Results 11  20
of
147
Geometric Containers for Efficient ShortestPath Computation
 ACM JOURNAL OF EXPERIMENTAL ALGORITHMICS
, 2005
"... A fundamental approach in finding efficiently best routes or optimal itineraries in traffic information systems is to reduce the search space (part of graph visited) of the most commonly used shortest path routine (Dijkstra’s algorithm) on a suitably defined graph. We investigate reduction of the se ..."
Abstract

Cited by 20 (8 self)
 Add to MetaCart
(Show Context)
A fundamental approach in finding efficiently best routes or optimal itineraries in traffic information systems is to reduce the search space (part of graph visited) of the most commonly used shortest path routine (Dijkstra’s algorithm) on a suitably defined graph. We investigate reduction of the search space while simultaneously retaining data structures, created during a preprocessing phase, of size linear (i.e., optimal) to the size of the graph. We show that the search space of Dijkstra’s algorithm can be significantly reduced by extracting geometric information from a given layout of the graph and by encapsulating precomputed shortestpath information in resulted geometric objects (containers). We present an extensive experimental study comparing the impact of different types of geometric containers using test data from realworld traffic networks. We also present new algorithms as well as an empirical study for the dynamic case of this problem, where edge weights are subject to change and the geometric containers have to be updated and show that our new methods are two to three times faster than recomputing everything from scratch. Finally, in an appendix, we discuss the software framework that we developed to realize the implementations of all of our variants of Dijkstra’s algorithm. Such a framework is not trivial to achieve as our goal was to maintain a common code base that is, at the same time, small, efficient, and flexible,
Level of Detail Generation of 3D Building Groups by Aggregation and Typification
 In: Proceedings of the XXII International Cartographic Conference, La Coruna
, 2005
"... The realtime visualisation of 3D city models requires the representation of the buildings in different levels of detail (LoD). This LoDs should be generated automatically by specific generalisation procedures. In this article we propose two approaches which extents cartographic generalisation algor ..."
Abstract

Cited by 18 (0 self)
 Add to MetaCart
The realtime visualisation of 3D city models requires the representation of the buildings in different levels of detail (LoD). This LoDs should be generated automatically by specific generalisation procedures. In this article we propose two approaches which extents cartographic generalisation algorithm for the application to 3D building groups generalisation. The problem of generalisation of object groups leads directly to the topic of aggregation and typification. Typification denotes the process of replacing a number of objects in a group by a smaller number of new objects, while leaving the main visual structure unchanged. We describe a typification approach which is able to detect grid like building structures to preserve this structure when reducing the number of involved objects. For the aggregation of 3D building groups we describe an approach based on the 2D aggregation program CHANGE.
Largest and Smallest Convex Hulls for Imprecise Points
 ALGORITHMICA
, 2008
"... Assume that a set of imprecise points is given, where each point is specified by a region in which the point may lie. We study the problem of computing the smallest and largest possible convex hulls, measured by length and by area. Generally we assume the imprecision region to be a square, but we d ..."
Abstract

Cited by 18 (4 self)
 Add to MetaCart
(Show Context)
Assume that a set of imprecise points is given, where each point is specified by a region in which the point may lie. We study the problem of computing the smallest and largest possible convex hulls, measured by length and by area. Generally we assume the imprecision region to be a square, but we discuss the case where it is a segment or circle as well. We give polynomial time algorithms for several variants of this problem, ranging in running time from O(n log n) to O(n^13), and prove NPhardness for some other variants.
Metric functional dependencies
 In ICDE
, 2009
"... Abstract—When merging data from various sources, it is often the case that small variations in data format and interpretation cause traditional functional dependencies (FDs) to be violated, without there being an intrinsic violation of semantics. Examples include differing address formats, or differ ..."
Abstract

Cited by 18 (1 self)
 Add to MetaCart
(Show Context)
Abstract—When merging data from various sources, it is often the case that small variations in data format and interpretation cause traditional functional dependencies (FDs) to be violated, without there being an intrinsic violation of semantics. Examples include differing address formats, or different reported latitude/longitudes for a given address. In this paper, we define metric functional dependencies, which strictly generalize traditional FDs by allowing small differences (controlled by a metric) in values of the consequent attribute of an FD. We present efficient algorithms for the verification problem: determining whether a given metric FD holds for a given relation. We experimentally demonstrate the validity and efficiency of our approach on various data sets that lie in multidimensional spaces. I.
No Quadrangulation is Extremely Odd
, 1995
"... Given a set S of n points in the plane, a quadrangulation of S is a planar subdivision whose vertices are the points of S, whose outer face is the convex hull of S, and every face of the subdivision (except possibly the outer face) is a quadrilateral. We show that S admits a quadrangulation if a ..."
Abstract

