Spatial search is the ability to find geometric information in a multidimensional space. Most information retrieval systems that support spatial data, including Solr, are limited to a two-dimensional Cartesian plane, with additional support for geospatial search in which two dimensions reference the location on the surface of a sphere.

That description is a bit abstract, so let's now review some common spatial requirements of an application. If your Solr documents represent businesses and you know where the business resides in terms of a latitude and a longitude, then you probably want to show search results (businesses) filtered to the vicinity of where the user is looking. The user interface might have a map centered at a region of interest, and/or you know approximately where the user is from the GPS of their mobile device, or you might even have a GeoIP database at your...