Acta Geodaetica et Cartographica Sinica ›› 2016, Vol. 45 ›› Issue (8): 983-991.doi: 10.11947/j.AGCS.2016.20150446

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Clutter-free Visualization of Large Point Symbols at Multiple Scales by Offset Quadtrees

ZHANG Xiang1, WANG Shaodong2, WANG Yuxia3   

  1. 1. School of Resources and Environmental Sciences, Wuhan University, Wuhan 430079, China;
    2. Institute of Software Chinese Academy of Sciences, Beijing 100190, China;
    3. Institute of Remote Sensing and Geographical Information Systems, Peking University, Beijing 100871, China
  • Received:2015-08-30 Revised:2016-06-23 Online:2016-08-20 Published:2016-08-31
  • Supported by:
    The National Natural Science Foundation of China (No. 41301410);The National High-tech Research and Development Program of China (863 Program) (No. 2015AA123901);The Project for National Basic Science Personnel Training Fund(No. J1103409)

Abstract: To address the cartographic problems in map mash-up applications in the Web 2.0 context, this paper studies a clutter-free technique for visualizing large symbols on Web maps. Basically, a quadtree is used to select one symbol in each grid cell at each zoom level. To resolve the symbol overlaps between neighboring quad-grids, multiple offsets are applied to the quadtree and a voting strategy is used to compute the significant level of symbols for their selection at multiple scales. The method is able to resolve spatial conflicts without explicit conflict detection, thus enabling a highly efficient processing. Also the resulting map forms a visual hierarchy of semantic importance. We discuss issues such as the relative importance, symbol-to-grid size ratio, and effective offset schemes, and propose two extensions to make better use of the free space available on the map. Experiments were carried out to validate the technique,which demonstrates its robustness and efficiency (a non-optimal implementation leads to a sub-second processing for datasets of a 105 magnitude).

Key words: clutter reduction, multi-scale visualization, large symbols, quadtree, real-time Web mapping

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