Sample with grid

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RivEX can generate a grid over your network and sample a fixed number of points per grid cell.  This is stratified sampling.  Such output could feed into further river analysis or compliment a survey strategy.


Stratified sampling using a generated grid

A portion of a network sampled using a grid cell size of 10Km with each cell sampled 3 times


The generated grid is based upon the extent of the river network layer. The maximum X and Y values of the grid extent are rounded up to fit the user specified cell size. This ensures all parts of the network intersect the grid.


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.


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

PolygonID

This is the unique ID given to the grid cell


Set the cell size in meters and enter the number of times you want to sample within each grid cell.


Sample with grid dialog


You have the option to keep the grid generated by RivEX otherwise it is destroyed when processing has completed. If you choose to keep the grid then it is stored in the same File GeoDatabase as the point layer and is name riv_cellsize_XXX where riv is the name of your river network and XXX was your chosen cell size. The grid layer has the following fields:


Field name

Description

ID

Unique ID given to the grid cell

Cell_Size

This is the user entered value and is the distance along the X and Y axis

Cell_Area

The area of the cell, calculated as X * Y

Min_X

The minimum X coordinate of the cell

Min_Y

The minimum Y coordinate of the cell

Max_X

The maximum X coordinate of the cell

Max_Y

The maximum Y coordinate of the cell

Centroid_X

The centroid X coordinate of the cell

Centroid_Y

The centroid Y coordinate of the cell

Count*

The number of sampling points generated within the cell


* = The Count field is the number of sampling points generated in the cell, if this is less than what you requested then RivEX was unable to sample that part of the network the required number of times. For example you may have specified an exclusion zone that stopped RivEX sampling a fragment of network that fell within the grid cell.