Research On 3D Terrain Viewshed Analysis Algorithm Based On WebGL

Viewshed analysis in three-dimensional Geographic Information System plays an important role in people's perception and recognition of geographical space,and is widely used in landscape analysis,urban planning,military,games and other fields.Therefore,this paper studies the related theory of viewshed analysis based on three-dimensional terrain.In view of the shortcomings of existing algorithms on browser side,an improved algorithm based on WebGL is proposed,which can be effectively applied in browser.Combined with practical cases,the important roles of visual analysis in landscape analysis are discussed.The main research contents and conclusions are as follows:This thesis summarizes the relevant theoretical knowledge of viewshed analysis,and analyses in detail the two basic factors of viewshed analysis,namely intervisibility and viewshed.Intervisibility analysis is the basis of viewshed analysis.How to efficiently calculate the direct intervisibility between two points and the complexity of viewshed analysis algorithm is also the core point of algorithm improvement.This thesis introduces the commonly used methods.The advantages and disadvantages of four kinds of intervisibility algorithms are compared,and corresponding methods are selected according to the appropriate situation.Then three commonly used Line-of-Sight-based viewshed analysis algorithms are analyzed,and the principles of each algorithm are elaborated in detail,which lays a theoretical foundation for the improved algorithm proposed later.Based on the traditional Line-of-Sight-based viewshed analysis algorithm,an efficient improved viewshed algorithm is proposed.The improved principle mainly relies on the reuse of front-point visibility under the condition of terrain continuity,which saves a lot of calculation.At the same time,the selection of sampling points is optimized according to the line-of-sight radius to maximize the efficiency of the algorithm.The improved algorithm is compared with traditional line-of-sight algorithm and viewshed analysis in software.Viewshed analysis is carried out in different terrain areas such as mountain,hill and plain.The execution time and calculation points of the improved algorithm are counted.The experimental result demonstrates that the execution time of the improved algorithm is obviously improved and the calculation amount is greatly reduced,especially in flat terrain.The effect of the improved algorithm is particularly obvious in areas with less occlusion.The comparison of the experimental results also proves the effectiveness of the improved algorithm.Combining with the improved algorithm and using Cesium platform,a viewshed analysis system based on WebGL is constructed.There are three key technical points,including image and terrain data loading,observation point selection,visual domain calculation and display.The three key points are analyzed and programmed with Cesium related technology.Finally,using the WebGL-based viewshed analysis system and practical cases,taking Wudang Mountain as an example,five famous scenic spots are selected to carry out the viewshed analysis of landscape,and the rationality of the new landscape spots is illustrated through the viewshed analysis of the newly added points,and the important role of viewshed analysis in landscape analysis,planning and design is clarified,meanwhile the site selection of Wudang Mountain forest-fire prevention observatory are analyzed and planned.