The site snapping tool

• RivEX can help you snap sites (points) to your river network.

1. You can specify a search tolerance (the default is 100m).
2. If you have available data; match the site with river using names.
3. RivEX can avoid snapping the site to polyline end nodes.

• This page deals with how to use RivEX for snapping sites.  Follow this link for a discussion on the types of snapping errors and their consequences.

• Your site data must be a point layer such as a Shapefile or Geodatabase FeatureClass.

• MULTIPOINT data is rejected by RivEX. You will need to convert it to single part form using the Geo-Processing tool Multipart to Singlepart.

• RivEX creates a Shapefile copy of your site dataset with each site snapped or rejected.  The name of this Shapefile is the name of your input dataset plus the suffix "_Snapped".  For example if your input site layer was a Shapefile called "WeirLocations.shp" then the output file will be called "WeirLocations_Snapped.shp" and is created in the default location C:\RivEX\Output.

• A site that fails a snap is left as it's original point. The quality control fields will be populated with why it failed.

• 4 quality control fields are appended to the output Shapefile and are described below.

• Select your network, polyline ID, From and To node fields and press Build to construct the data dictionaries.

Whilst the data dictionaries are not used during the snapping process they do provide a useful validation of the network.

• The site layer must be loaded into ArcMap before you can select it from the drop down menu.

• The basic snap is simply to snap a site to the nearest polyline within a specified search tolerance.  No contextual information is used to quality control the snap and can thus produce unreliable results.  You must enter a search tolerance, the default is 100m.

• To be able to use name matching both your river network and site data must have string fields populated with river names.  You can choose the very strict quality control of exact name or use similar names by doing an in-string search for the river name.

• Exact matching will convert all text to lower case before attempting to match and will fail a site if they don't match exactly.  Be aware that spelling mistakes and local variations in the name will fail a site.  Whilst this is a less forgiving method it does ensure the highest quality snap.  Examples are:

• Similar name matching attempts to reduce the site's river name to a simplified version.  If this simplified version is found within the river name from your network layer then this is considered a good quality snap.  If it is not found then the site is rejected.  Examples of name reduction are

• If you have chosen to use name matching you must select the correct fields from the drop down menus as shown below.

• Click Go! to start the snapping process.

• When using similar matching you will be offered a dialog box to add/remove extra synonyms as shown below. Here you can add common words used in your language.

• The technique used by RivEX is not perfect and can accept/reject sites when the site name is radically different or the name attached to the site contains a synonym within a word used to name the river.

• It could be possible for RivEX to reject a genuine site due to inconsistent naming.  For example a site labelled "R. Lune (near farm)" will be reduced to "Lune near farm" which is not a sub string of "River Lune".  The appended SrchName field will help you identify this situation.

• Another issue with the sub-string  search technique used by RivEX are words that include  synonyms themselves.  Thus a site on "Rock River" would be reduced to "Rock" and is a sub-string within a word such as "Brock Lake".  The appended SrchName field will help you identify this situation.

• Below is a list of the current combination of values that are used to populate the appended snapping fields.

X = The distance the point was moved from initial location to point on network

• The check box avoid node () if ticked will force a potential snap 1% along the length of the polyline to avoid snapping to a node.  This avoids the later situation of you using the point in a spatial selection and selecting 2 or more polylines because it snapped to a tributary node when you really wanted to select a single starting polyline.