Options that bias random selection

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RivEX offers a variety options for influencing the outcome of the random selection process.  These will introduce a bias to the selection process and it is important that you understand these.




Avoid sampling same location


This option stops RivEX from generating a point directly on top of a point that has already been identified. If by chance it does create a point on top of another and this option is ticked then it will abandon the point and choose another random location.



Avoid nodes


If this option is ticked, RivEX will abandon a random point if it hits a network node (polyline end points).  It will keep on generating new XY coordinates until it succeeds or hits the 200th attempt limit.  This option is useful as it stops points from intersecting more than 1 polyline.  This is a situation you will want to avoid if you are to use the points in future network searching.



Apply an exclusion zone


When a point is generated a user defined buffer is created and used to check if any existing sampling points fall within the buffer.  If they do then the point is abandon and a new set of coordinates generated. This option influences the random sampling in one of two ways:


oFirstly it has the affect of spacing out points, this is most noticeable if you have requested a large number of sampling points. They end are being regularly spaced.

oOnce a point has been placed upon a network then no other point can be placed near it (within the specified buffer distance).  For example a point generated on a different tributary that creates an intersecting buffer zone on an existing point would be abandoned, even though its on another river.


You would want to use this tool to ensure a minimum distance between points which may end up representing habitat or geomorphological survey sites.