in R Binbin Lu, Martin Charlton National Centre for Geocomputation National University of Ireland Maynooth The R User Conference 2011

Transcription

1 Converting a spatial network to a graph in R Binbin Lu, Martin Charlton National Centre for Geocomputation National University of Ireland Maynooth Maynooth, Co.Kildare, Ireland The R User Conference 2011

2 Contents Motivation Key points in the conversion R package shp2graph Future work

3 Motivation How should I get all the shortest t paths between any pair of points? SANNET pgrouting Coding with Dijkstra algorithm But I am an R user and I am an R user and I am lazy

4 Search in R packages Shape file maptools sp graph igraph RBGL sp.between() shortest.paths() Solution: Converting a spatial network to a graph object Key points for this conversion: Modelling Road network with graph data model Attribute heritage Topology Structure optimization Integration of points into network

5 Graph data model Edge (Arc) Node (Vertex) Edge list NodeID1 NodeID2 Node list ID X Y Graph data model: ID 1 ID 2... ID n Matrix A kind of data model to abstract away ID1 ID 2... ID n geographical details and represent a aij 0or 1 Edgelength... network as a graph, that is, as a finite set of points called nodes (i.e. vertices) and relations between these points called edges (i.e. arcs).

6 Attributes inheritance and translation Geometry Spatial coordinates of nodes Attributes IDsofnodes andedges; Lengths of edges; Orientation of edges

7 Topology errors (a) Dangling arc (b) Undershoot (c) Overshoot (d) Missing intersection

8 Connectivity Problem

9 Solutions to correct topology errors GIS software ArcGIS ArcInfo GRASS AutoCAD Map Topology correction tools/functions Extend Polyline Tool, Intersect Polylines Tool, Remove Redundant Nodes Tool and Clean Dangles Tool in ET Geo tools Clean Polyline, Clean Dangling Nodes, Split Polylines(With a Layer) in ET Geo Wizards Clean {Coverage}, Validate Entire Topology tool, Fix Topology Error tool Cleaning tools in v.clean Toolset: break,rmdangle, snap Creatinganetworktopology, EditingTopologies

10 Over correction

11 Structure Optimization Pseudo-node Multiple-edge Self-loop

12 Test on Ontario Road Network data Test on Ontario Road Network data

13 Test on Ontario Road Network data Comparison between original ORN and optimized graph Data Number of nodes Number of edges Original vector data Optimized graph % %

14 Integration of point data into network Two guidelines: 1. Find a representative node for each point 2. Add a new edge to connect each point with the network

15 iv. Add a new edge between eac point and the nearest point h ii. Mapping each Point to the nearest point i. Mapping each point to the nearest node iii. Add a new edge between each point and the nearest node

16 Edge Reconstruction Edge split Edge merge Functions for recalculating attributes New _ a i A li New _ ai A L New _ a max Ai New _ a minai New _ a sum Ai New _ a mean A i

17 R package shp2graph p

18 R package shp2graph p

19 After conversion, you can compute the shortest paths visualize the graph with igraph, shp2graph, pajek do network analysis with ih various kinds of methods available in graph, igraph, RBGL, etc

20 Future work I was an R beginner when I started to develop this package, so I make lots of mistakes! Awkward parameter settings in functions Poor coding style Topology correction functions will be developed in the near future.

21 I will keep working on this, and welcome any contribution, suggestions or comments.

22 Acknowledgments To Centre for Research into Statistical Methodology (CRISM) for funding me to attend this conference. Research presented in this paper is jointly funded by a Strategic Research Cluster grant (07/SRC/I1168) by Science Foundation Ireland under the National Development Plan and China Scholarship Council (CSC).

23 Any comments or questions?