageway, connector, or other feature.
Error: Lines outside channels.
Issue: Lines fall outside of the elevation surface channel.
Solution: Inspect linework and adjust the lines if they fall outside of the lowest point of the channel in profile.
Error: Missing culvert.
Issue: Culverts should be added to continue stream flow under roadways.
Solution: Culverts should be added to identify where streams flow under roadways. Don't stop stream segments at culvert locations; identify culverts so that flow path modeling will be more accurate.
Error: Number of Vertices.
Issue: Vertices should be placed no closer than 1.5 meters apart. The figure shows a 1 meter lidar digital elevation model surface with a stream overlain with vertices closer than 1.5 meters apart, which is incorrect.
Solution: Reduce the number of vertices by generalization.
Error: Artificial paths crossing islands.
Issue: Artificial paths should take the shortest path from the inlet to the outflow from a waterbody. Artificial paths should not cross island. Point A in the example shows an artificial path crossing an island.
Solution: Delineate artificial path so that it takes the shortest path from the inlet to the outflow from a waterbody. Paths may be delineated to show flow around islands, but care must be taken not to form loops, where the direction of flow doubles back on itself.
Error: Artificial path.
Issue: No artificial path inside a lake/pond.
Solution: The segment that flows through the lake/pond should be coded as an artificial path, not stream/river. The flow of water through a waterbody should always be indicated with an artificial path that begins and ends at the end nodes of the connecting flowline segments, and is snapped to the waterbody boundary.
Error: Waterbody boundary does not match surface topography.
Issue: Waterbody boundaries must match the apparent boundary on the surface, but must have a z-value that equals the hydroflattened surface of the DEM. In the example provided, the area within the red box highlights an example of where the waterbody boundary does not fit the boundary of the hydroflattened area.
Solution: Delineate the waterbody so it follows the edge of the hydroflattened surface, retaining the z-values of the hydroflattened surface. The waterbody boundary should not drift on the bank area.
Error: Misattributed Drainageway.
Issue: Drainageways are locations where no definite channel can be found over a distance for a stream/river feature, but it is clear there is overland flow that connects downstream. A drainageway is used to connect these features.
Solution: Split the stream/river feature and re-attribute the drainageway accordingly.