Generating sampling points with RivEX+

RivEX+ is a tool which can generate sampling points on your river network using a variety of techniques. You could use this tool to generate points at national or catchment level for developing habitat, water quality or even geomorphological surveys. The current tool can generate points snapped to the network by:

• Sampling every polyline in the network

• Stepping along the network using a user defined distance

• Random points (a set number or as a % of the network)

• Stratified sampling using a grid (samples a set number within each grid cell)

• Stratified sampling using a user supplied polygon layer

It is important to appreciate, the techniques which sample the network randomly can be biased by the settings you choose and these are discussed in the appropriate sections of the manual.

A random location on the river is generated by reading the network and constructing a virtual line which it then generates a random distance along.  RivEX+ will try up to 200 times to place the point on the network; but it is possible for RivEX+ to fail, especially if you have set up an illogical sampling scenario.

Sampling every polyline

RivEX+ can sample each polyline within your network and generate points at set percentages along polyline or randomly along each polyline. You can filter your data using an existing polyline selection.

The image below shows the output created when sampling each polyline at 25%, 50% and 75% along length of polyline. Points are labelled with their percentage position along length.

Note: This network could not be processed by RivEX for Strahler Order as the polylines are flowing in the wrong direction (as indicated by the sequence of sampling points)!

The image below shows the same network processed, but each polyline is randomly sampled 3 times along its length. Points are labelled with their percentage position along length.

For each point created, the following information is written to the Shapefile:

• A unique ID number
• The intersecting polyline ID
• XY coordinates
• Percentage along polyline length

An example of the attribute information generated is shown in the table below.

Regularly spaced point

For each point created, the following information is written to the Shapefile:

• A unique ID number
• XY coordinates
• Distance from the network mouth
• The catchment ID the point is within
• The ID of the polyline the point is intersecting

With the spacing distance set to 1Km a typical output would be:

RivEX+ will generate metadata for the above process which you can read in ArcCatalog.  Remember the style sheet filters what you see, so you will need to swap between different style sheets to see specific information generated by RivEX+.

This tool will generate incorrect points if applied to data that is in a geographic coordinate system (i.e. latitude/longitude).  For accurate results your network must be projected into a coordinate system which uses meter units.

Random sampling

RivEX+ can generate random points across the whole network or a subset identified by a selection.  It can generate a fixed number of points or a percentage of network where each point has a footprint (fixed reach length). The selection process can be modified so as to avoid sampling the same point or by imposing an exclusion zone around each identified point.

The image below shows a portion of network which has been randomly sampled as 10% of the network where each point has a 1Km footprint, each point is labelled with its unique ID. 

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).

For each point created, the following information is written to the Shapefile:

• A unique ID number
• The intersecting polyline ID
• XY coordinates
• Percentage along polyline length

Stratified sampling using a grid

RivEX+ can generate a grid to stratify the sampling points. The user enters the size of the grid cell in meters and the number of points per cell. RivEX+ will then construct a grid to the extent of the dataset (or selection) and randomly sample rivers within each cell by the number entered by the user. The user can choose to keep the grid after processing has finished.

The image below shows a portion of a network sampled using a grid cell size of 10Km with each cell sampled 3 times.

For each point created, the following information is written to the Shapefile:

• A unique ID number
• The intersecting polyline ID
• XY coordinates
• Percentage along polyline length

Stratified sampling using a polygon layer

RivEX+ can stratify its sampling using a supplied polygon layer.  The polygon layer will typically represent catchment areas or administrative boundaries.  RivEX+ can use an existing field within the polygon layer for supplying the number of points per polygon, generate a fixed number of points or a percentage of network within each polygon. The selection process can be modified so as to avoid sampling the same point or by imposing an exclusion zone around each identified point.

The below image shows 3 sampling points per polygon. The points are labelled with the polygon ID.

For each point created, the following information is written to the Shapefile:

• A unique ID number
• The intersecting polyline ID
• XY coordinates
• Percentage along polyline length
• The polygon ID