一种实时的点对点客户端在线地图动态投影方法

Study on the Real-time and Dynamic Display of Online Map Based on Point-to-point Projection

在线地图信息丰富、成本低、时效性强,逐渐成为GIS系统的重要数据源.目前,在线地图的客户端展示一般采用在线地图的坐标系统,这种方式成图速度快,但不能改变、显示坐标系统;如果采用所有像素投影转换,成图速度慢,且难以满足实时性要求.为解决该问题,提出一种实时的点对点客户端在线地图动态投影方法.首先,将地图当前显示范围转换为在线地图坐标系统下的矩形范围,根据转换后的矩形范围和客户坐标范围计算在线地图最佳显示层级和待绘制的瓦片数据单元序列.在客户端,对于每一个瓦片单元,反算地图显示坐标系统下的显示范围,确定该单元在客户端地图上显示的客户坐标矩形,并生成与客户坐标矩形同等大小的位图.对内存位图在水平和垂直方向每隔2个像元采样坐标点,将待转换的坐标点数量降低为原来的1/4,提高瓦片投影转换效率.最后,利用双线性内插算法生成位图其他位置上的像素值,从而得到完整的瓦片显示位图.该方法不仅实现了瓦片地图客户端无缝高精度投影显示,还提高了投影转换效率,具有一定的理论意义和实践价值.

Online thematic map has become an important GIS data resources owing to its abundant information,low cost and strong time effectiveness. At present,the coordinate system of online map is often adopted when the online map is integrate into other map layers. Because of not considering the projection conversion,it＇s impossible to display the online map on other map layers with different coordinate system. On the other hand,it will be time-consuming when all the pixels are projected to determine the pixel value from origin tile map. In order to solve this problem,this paper proposes a method of dynamic coordinate transformation by point-to-point projection. Firstly,transforming the display rectangle to the coordinate system of online map. According to the transformed rectangle,the best display level of tile map and the tile data unit sequence is computed. For each the unit,the client rectangle is back calculated under the display coordinate system which is used to determine the size of memory image of tile map. Then,the value of each pixel of the memory image is found from tiled map. In order to improve the efficiency of calculation,the points are resampled every two pixels in horizontal and vertical directions and bilinear interpolation is used to get the full tile data on the display map. The experimental results show that the method can realize the seamless projection display of online map under the condition of keeping the accuracy and efficiency,which has some theoretical significance and practical value.