1. A method comprising: accessing, at an online system, a set of geo-tiles associated with a map of a physical location, each geo-tile representing a section of the map; determining, for each geo-tile, whether a threshold portion of the geo-tile represents a portion of a geographic region; responsive to determining at least the threshold portion of the geo-tile represents a portion of the geographic region: computing a value associated with the geo-tile, the value based at least in part on the portion of the geographic region represented by the geo-tile, and storing the computed value with an identifier associated with the geo-tile; responsive to determining less than the threshold portion of the geo-tile represents the portion of the geographic region: accessing an additional set of geo-tiles, the additional set of geo tiles collectively representing the section of the map of the physical location region represented by the geo-tile, determining, for each of the additional set of geo-tiles, whether a portion of the geo-tile representing the portion of the geographic region is at least the threshold portion, responsive to determining at least the threshold portion of the geo tile of the additional set represents a portion of the geographic region, computing the value associated with the geo-tile, and storing the computed value with an identifier associated with the geo-tile; determining the portion of the geographic region included in the map based at least in part on the stored values; and storing the determined portion in an index associated with the geographic region and the map of the physical location.