Sample whole network

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RivEX allows you to generate a fixed number of points or as a percentage of the network length. This tool is ideal for generating a layer of random points across a network or a sub-set identified by an existing selection.  Such output could feed into further river analysis or compliment a survey strategy.


You have a variety of options that can influence (bias) the selection process and it is important that you understand these. Click here to review these options. Avoid sampling same location and nodes are ticked on as default.


RivEX will attempt to locate a random point but if it fails it will try up to 200 times.  If after the 200th attempt it is still unsuccessful it will bail out.  This scenario can occur when for example you have included an exclusion zone around each point, but this was set so large that no other points could be placed on the same network


The output is written to the default directory File GeoDatabase.  Each point stored will contain the following information:


Field name

Description

SampleID

The unique ID given to the sampling point

PolylineID

The polyline ID. This is from the ID field you chose to build network topology with

X_Coord

The X coordinate of the point

Y_Coord

The Y coordinate of the point

Per_Along

The percentage along the length of polyline the sampling point is located


If you have a selection on your network you can choose to process only those polylines by ticking the appropriate check box.  This check box becomes active if there is a selection.



Sample network dialog


The output name should be a valid File Geodatabase FeatureClass name



Fixed number sampling


This option samples the network the number of user specified times. In the example image above the user has specified that 25 sampling points should be generated on selected polylines only and each new point will impose a 50m exclusion zone.



Percentage of network sampling


If you are sampling a percentage of network then you need to provide a footprint size ( in meters) and percentage cover of network.  The footprint is a linear length that the sampling point represents.


The number of points created is based upon the footprint size and the desired percentage of network cover.  For example a network with 8900Km of river would generate 890 sampling points if each point had a footprint of 1Km and the user wanted 10% coverage.  This calculation treats each footprint as non-overlapping when determining the number of points required but the output does not displace points when choosing the random XY coordinates.  Therefore large sample sizes will create the required number of points to sample  a network with non-overlapping footprints but can actually sample less network because the placement of the point does not consider existing footprints (unless you choose the exclusion zone option).