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Key words: Spatial Data Representation, Spatial Data Model, GIS, Spatial Database
Maps are the primary media of geographical information and are the elementary objects manipulated in GIS. Almost all existing GIS adopt the layer-based approach to represent geographic information on maps.
In the layer-based approach, spatial data are represented by a set of thematic maps, named layers, which denote some given themes such as roads, buildings, subways, contours, borders, and so on. The layer-based approach has advantages in avoiding the superposition (overlap) of spatial objects, which is difficult to process using spatial queries and spatial analysis. However, the map represented as a layer is difficult to extend. Furthermore, in a web-based GIS, transmission of spatial data for map viewing is slow.
In this paper, we propose a new method, a scale-based approach, for representing spatial data to solve the problems noted above. In this approach we organized the maps in a set of maps, named Scale-View, which associates some given scales. Traditionally, maps are represented in two levels, as a layer and as a spatial object. Here we represent maps in three levels: scale-view, block, and spatial object. A "block" is an abstract representation of a set of spatial objects. Inside it, are a set of spatial objects named child objects, or a set of blocks named child blocks. So it is also an entity of these spatial objects. A "Scale-View" is a set of blocks and a set of spatial objects that associate with a certain scale.
We organize a map into a tree structure depending on a series of scales. In the boot of tree, is the first scale-view, which is a pure "block," the entity of the whole map that abstractly represents the shape of this map as a polygon. In the second scale-view, we separate the boot block into several blocks (or a set spatial objects) depending on certain separation rules such as "to separate by administrative boundary." The spatial object that cannot be separated using this rule (such as mountains, etc.) link directly to the parent block as a child object. The third scale- view, fourth scale-view, etc. are created until the lowest scale is satisfied by our request. All elements of the lowest scale-view are spatial objects.
In this project, we have created a prototype web-based GIS using the proposed spatial data representation, and have completed system response time and storage space tests. System performance is improved throughout the experiment.