The effective depth of a search tree may be reduced by doing one search in an abstract space to get a rough solution, then doing another search in the actual space to refine the solution.
Example: Route Finding Search for a path between two locations first on a coarse map that includes only major cities and interstate highways; then refine paths to get to the interstate highways.
Advantage: Can make an otherwise very large search tractable.
Disadvantage: The path found may be sub-optimal, depending on how good the abstraction is.