adl:geographic locations

The adl:geographic locations bucket contains the footprints of library objects. Each object in an ADL collection should be associated with at least one footprint (region on the surface of the Earth). The adl:geographic location bucket contains the geographic coordinates (latitude and longitude) of the vertices of the geographic polygons bounding the object's footprint. Note that a single object may have multiple disjoint footprints.

ADL uses the adl:geographic locations bucket to support its geographic search capability. Three query operations are defined on the bucket, contains (the footprint in the bucket's wholly contained in a query footprint), within (the footprint is wholly within the query footprint), and overlaps (the footprint in the bucket at least partially overlaps a query footprint). Note that contains is a proper subset of overlaps.

In practice, the footprints assigned to the adl:geographic location bucket usually fall into one of two special cases: points (single vertices) and "bounding boxes" (rectangles). Points are often the only footprint information available for relatively small features (e.g., cities on a world map). Bounding boxes have two advantages. If a feature does not have a polygon associated with it, but is associated with more than one point, then the bounding box may be trivially computed. Moreover, even if polygonal footprints are available, a bounding box may be preferable to search on, for reasons of index efficiency.