A Raster Approximation for Processing of Polyline Joins

The main subject of spatial joins is polygons and polylines. The processing of spatial joins can be greatly improved by the use of filters that reduce the need for examining the exact geometry of spatial objects in order to find the intersecting ones. Approximations of candidate pairs of spatial objects are examined using such filters. As a result, three possible sets of answers are identified: the positive one, composed of intersecting pairs; the negative one, composed of non-intersecting pairs; and the inconclusive one, composed of the remaining pairs of candidates. To identify all the intersecting pairs of spatial objects with inconclusive answers, it is necessary to have access to the representation of them so that an exact geometry test can take place. This is true for both polygons and polylines. There are many approximations designed for polygons, but few of them are suitable for approximating polylines. This article presents a polyline approximation for spatial join processing, which we call raster signature for polylines. The performance of a filter using this approximation was evaluated with real world datasets. The results showed that our approach, when compared to other approaches presented in the related literature, reduced the inconclusive answers by a factor of more than two. As a result, the need for retrieving the representation of polylines and carrying out exact geometry tests is reduced by a factor of more than two, as well.