A straightforward approach adopted for comparison purposes is to use Independent Linear quadtrees (IL-trees). As its name states, a separate Linear quadtree is used for each feature resulting in as many Linear quadtrees as the number of features in the thematic map. This approach could present a substantial space overhead since multiple indices have to be stored. This fact is also a weak point during concurrent manipulation of multiple features because of the need to traverse and join the results from multiple indices.