Progressive Transmission Method For Vector River Networks

Progressive transmission is a new type of network transmission mode on spatial data. It is an important research topic in GIS at present. Progressive transmission achieves that transmission synchronize with display. It not only follows the cognitive law of human beings from coarse to fine form, but also shortens the waiting time for users, and realizes the interaction with users. It is an effective method to solve the contradiction between the limited network bandwidth and transmission and display of massive data.Geographic information systems exist two types of spatial data, i.e. raster and vector. The data structure of raster data is relatively simple. Progressive transmission based on raster data studies have been relatively mature, and achieved remarkable results. Vector data has its own diversity and complexity. The research of progressive transmission is not yet mature. There are still many problems need to be solved.Study on progressive transmission of vector data mainly focuses on three aspects that including progressive transmission frame, vector data generalization, data organization method based on multi resolution. This paper carried on the thorough analysis and research from the angle of compression and organization of curve with the river elements as the research object, propose a method for curve compression and data organization, based on the theory that utilize axis to divide bend, which applies to progressive transmission. The basic idea is that, first of all, the endpoint connection as the initial axis of the curve, according to the screening threshold of the corresponding axis to screening bends and extract node and the feature points are maintained. Later, according to the type of combination of adjacent bends to generate new axes and extract nodes from the screening of bending. After that, it utilize multi tree to record extraction data, and adjust the multi tree in order to ensure the rationality of curve profile. Finally, in order to improve the efficiency of data retrieval, storage node data linearly. The method takes into account the hierarchical characteristic of curve from the angle of composite bends. It not only maintains the topological relation of spatial elements, but also realizes multi scale expression of vector river network in geometric detail layer.Multi scale expression of river network in target layer is based on river-tree, using the square root rule model to calculate the number of selected river, and comprehensive index which comprehensive consideration of three indexes including river length, grade and level was the importance coefficient of river selection.Based on the above achievements, this paper realized a progressive transmission experimental system of vector river network based on C/S model. Combined with the multi-scale representation of target layer and geometric details layer, the river network was transmitted and displayed hierarchically. Experimental analysis shows that the method of river curve compression and multi scale expression which based on the curve axis and bends as the classification standards can better maintains river curves shape of geometric details layer when it was displayed progressively. In this paper, the progressive transmission method that was proposed reduces the amount of data transmission and improves the efficiency of visualization, which can better support for progressive transmission of vector river network.