Options that bias random selection

• 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.

Don't sample the same point

• 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 probably want to avoid if you are to use the points in future network searching.

Avoid polylines comprised of 2 vertices

• Polylines comprised of only 2 vertices are typically very small within river networks and usually indicate the limit of data capture when the rivers were digitised.  But there are always exceptions to this rule, especially if the polyline represents a very straight drainage ditch, so caution should be used when using this option. If a point identifies such a polyline and this option is ticked then the point is abandon and a new set of coordinates are generated.

• Ticking this option excludes 2 vertex polylines from the random selection process.

Use exclusion zone

• When a point is generated and snapped to the polyline a user defined buffer is created and used to check if any other 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:

• Firstly 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.

• Once a point has been placed upon a network then no other point can be placed near it within the buffer zone.  Therefore a point placed on a different tributary that falls within the buffer zone of 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.