Understanding your data!

River networks can be displayed in several formats and it is important that you supply RivEX with the correct type of network.  RivEX works only with data that is in vector format.  This page explains what type of network RivEX requires. These specifications also apply to RivEX+.

Example of a correct network                     Examples of incorrect networks

The correct network that RivEX can process

To be a valid network for RivEX to process it must have the following:

• Your dataset must be a vector dataset
• Your vector dataset must be of polyline (arcs) feature type
• Polylines must be connected to each other only at their ends (nodes)
• Polylines must be digitised in a source (From) to mouth (To) direction
• The polyline represents the centre line of the river and not the banks
• Lakes and reservoirs should be reduced to centre lines so that a connected network is formed

What you see on a map	The centre line network

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River networks that RivEX cannot process

River networks can be displayed in a variety of formats.  Depending upon scale and symbology a river may appear to be a network which RivEX could process.  The following are examples of networks which are not topologically correct vector networks.

Raster

Many rivers are displayed and analysed in raster format.  If your river network has a pixellated appearance as shown in the image below then your data is in raster format and RivEX cannot process this.

Polygon

Rivers are often captured on maps as polygons, especially wide rivers.  If your data is in polygon format then this cannot be processed by RivEX. The images below show a section of river that first appears to be a valid network but when zoomed in it is clearly a polygon and cannot be processed.

Section of river which appears to be a single line	Section of river zoomed in clearly shows that it is a polygon

Polyline (but not topologically correct)

Rivers may be captured as lines but the network is not topologically correct.  Below is an image of a network that should not be processed by RivEX.  The RED circle indicates the point at which a river has exceeded a threshold size and is now being displayed with its left and right bank.  Technically this is a network as it is still composed of polylines but the stream ordering algorithm expects a single line to represent the channel not left and right banks.  The GREEN circle indicates water bodies, most likely lakes, that are not part of the network.  Such polylines should be removed from the dataset.

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