Cited by 18 (3 self)
 Add to MetaCart
Given a set S of n points in the plane, a quadrangulation of S is a planar subdivision whose vertices are the points of S, whose outer face is the convex hull of S, and every face of the subdivision (except possibly the outer face) is a quadrilateral. We show that S admits a quadrangulation if and only if S does not have an odd number of extreme points. If S admits a quadrangulation, we present an algorithm that computes a quadrangulation of S in O(n log n) time even in the presence of collinear points. If S does not admit a quadrangulation, then our algorithm can quadrangulate S with the addition of one extra point, which is optimal. We also provide an\Omega (n log n) time lower bound for the problem. Finally, our results imply that a kangulation of a set of points can be achieved with the addition of at most k \Gamma 3 extra points within the same time bound.
Measuring rectangularity
 Machine Vision and Applications, 11:191
, 1999
"... Three new methods for measuring the rectangularity of regions are developed. They are tested together with the standard minimum bounding rectangle method on synthetic and real data. It is concluded that while all the methods have their drawbacks the best two are the bounding rectangle and discrepanc ..."
Abstract

Cited by 17 (8 self)
 Add to MetaCart
(Show Context)
Three new methods for measuring the rectangularity of regions are developed. They are tested together with the standard minimum bounding rectangle method on synthetic and real data. It is concluded that while all the methods have their drawbacks the best two are the bounding rectangle and discrepancy methods. 1
Computing Constrained MinimumWidth Annuli of Point Sets
, 2000
"... We study the problem of determining whether a manufactured disc of certain radius r is within tolerance. More precisely, we present algorithms that, given a set of n probe points on the surface of the manufactured object, compute the thinnest annulus whose outer (or inner, or median) radius is r and ..."
Abstract

Cited by 16 (5 self)
 Add to MetaCart
We study the problem of determining whether a manufactured disc of certain radius r is within tolerance. More precisely, we present algorithms that, given a set of n probe points on the surface of the manufactured object, compute the thinnest annulus whose outer (or inner, or median) radius is r and that contains all the probe points. Our algorithms run in O(n log n) time.
Quadrangulations of Planar Sets
 In Proceedings of the 4th International Workshop on Algorithms and Data Structures
, 1985
"... Given a set S such as a polygon or a set of points, a quadrangulation of S is a partition of the interior of S, if S is a polygon, or the interior of the convex hull of S, if S is a set of points, into quadrangles (quadrilaterals) obtained by inserting edges between pairs of points (diagonals betwee ..."
Abstract

Cited by 14 (2 self)
 Add to MetaCart
(Show Context)
Given a set S such as a polygon or a set of points, a quadrangulation of S is a partition of the interior of S, if S is a polygon, or the interior of the convex hull of S, if S is a set of points, into quadrangles (quadrilaterals) obtained by inserting edges between pairs of points (diagonals between vertices of the polygon) such that the edges intersect each other only at their end points. Not all polygons or sets of points admit quadrangulations, even when the quadrangles are not required to be convex (convex quadrangulations) . In this paper we briefly survey some recent results concerning the characterization of those planar sets that always admit quadrangulations (convex and nonconvex) as well as some related computational problems. 1. Introduction In the field of computational geometry a triangulation of a finite planar set such as a set of points, line segments or polygon, is a well studied structure [O'R94], [PS85]. For one thing, a triangulation always exists and for anothe...
Remote sensing image thresholding methods for Determining landslide activity
 Vol
, 2005
"... Detecting landslides and monitoring their activity is of great relevance for disaster prevention, preparedness and mitigation in hilly areas. To this end, change detection techniques are developed and applied to multitemporal digital aerial photographs, simulating the very high spatial resolution o ..."
Abstract

Cited by 12 (0 self)
 Add to MetaCart
(Show Context)
Detecting landslides and monitoring their activity is of great relevance for disaster prevention, preparedness and mitigation in hilly areas. To this end, change detection techniques are developed and applied to multitemporal digital aerial photographs, simulating the very high spatial resolution of new satellite sensor optical imagery, over the Tessina complex landslide in northeastern Italy. Several automatic thresholding algorithms are compared on the difference orthorectified and radiometrically normalised images, including some standard methods based on clustering, statistics, moments, and entropy, as well as some more novel techniques previously developed by the authors. In addition, a variety of filters are employed to eliminate much of the undesirable residual clutter in the thresholded difference image, mainly as a result of natural vegetation and manmade land cover changes. These filters are based on shape and size properties of the connected sets of pixels in the threshold maps. This has enabled us to discriminate most ground surface changes related to the movement of a preexisting landslide. 1