Transform - Select Shortest Path

Given a selection that includes a system of lines that can be treated as a network plus two points this transform operator selects those lines that form the shortest path through the network between the two points.

Example

We begin with a map with three drawing layers:

· A background map of Mexico. This has been set to low opacity so it is faint and unobtrusive.

· A drawing called Nodes that contains points. These were created for this example by running the Centroids transform operator on the Mexican province areas.

· A drawing called Links that contains lines. These were created for this example by running the Relative Neighborhood Network transform operator on the points in the Nodes layer.

$images\eg_shortest_path_01.gif$

In the Nodes layer we select two points. If we like, we can save this selection in the Selections pane so we can later select exactly these two points again.

$images\eg_shortest_path_02.gif$

In the Links layer we select all of the lines. This may be done rapidly by enabling Edit - Select Objects - Lines (or simply pushing in the Select Lines button) and then choosing Edit - Select by Type .

$images\eg_shortest_path_03.gif$

Run the Select Shortest Path transform using the [Selection] as the scope. Press Apply to select those lines that comprise the shortest path between the two points through the network.

$images\eg_shortest_path_04.gif$

The result is that the lines on the shortest path are selected, and highlighted in red selection color. (In the illustration above we have also used the saved selection to select the two points as well.) We will often save this selection, or copy the lines involved and paste them into a new drawing.

Troubleshooting

When running this transform operator and getting unexpected results, check the following:

· Are you using an object set that contains only the two points between which you want a shortest path? Running this operator on a set of objects that contains more than two points will select two random points and select a shortest path between them.

· Do the lines in use form a network? That is, are they contiguous or are there breaks in the lines? Use Normalize Topology to make sure the various ends of lines are coincident with the beginnings of the next lines.

· Are the points in use coincident with the ends of the lines? If not use Normalize Topology to make sure they are coincident.