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Topology and Network Errors

Topology and network errors are described in detail accompanied by visual representations with solutions provided.

Incorrect Topology Errors:

Lines must only touch at their ends and not intersect or overlap

Must not have dangles

Lines must not self-overlap

Lines must not self-intersect

Line must be single part

Must be inside

Overlap and no junction

Islands overlapping waterbodies


Isolated polygon featureThis page 


Network Related Errors:

Flowline direction

Disconnected network

Node to edge junction

XYZ Error

Missing segment

* See below for more information about the errors. Click the image for a larger view


Lines Must Not Intersect or Touch Interior
Example of intersecting/overlapping lines topology error.  Line C is overlapping line B, which is an error, and should be snapped at the confluence of A and B (red box).  The correct solution to this error is seen in the green box of the inset. 

Error: Lines must only touch at their ends and not intersect or overlap 

Error Type: Incorrect Topology  

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, with the exception of where a bridge, tunnel, or pipeline is used to convey water over or under another hydrographic feature and there is no interaction of the water in the intersecting features.  Dam lines may also intersect other features, but must use a “connector” line to indicate the conveyance of water. 

Solution: Snap features at intersecting nodes so they have identical X, Y coordinates and elevation values. 







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 start of the red line.

Error: Must not have dangles

Error Type: Incorrect Topology 

Issue: A line from one layer must touch lines from the same layer, where end nodes snap to start nodes and have identical X, Y coordinates and elevation values, unless it is a headwater or termination point of the network.

Solution: Snap features at intersecting nodes so they have identical X, Y coordinates and elevation values.










Lines Must Not Self-Overlap
Lines must not self-overlap

Error: Lines must not self-overlap

Error Type: Incorrect Topology 

Issue: Lines with the same feature code can not overlap themselves. The figure illustrates a stream/river with an overlapping section, probably due to a digitizing or generalization error.  

Solution: Remove the overlapping section and correct the path.  








Incorrect topology example 5
Example of self overlapping polygon boundary.

Error: Must not self-overlap

Error Type: Incorrect Topology 

Issue: A polygon feature from one layer must not intersect or overlap itself. Any polygon where the feature overlaps itself is an error. This is often a digitizing error, which may be found by topology checks.

Solution: Re-delineate the polygon so no portion of the polygon overlaps any other portion.












Incorrect topology example 6
Example of self-intersecting line.

Error: Lines must not self-intersect

ErrorType: 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: Re-delineate the line so no portion of the line overlaps any other portion. 









Lines Must Be Single Part
Example of multipart line with pseudonodes (Black lines represent culverts and blue lines represent stream segments).  Line segment 2 has different attributes from line segments 1 and 3, so should be separate lines, although they form part of the same drainage flowpath.  Lines 3 and 4 have identical attributes and should not be split (red box).

Error: Line must be single part

Error Type: Incorrect Topology 

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

Solution: Remove pseudonodes. End nodes should only be found when attribution changes.













Must Be Inside
Example of line extending beyond the edge of a polygon, which it should be snapped to.

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. Certain lines 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, with nodes snapped to the boundary of the polygon and identical X, Y coordinates and elevation values.








Overlap and No Junction
Example of overlapping line and no junction at intersection.  Example A shows an overlap error, and example B shows the solution where the junction has been added and the overlapping segment removed.

Error: Overlap and no junction

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

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













Islands overlapping waterbodies
Example of islands correctly removed from a polygon representing a lake/pond feature.  If the islands were simply drawn on top of the lake/pond feature, this would be an overlap error.

Error: Islands overlapping waterbodies

Error Type: Incorrect Topology 

Issue: Island polygons must be removed from a hydrography polygon. The island is not a hydrography feature, but is defined by the outline of the waterbody polygon. 

Solution: Erase island polygon extents from the waterbody polygon.












Example of remnant slivers (Green areas, A and B) that should have been merged into the new polygon (brown polygon).

Error: Slivers

Error Type: Incorrect Topology

Issue: Remnant slivers from old features that haven’t been merged with new features after certain geoprocessing steps are performed that divide, cut, merge, or intersect two features together.

Solution: When new features are created using geoprocessing steps such as “clip” or “union,” slivers of the old features may be created and should be removed or merged with adjacent features with identical attributes. 














Flowline Direction
An artificial path (A) is flowing the wrong direction. The artificial path should be flipped so all of the segments flow in the same direction downstream.

Error: Flowline direction

Error: Flowline direction

Error Type: Network Issues 

Issue: Flowline segment digitized against flow
Solution: Flip segments to have all segments flow in a downstream direction.









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: Disconnected network

Error Type: Network Issues

Issue: Hydrographic drainage networks should be completely connected, unless there is a legitimately isolated portion of the network, such as a stream that ends at a sink.

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. Stream segments should be snapped so intersecting nodes have identical X, Y coordinates and elevation values.




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

Error: Node to edge junction

Error Type: Network Issues

Issue: End node to edge connection error. While a line segment may touch a polygon boundary, it may not touch another line along any point other than its start or end nodes.

Solution: Segment should be split at point of intersection.








XYZ Errors
Features should not only be snapped together, but tests should be run to ensure X, Y, and Z values are identical.  If not, this should be corrected.

Error: XYZ Error

Issue: Intersections between features should match in X, Y, and Z coodinate values.  If they do not match in any of these dimensions, it is an error.

Solution: Features should not only be snapped together, but tests should be run to ensure X, Y, and Z values are identical.  If not, this should be corrected.

















 Isolated polygon feature
Isolated polygon feature with no connection to the network.  In this example, a connector would be placed at location A.

Error: Isolated polygon feature

Issue:  Polygons should not be isolated from the drainage network unless there is a valid disconnection.

Solution:  Polygon features should be connected to the network if there is an apparent channel connecting the feature to the drainage network.  If there is no connecting channel, the location should be evaluated to determine if an indefinite surface or subsurface flow feature should be used such as: connector, culvert, drainageway, or underground conduit.