Extract nearby network

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RivEX can take a set of snapped points and extract the upstream and downstream nearby network into a new dataset. It can optionally generate a buffer dataset of the extracted network. An example of a site extracting 500m of upstream nearby network and its buffer is shown below. The extraction algorithm can deal with multi-threaded networks.



Extracted nearby network example

A 500m upstream nearby network and its buffer. Arrows indicate the 500m limit.


Such a dataset could be used in habitat analysis, riparian analysis or studies in migration. With the data extracted out as separate spatial datasets you would be able to use these in further spatial analysis or simply display them.


WarningThe network and site data should be in a coordinate system that uses meters. This tool will yield incorrect results if applied to data in a geographic coordinate system (i.e. data in latitude/longitude).


A site dataset must be snapped to the network and have the following attributes for each site:

o  Unique numeric ID

o  A field indicating the upstream\downstream limit to be extracted; units must be in meters.

o  An optional field that gives the buffer width in meters for the upstream extracted network.

o  An optional field that gives the buffer width in meters for the downstream extracted network.


Every site in your dataset could potentially have a different upstream and downstream distance and varying buffer size; you just need to set your data up as such. If you wanted to extract a standard 1Km upstream network for every site then you need only create the field and set all rows to be 1000m.


You can use the same field for the network distance and buffer width for both upstream and downstream direction. An example is shown below.


Same both direction

A standard 500m distance and 50m buffer width applied to all sites


As RivEX traverses the network to the limit specified in the site data it will include any tributaries with sources less than the specified distance. This is demonstrated in the image below, these are the sources marked by the arrows.


Sources less than the required distance

A 500m distance and 50m buffer width applied to a site identifies tributaries that fall short of the 500m limit (marked with arrows)


If the section of network contains a multi-threaded section then the final limit reached is dictated by the digitizing order of the polylines. This is shown in the image below where the green and red bars indicate the limit of the extracted network. If the traverse up the network happened to follow the green (right hand side) route then as this is straighter it will achieve a further distance upstream. If it had followed the red (left hand side) route the final distance would appear shorter due the sinuosity of the left side, yet it has still travelled the required distance.


Influence of loop on nearby network

Which side of the loop RivEX traverses first will influence the final position of the network limit


An example of the completed dialog is shown below. If you choose to create a buffer then you must supply the buffer field. You can also limit the processing to just the selected sites.



The output is stored in the File GeoDatabase fGDB_Reaches.gdb which is found in the ..\RivEX_Workspace\Output folder. The output names take on the site name with a suffix of "_NearestNet" and "_NearestNetBuffer". For example if your site data was called Sites95 then the output would be Site95_NearestNet stored in the File GeoDatabase fGDB_Reaches.gdb in the  RivEX workspace output folder.


The NearestNet output FeatureClass will contain the following fields:


Field Name

Description

OBJECTID

The row ID

SHAPE

The Polyline geometry

SiteID

The site ID the network was extracted for

Direction

Indicates which direction the network was extracted, this will only ever be DS, US

Truncated

A Y/N flag indicating if the network was truncated

Shape_Length

The length of the polyline


A network is considered "truncated" if it never reaches the desired limit specified for the site. This will happen for sites that are near the source\mouth of river and have a large network distance. You can use this field to quality control your data.


The extracted network will be composed of polylines that originate from the network or be clipped backed versions of them. Thus the nearby network will never have the same topology as the base network that it is extracted from. This means you will not have the ID or node ID information that are encoded into the river network.


If you wanted to know the total network length for each site you could process this dataset using the summary statistic tool, grouping by site ID and summing by Shape_Length.


The NearestNetBuffer output FeatureClass will contain the following fields:


Field Name

Description

OBJECTID

The row ID

SHAPE

The Polygon geometry

SiteID

The site ID the buffer was computed for

Direction

Indicates which direction the network was extracted, this will only ever be DS, US

Buffer Distance

This was the distance used to buffer the extracted network

Shape_Length

The perimeter length of the polygon

Shape_Area

The area of the polygon