Issue: A line must not intersect or overlap other lines from the same layer. Any line where features overlap or any point of intersection is an error.

Solution: remove duplicate linework so only one line segment represents the feature.

Error: Must not have dangles.

Incorrect topology example 3 — Example of a dangling line. The end of the line blue line represented by an arrow should be snapped to the end of the red line representing an artificial path.

Error Type: Incorrect Topology.

Issue: A line from one layer must touch lines from the same layer at both endpoints, unless it is a headwater or termination point of the network (fig. #).

Solution: snap segments at nodes.

Error: Lines must not self-overlap.

Error Type: Incorrect Topology.

Issue: A line feature from one layer must not intersect or overlap itself. Any line where the feature overlaps itself is an error. Often occurs when merging two data sets with similar features.

Solution: create a node at the point of intersection and either recode or remove the dangling nodes.

Error: Polygons must not self-overlap.

Incorrect topology example 5 — Example of self overlapping polygon boundary.(Public domain.)

Error Type: Incorrect Topology.

Issue: A polygon eature from one layer must not intersect or overlap itself. Any line where the feature overlaps itself is an error. Often occurs when merging two data sets with similar features.

Solution: remove duplicate linework so only one line or polygon represents the feature.

Error: Lines must not self-intersect.

Incorrect topology example 6 — Example of self-intersecting line.

Error Type: Incorrect Topology.

Issue: A line feature from one layer must not intersect itself. Any line where the feature overlaps itself or any point where the feature intersects itself is an error.

Solution: remove duplicate linework so only one line or polygon represents the feature.

Error: Line must be must be single part.

Lines Must Be Single Part — Example of multipart line with pseudonodes.

Error Type: Incorrect Topology.

Issue: A line feature from one layer must not have more than one part. Any line feature with more than one part is an error.

Solution: Remove pseudonodes. End nodes should only be found at the end of reaches as defined by NHD.

Error: Line must not intersect or touch interior.

Incorrect topology example 8 — Example of line incorrectly intersecting another line.

Error Type: Incorrect Topology.

Issue: A line from one layer must touch lines from the same layer at its endpoints. Any line where features overlap or any point of intersection is an error.

Solution: Remove vertices that create the overlap.

Error: Must be inside.

Error Type: Incorrect Topology.

Issue: This rule applies to artificial path segments or other linear features that must be inside of polygon water features. A line from one layer must be contained within a polygon feature from another layer. Any line in the first layer that is not contained within a polygon from the second layer is an error.

Solution: align artificial paths so that they are within the waterbodies or area features.

Error: Overlap and no junction.

Incorrect topology example — Solution to overlapping line and no junction problem. Remove overlap and create node/junction at the intersection of the two lines.

Error Type: Incorrect Topology.

Issue: Sometimes lines that should form a junction are drawn as two separate lines that overlap for a distance and then diverge. This is an error.

Solution: remove overlap and create a node/junction at the intersection of the 2 lines.

Error: Islands overlapping waterbodies.

Error Type: Incorrect Topology.

Issue: An area must not overlap another area from the same layer. Any area where features overlap is an error.

Solution: create additional features at places of overlap.

Error: Slivers.

Error Type: Incorrect Topology.

Issue: Remnant slivers from old features merged with new ones.

Solution: When new features are created and clipped or unioned with existing features, slivers of the old features may be created and should be removed.

Error: Flowline direction.

Error Type: Network Issues.

Issue: flowline segment digitized against flow.

Solution: flip segments to have all segments flow downstream direction.

Error: Disconnected network.

Incorrect topology example 14 — Stream network is incomplete. The red lines represent connected flow and the blue lines have been inappropriately disconnected from the network.

Error Type: Network Issues.

Issue: General.

Solution: A network should be built and checked for errors. If the network contains errors, a flow trace upstream or downstream will not function correctly. All line topology rules must be followed for network connectivity to be complete.. If there are valid reasons for disconnections, this should be taken into consideration.

Error: Node to edge junction.

Incorrect topology example 15 — Example of end node connecting to a line. The red line should be split for node to node connection.

Error Type: Network Issues.

Issue: End node to edge connection error.

Solution: Segment should be split at confluence.

Error: Overshoot.

Incorrect topology example 16 — Example of overshoot. The blue line should end at the red line and snap to nodes on the red line.

Error Type: Network Issues.

Issue: Overshoot of end node, not snapped.

Solution: Remove overshoot; snap at node on other segments.

Error: Missing segment.

Incorrect topology example 17 — This example shows a broken network because the red line should connect with the blue lines.

Error Type: Network Issues.

Issue: A missing line segment breaks connectivity.

Solution: connect all segments so that gaps are removed.

Elevation-derived Hydrography Topology and Network Error Examples and Solutions

Incorrect Topology Errors:

Network Related Errors:

Error: Lines must not overlap.

Error: Lines must not intersect.

Error: Must not have dangles.

Error: Lines must not self-overlap.

Error: Polygons must not self-overlap.

Error: Lines must not self-intersect.

Error: Line must be must be single part.

Error: Line must not intersect or touch interior.

Error: Must be inside.

Error: Overlap and no junction.

Error: Islands overlapping waterbodies.

Error: Slivers.

Error: Flowline direction.

Error: Disconnected network.

Error: Node to edge junction.

Error: Overshoot.

Error: Missing segment.

U.S. Geological Survey

U.S. Department of the Interior