Modeling and Expression of Vector Data in the Hexagonal Discrete Global Grid System
The Discrete Global Grid System (DGGS) is a new type of global spatial data model and is the extension of the plane grid on a sphere. The hexagon is usually used in the construction of DGGS for its advantageous geometric structure. The paper principally focuses on the issue of modeling and expression of vector data in the hexagon DGGS. The precision of vector data is the basis of data recording and data expression, and data with different precision fall into the grid cells of corresponding sizes, making the gridding data themselves contain the precision and scale information. The present method of data recording is reserved, as far as possible, in the data recording process, and only the geometric information of vectors is substituted by the one-dimension coding of grids. This approach is more simple and effective than the digital coordinate recording method. The gridding expression of vector data differs from the traditional technique, mainly due to the subdivision of the durative space by grids as well as the obedience of the subdivision special rules, among which the point expression should activate the corresponding grid cells in the light of the point coordinates. Linear expression should activate the corresponding grid cells of every coordinate as well as the connected grids between every two node cells, and area expression should express both the boundary and internal regions by virtue of grid cells. For spherical expression, vector data have to solve the cell filling problem, but also the extension from planum to sphere. This paper puts forward a reasonable sphere extension approach, in which the vector data expression on the spherical grids was accomplished by the dismantling of vector data on different extended areas and the multi-times transformation. Besides, the algorithm in connection with the vector data was verified through experiments for its effect and efficiency. Moreover, the distance and direction of vector data on the grids would change in the mapping process from planum to sphere girds, leading to an inaccurate spherical gridding expression. So, the effects on the rectilinear direction in grids of the hexagon from the planum-sphere mapping process was investigated, and accuracy control of the spherical expression was processed to make sure that the drawing error of the spherical grids for vector data should be limited within one cell.