Determining Objects within Isochrones in Spatial Network Databases

Isochrones are generally defined as the set of all space points from which a query point can be reached in a given timespan, and they are used in urban planning to conduct reachability and coverage analyzes in a city. In a spatial network representing the street network, an isochrone is represented as a subgraph of the street network. Such a network representation is not always sufficient to determine all objects within an isochrone, since objects are not only on the network but might be in the immediate vicinity of links (e.g., houses along a street). Thus, the spatial area covered by an isochrone needs to be considered. In this paper we present two algorithms for determining all objects that are within an isochrone. The main idea is to first transform an isochrone network into an isochrone area, which is then intersected with the objects. The first approach constructs a spatial buffer around each edge in the isochrone network, yielding an area that might contain holes. The second approach creates a single area that is delimited by a polygon composed of the outermost edges of the isochrone network. In an empirical evaluation using real-world data we compare the two solutions with a precise yet expensive baseline algorithm. The results demonstrate the efficiency and high accuracy of our solutions.