Finding the source upstream of a site

• RivEX can take a point layer which could represent the locations of survey sites and calculate the distance to source.  The source is defined as the point furthest away from the network mouth upstream of your input site.  In the image below the green point represents a site and the red point is the source that is identified.  The magenta dashed line has been added simply to visualise the route taken, it is not a "shape" that is created.

• Your points must be snapped to the river network and can be distributed throughout all sub-catchments.

• Your network must have be run through RivEX and processed for catchment ID and distance to mouth.  You will have also run your network through the segment identifier process.  Please refer to the appropriate section of the manual for how to calculate these values.

• This tool locates sources by subtracting the differences in distance to network mouth encoded into each polyline.  It is not a route finding tool, such as network tracing and thus does not encode the network with the route to source.  It is possible that the distance reported from site to source is not the shortest when there are braids or loops within the network.

• In special cases RivEX will return a negative distance to source, this is clearly incorrect!  The assumption behind the processing algorithms are that the network drains to the a single mouth at the coast.  Negative values are generated because a network mouth is located within a catchment and not at the coast.  These can be genuine channels draining into a cannel or sink holes.

• As RivEX cannot distinguish between mouths within catchments and mouths at the coast the distance to mouth algorithm overwrite values.  The image below demonstrates such an example. Polylines upstream of mouth "A" have ended up being coded to drain to "A". The polyline the site is on drains to Mouth "B" which is much further away.  When it locates the furthest source upstream this is much closer to "A" and when RivEX subtracts the distance to mouth of the polyline that the site is on this leads to a negative value.

• To use the source locating tool you must complete the parameters section of the main interface and press the Build button.

• Click on the Find Sources tab.

• In the select point layer frame, identify the point layer that represents the sites and the unique ID field as shown below.  This point layer must be loaded into the map document before you open the main RivEX interface.

• You will asked to confirm that the point layer is a layer with points snapped to the network. Go here to find out about using RivEX for snapping points to the network.

• If your point layer has an existing selection, then ticking the check box "Use only selected sites" will ensure only these points will be used in the analyses.

• You must now identify the catchment ID field and distance to mouth fields fields.  These are RivEX generated fields and must have been created before you use the site spacing tool.

• When you tick the check box you will be confronted with the following message box.  This is simply to ensure that you have computed the important segment identification tables and stored them as dbase files in C:\RivEX\Output.

• You must not have changed the name of your network since you built the segment ID tables as RivEX searches the C:\RivEX\Output directory.  For example your network was called MyRivers.shp and consisted of 2 catchments numbered 1 and 2.  When running find sources, RivEX searches the network and will determine there are 2 catchments numbered 1 and 2.  With this knowledge RivEX builds a path name to the segment table, for example C:\RivEX\Output\MyRivers_Tables.mdb/Catch_1_Upstream_IDs.

If you had renamed the dataset to MyRiversx2.shp then RivEX would try and find.  C:\RivEX\Output\MyRiversx2_Tables.mdb/Catch_1_Upstream_IDs which obviously does not exist!

• Click on Go! and the tool will run and and when finished, announce that it will send the data directly to MS ExcelXP/2003.  The tool will terminate early if any of the sites do not have a unique ID or identify a single starting polyline (i.e. not snapped to the network).

• The output is a single MS Excel sheet which lists for each input site:  Site ID, the catchment ID, the ID of the source (this is the From-node ID of the polyline), the distance the source is from the mouth, the distance the input site is from the mouth, the distance the source is from the input site and the polyline ID that the site intersected.

• If the output exceeds the capacity of MS ExcelXP/2003 then RivEX will automatically write the data to a dbase file which will have the name of the input site layer post fixed with "_USS" . The field headings have to be 10 characters or less to be a valid dbase field therefore abbreviations are used and are described below.