Quality controlling your data

RivEX requires your river network to be topologically correct for it to successfully process. Various errors can occur when a network is created and/or updated. 

RivEX provides a set of quality controls which you can run on your network.  The output of a quality control is a dBase file identifying the polylines which failed the quality control.  It is then up to you, the user, to correct your data.

Please refer to online manual on how to deal with such errors.

Null length polylines

This is simply an entry in your spatial dataset (featureclass/shapefile) that exists as a row in your table and quite possibly has attributes associated with it but the poyline is of zero length.

Multi-part polylines

Multi-part polylines are polylines composed of more than one polyline.  They appear to be connected but are in fact invalid network elements.  Multi-part features are common within GIS, a good example would be a group of islands which you wish to treat as an individual country.  In such an example the polygons are aggregated into a single multi-part polygon, so you would get a single row in your attribute table.  Unfortunately multi-part polylines invalidate the topology of a network and need to be disaggregated or "exploded" into their individual polylines.  The image below demonstrates a braid which is in fact a multi-part polyline.  The red dots are the nodes of the network.  The green circles indicate junctions where you would expect to see nodes to ensure a valid network.  Here we see none so this must be a multi-part polyline.  It could be possible, but less likely, that there are two polylines digitised which share a common path upstream and downstream of the braid.

Self-intersection

A river channel does not meander in such away that it ends up flowing on top of itself!  A polyline that is digitised in such away is termed "self-intersecting".  These are errors that should be removed.  The image below shows a classic example of a self intersecting polyline.  The red dots are the nodes of the network and in this example it is clear that a single vertex ( green dot) has been misplaced causing the polyline to self-intersect.

Closed polylines

A river flows from the source to the mouth.  It may divide due to braiding or artificial side channels but a single segment of channel does not loop.  That is, a channel does not end where it started!  Such errors are hard to identify and can be caused by poor snapping when the polyline was created or edited.  The image below shows some examples of polylines that are looping.

Polyline displays only one node	Polyline displays only one node but appears to be two very close polylines	Polyline displays one node and appears to be a tributary

Very small polylines - use caution when interpreting the results

A river network may consist of thousands of polylines.  Most will form familiar drainage patterns whilst others will form grid like patterns typical of highly managed water ways.  Experience with editing such networks has identified a problem of very small polylines, usually consisting of 2 vertices.  These are introduced into the network during editing of typically grid like channels.  Such polylines may not always be an error and could be genuine.  RivEX gives you the opportunity to identify such polylines for you to make a choice of whether they are genuine or simply digitising errors.  The default threshold length is 1m but you can alter this to your requirements.

Disconnected polylines

For polylines to participate in a topologically correct network they must always join each other at their nodes.  A common digitising error is to snap the end of a polyline to another polyline, but not to the node.  These polylines are intersecting  but they do not participate in the topology of the network.

Double digitised polylines

The same polyline can sometimes be digitised more than once.  Depending upon how similar the erroneous polyline is, these can be very hard to spot.  RivEX searches the river network for polylines which share the same start and end node and intersect each other.

Intersecting polylines

To be a valid river network polylines must connect to each other only at their nodes.  A network can fulfil this requirement yet still have polylines that intersect each other along their length.  This is a fairly common type of error that can be difficult to spot as the intersection is often very small.  These errors are often over looked as the connectivity of the network is not compromised.  They do influence other types of processing.  For example, if you use RivEX to build a pseudo node free network then such intersections will cause multi-part features to be created when the GIS attempts to union the polylines for removing the pseudo nodes.  In the image below we see a polyline intersecting another yet they still join at the red node.

Are M-values Monotonic?

This tool will report for each polyline in a PolylineM FeatureClass the M-value trend.  A polyline can be described as monotonic if the M - values always increase (or decrease) with direction of the polyline.  If a polyline's M - values  have become mixed such that they decrease then increase this would lead to errors in your event layers and any other data relying on the measure values.

If you have used RivEX to build a measured network then as polylines flow in a source to sea direction the M - values will monotonically decrease as these values are distance from river mouth.