StreamOrderCalculator

Categories

Calculated Values

Related Transformers

NetworkFlowOrientor

NetworkTopologyCalculator

ShortestPathFinder 

StreamPriorityCalculator

Computes the Strahler order and/or Horton order of streams in a river network.

This recursive algorithm processes vector river networks for Strahler stream order values. The algorithm requires the vector network to be topologically correct to successfully process. The network must be a center-lined network where each arc (sometimes referred to as an edge) must be joined at their node (sometimes referred to as a junction). No left and right banks or lake side shores should be present.

Example

The image below demonstrates a map representation of a river network, an invalid network where lake and river bank sides have been digitally captured and a valid, topologically correct, center-lined river network which the algorithm can process.

If the network is "broken" (arcs not connecting) then the output will be incorrect. The algorithm would treat the disconnected catchment as a separate river system, so it is important to check the connectivity of the river network before attempting to compute Strahler order values.

Configuration

Input Ports

This transformer only takes linear features and one destination node per group.

Input Line features must be a topologically noded network with features connecting at line ends only. That is, all features must be split at junctions.

Output Ports

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.

Parameters

Group By

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.
Complete Groups

When All Features Received: This is the default behavior. Processing will only occur in this transformer once all input is present.

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.

There are two typical reasons for using 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 When All Features Received 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.
Fix Flow Direction of Input

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.

Detect Cycles

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.
Stream Order Type
Strahler

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 written to the output attribute Strahler Order .
Horton

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 written to the output attribute Horton Order.

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.

Editing Transformer Parameters

Using a set of menu options, transformer parameters can be assigned by referencing other elements in the workspace. More advanced functions, such as an advanced editor and an arithmetic editor, are also available in some transformers. To access a menu of these options, click beside the applicable parameter. For more information, see Transformer Parameter Menu Options.

Defining Values

There are several ways to define a value for use in a Transformer. The simplest is to simply type in a value or string, which can include functions of various types such as attribute references, math and string functions, and workspace parameters. There are a number of tools and shortcuts that can assist in constructing values, generally available from the drop-down context menu adjacent to the value field.

Using the Text Editor

The Text Editor provides a convenient way to construct text strings (including regular expressions) from various data sources, such as attributes, parameters, and constants, where the result is used directly inside a parameter.

Text Editor

Using the Arithmetic Editor

The Arithmetic Editor provides a convenient way to construct math expressions from various data sources, such as attributes, parameters, and feature functions, where the result is used directly inside a parameter.

Arithmetic Editor

Conditional Values

Set values depending on one or more test conditions that either pass or fail.

Parameter Condition Definition Dialog

Content

Expressions and strings can include a number of functions, characters, parameters, and more.

When setting values - whether entered directly in a parameter or constructed using one of the editors - strings and expressions containing String, Math, Date/Time or FME Feature Functions will have those functions evaluated. Therefore, the names of these functions (in the form @<function_name>) should not be used as literal string values.

String Functions

These functions manipulate and format strings.

Special Characters

 A set of control characters is available in the Text Editor.

Math Functions

Math functions are available in both editors.
Date/Time Functions Date and time functions are available in the Text Editor.

Math Operators

These operators are available in the Arithmetic Editor.

FME Feature Functions

These return primarily feature-specific values.

FME Parameters

FME and workspace-specific parameters may be used.
Creating and Modifying User Parameters Create your own editable parameters.

Dialog Options - Tables

Transformers with table-style parameters have additional tools for populating and manipulating values.

Row Reordering

Enabled once you have clicked on a row item. Choices include:

  • Add a row
  • Remove a row
  • Move current row up one
  • Move current row down one
  • Move current row to top
  • Move current row to bottom

Cut, Copy, and Paste

Enabled once you have clicked on a row item. Choices include:

  • Cut a row - delete and copy to clipboard
  • Copy a row to the clipboard
  • Paste a row from the clipboard

Cut, copy, and paste may be used within a transformer, or between transformers.

Filter

Start typing a string, and the matrix will only display rows matching those characters. Searches all columns. This only affects the display of attributes within the transformer - it does not alter which attributes are output.

Import

Import populates the table with a set of new attributes read from a dataset. Specific application varies between transformers.

Reset/Refresh

Generally resets the table to its initial state, and may provide additional options to remove invalid entries. Behavior varies between transformers.

Note: Not all tools are available in all transformers.

 

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Keywords: Strahler Horton