StreamOrderCalculator

All river streams connected to the destination node are output through the Network port with the Strahler order and/or Horton value assigned to the attribute(s) specified in the Strahler Order Attribute and/or Horton Order Attribute.

All river streams that are not connected to the destination node are output through the Unused port.

If any cycle exists, stream order is not computed and all lines are output through the Unused port. All nodes in which the cycles occur are output through the Cycle port.

All non-linear features and extra destination nodes are output through the <Rejected> port.

The default behavior is to use the entire set of input features as the group. This option allows you to select attributes that define which groups to form. Each set of features that have the same value for all of these attributes will be processed as an independent group.

Process At End (Blocking): This is the default behavior. Processing will only occur in this transformer once all input is present.

Process When Group Changes (Advanced): This transformer will process input groups in order. Changes of the value of the Group By parameter on the input stream will trigger processing on the currently accumulating group. This may improve overall speed (particularly with multiple, equally-sized groups), but could cause undesired behavior if input groups are not truly ordered.

Considerations for Using Group By

There are two typical reasons for using Process When Group Changes (Advanced) . The first is incoming data that is intended to be processed in groups (and is already so ordered). In this case, the structure dictates Group By usage - not performance considerations.

The second possible reason is potential performance gains.

Performance gains are most likely when the data is already sorted (or read using a SQL ORDER BY statement) since less work is required of FME. If the data needs ordering, it can be sorted in the workspace (though the added processing overhead may negate any gains).

Sorting becomes more difficult according to the number of data streams. Multiple streams of data could be almost impossible to sort into the correct order, since all features matching a Group By value need to arrive before any features (of any feature type or dataset) belonging to the next group. In this case, using Group By with Process At End (Blocking) may be the equivalent and simpler approach.

Note: Multiple feature types and features from multiple datasets will not generally naturally occur in the correct order.

As with many scenarios, testing different approaches in your workspace with your data is the only definitive way to identify performance gains.

You can choose to fix the direction of the streams to fit the downstream direction to the destination node by setting this parameter to Yes.

You can choose to detect cycles by setting Detect Cycles to Yes. This is useful to make sure that no cycles exist in the network prior to fixing flow direction or computing stream order.

If any cycle exists, stream order is not computed and all lines are output through the Unused port. All nodes in which the cycles occur are output through the Cycle port.

The Strahler order of the streams is calculated as follows:

• When two or more streams with the same Strahler orders join, the outflow streams are assigned this Strahler order plus 1.
• When two or more streams with different Strahler orders join, the outflow streams are assigned the maximum Strahler order.

Strahler order is output to an attribute specified by Strahler Order Attribute.

If Stream Order Type is set to Horton, then Strahler order is calculated internally before Horton order can be computed. Horton ordering is based on the idea of a main stream. It starts with finding out the main stream that flows to the sink node; the Horton order of the arcs in this main stream will be the maximum Strahler order of these arcs. This process continues for each of the remaining tributaries until all streams have been assigned a Horton order, which is output to an attribute specified by Horton Order Attribute.

Users can specify the rules by which main streams are determined. At each junction in the network, the main stream of the incoming streams is selected based on the following rules:

• If Horton Class Attribute is specified, always match the value of this attribute of the incoming streams to the main stream determined previously. If there is only one stream that matches, it is part of the current main stream.
• Otherwise, pick the stream with the highest value based the following formula:

priorityValue = (Horton Priority Weight) * (Horton Priority Attribute value) + (Horton Angle Weight) * (angle deviation between this stream and the previous main stream) + (Horton Length Weight) * (ratio of longest length to a source node)

The angle between this stream and the previous stream is normalized to a value between 0 and 1. The value is 1 if the incoming stream is 0 degrees (straight) away from the previous main stream. If the stream is 180 degrees (exact opposite direction) away from the previous main stream, then the value is 0. If the angle between the streams is 45 or 90 or 135 degrees, the value is computed as 0.75, 0.5 and 0.25 respectively.

The ratio of the longest length to a source node for a stream is calculated as follows:

ratio = (longest length of this stream to a source node) / (sum of all longest lengths of the incoming arcs to a source node)

If none of the Horton Priority Weight, Horton Angle Weight and Horton Length Weight are specified, then by default, a main stream is determined by the longest branch. All these weights must be real values greater than or equal to zero.