Map Symbol Representation And Map Exporting Based On Run-Length Encoding

The study of cartographic representation concentrates mainly on storage structure of map in the computer. To display the map, it is necessary to representation map as grating structure. While current GIS systems mostly take advantage of vector symbol library to accomplish symbolization for vector data, it is undoubtedly require a lot of computing when conversion between raster and vector. Thus, the exporting efficiency is obviously decreased, which is unacceptable. The main cause of this problem is the dual structure of cartographic representation during map generating. There are completely different structure for symbolization and rasterizing, though the relation of symbolization is easy to complete, but the exporting rate is limited. For this reason, a new scheme for cartographic representation is proposed in this paper, which execute symbolization accomplish with rasterizing to avoid time consume on vector-grid conversion. While expressing the result of symbolization directly using raster structure has obvious deficiencies in data storage and data accuracy, the method of Run-Length encoding can compensate for this. As an important method for raster data compression, Run-Length encoding settles the problem of highly rely on memory for raster data. Compared to vector method, spatial operation based on Run-Length encoding is more efficiency. It is convenient for Run-Length decoding, thus, less computation is required to complete rasterizing. On the basis of above analysis, the method of express symbolized map using Run-Length encoding is put forward. That is, symbolization and Run-Length expression are executived simultaneously. Therefore, the efficiency of map exporting is improved fundamentally. The research is developed from the following aspects:1. Established a kind of Run-Length encoding link data structure oriented to cartographic and simplified the run-length operation. Map symbol is organized as run-length block, and some run-length block make a run-length layer. Finally, several run-length layers come to be a map. Thus, map is completed expressed by the nested run-length structure. Only through six "split" pattern, coordinate with different attributes, it is easy to achieve all kinds of operation.2. Design the run-length expression algorithm for point symbol, linear symbol and area symbol. The run-length expression of point symbol is inserting run-length symbols at locating points. For linear symbol, vertical superposition method is utilized to decomposed symbol. Then run-length buffer area is established separately according to positioning line of each element. Finally, the symbolization result is generated by superimposing of buffers. Run-length expression for area symbol is based on the above method. Overlay algorithm between point and polygons is used to select points inside the polygon region, and then call the method of point symbol run-length expression to achieve point symbol stuffing.3. Based on run-length operation of "update", provide the algorithm of run-length layers overlay. Thus, it is easily to complete multi-layer exporting. The screen output process for run-length represented map is introduced. Followed is the discussion for speeding up of redraw of cartographic. At last, take BMP file for example to explain the hard copy exporting for run-length map.The results show that symbolization can be replaced by run-length expression, especially the use of run-length buffer to achieve linear symbolization has advantage in efficiency and appearance. Based on run-length spatial operation, the run-length represented map can finish all kinds of manipulation. The final test further proof that RLE approach can reduces data redundancy. The advantages of the approach in speed and memory consumption make it highly suitable for dealing with high precision map output.