Closest facility is not returned when using Solver with the Hierarchy option

Description

The Closest Facility Solver returns facilities that are not closest when using the Hierarchy option and the following conditions:

- Less than 20% of the total number of facilities is being sought.
- The network being analyzed is an SDC (smart data compression) network with four hierarchy levels.
- The highest hierarchy level in the network is very sparse.

Cause

When the Hierarchy option is selected, the Closest Facility solver determines which facilities are closest to each incident. This is done by growing a shortest path tree from the incident until the number of facilities found is completely within the spatial extent of this shortest path tree.

All facilities outside of the spatial extent are assumed to be further than those within the spatial extent, even though some of these facilities actually can be closer.

Workaround

• Solve with Hierarchy option off. This will make the solver use the exact route lengths to determine which facilities are closest.
• Use custom ranges for the hierarchy. For example, if you use commercial SDC data with four levels of hierarchy, set the range for Primary Roads to cover the two highest hierarchy levels. This will make the shortest path tree less sparse, allowing the solver to find more facilities.
• Increase the number of facilities to find. For example, if you want to find 10 closest facilities, solve with the number of facilities to find equal to 15 and then look at the 10 that are closest. Finding a larger number of facilities will increase the spatial extent of the shortest path tree and make more facilities candidates for being the closest.