When importing spatial data (ArcGIS Geodatabases or shapefile data contain spatial geometry data that ModelBuilder can use to establish network connectivity by connecting pipe ends to nodes, creating nodes at pipe endpoints if none are found.), ModelBuilder provides two ways to specify network connectivity:

• Explicit connectivity--based on pipe Start node and Stop node (see "Step 4--Additional Options" ).
• Implicit connectivity--based on spatial data. When using implicit connectivity, ModelBuilder allows you to specify a Tolerance, and provides a second option allowing you to Create nodes if none found (see "Step 2--Specify Spatial Options" ).

The method that you use will vary depending on the quality of your data. The possible situations include (in order from best case to worst case):

• You have pipe start and stop information--Explicit connectivity is definitely the preferred option.
• You have some start and stop information--Use a combination of explicit and implicit connectivity (use the Spatial Data option, and specify pipe Start/Stop fields). If the start or stop data is missing (blank) for a particular pipe, ModelBuilder will then attempt to use spatial data to establish connectivity.
• You do not have start and stop information--Implicit connectivity is your only option. If your spatial data is good, then you should reduce your Tolerance accordingly.
• You do not have start and stop information, and you do not have any node data (e.g., you have GIS data that defines your pipes, but you do not have data for nodes)--Use implicit connectivity and specify the Create nodes if none found option; otherwise, the pipes cannot be created.

Other considerations include what happens when the coordinates of the pipe ends do not match up with the node coordinates. This problem can be one of a few different varieties:

1. Both nodes and pipe ends have coordinates, and pipes have explicit Start/Stop nodes--In this case, the node coordinates are used, and the pipe ends are moved to connect with the nodes.
2. Nodes have coordinates but pipes do not have explicit Start/Stop nodes--The nodes will be created, and the specified tolerance will be used to connect pipe ends within this tolerance to the appropriate nodes. If a pipe end does not fall within any node’s specified tolerance, a new node can be created using the Create nodes if none found option.
3. Pipe ends have coordinates but there are no junctions--New nodes must be created using the Create nodes if none found option. Pipe ends are then connected using the tolerance that is specified.

Another situation of interest occurs when two pipes cross but aren’t connected. If, at the point where the pipes cross, there are no pipe ends or nodes within the specified tolerance, then the pipes will not be connected in the model. If you intend for the pipes to connect, then pipe ends or junctions must exist within the specified tolerance.