Key technology for selecting depressions as sites of FAST-type radio telescopes
ZHU Boqin1,2(), YAN Zhaojin3(), XIE Jing3, LIU Hong4, SONG Xiaoqing3, XIANG Xiqiong4
1. National Astronomical Observatory, Chinese Academy of Sciences, Beijing 100101, China 2. Key Laboratory of FAST, Chinese Academy of Sciences, Beijing 100101, China 3. 111 Geological Brigade of Guizhou Provincial Bureau of Geology and Mineral Exploration and Development, Guiyang 550008, China 4. Key Laboratory of Karst Georesources and Environment (Ministry of Education), College of Resources and Environmental Engineering, Guizhou University, Guiyang 550025, China
The Five-hundred-meter Aperture Spherical radio Telescope (FAST), also known as Tianyan (meaning the Eye of the Sky), has attracted worldwide attention and is the largest single-dish radio telescope in the world. The joint observations of FAST and several more FAST-type radio telescopes allow detection sensitivity and resolution to be further improved and the research fields to be expanded. Therefore, Chinese radio astronomy scientists have the expectation of building more FAST-type radio telescopes in China, which should be achieved based on the preceding research on depressions as the sites of FAST-type radio telescopes. Presently, the shared digital elevation model (DEM) data enjoy intercontinental coverage and different ground resolutions. The development of computer processing technology has greatly enhanced the processing and analysis capacities of DEM data and continuously innovated the processing technologies. Moreover, relevant analyses and expressions can be simulated. Therefore, based on a comparative analysis of the structural scales of the projects of the Arecibo radio telescope and the FAST, as well as the morphological characteristics of karst depressions, this study proposed the conditions of ideal depressions as the sites of FAST-type radio telescopes. Moreover, by analyzing the resolution and data quality of shared DEM data on the Internet, it is concluded that areas with ASTER_GDEMV3 data with a resolution of 30 m are suitable as sites of large radio telescopes in provincial-level regions. In search of large-scale depressions in Guizhou Province, this study developed special modules for quantitative analyses, such as extracting the characteristic parameters of depressions and the fitting of filling, excavation, and superimposed sections, based on the ArcGIS platform and summarized the key steps to organize and apply the major tools of ArcGIS in the special modules. The results of this study determined key technology in search of large Karst depressions in provincial-level regions. Furthermore, this study proposed several issues that are noteworthy in the application.
①[焦点统计],应用 MAXIMUM 统计类型处理 DEM 数据; ②[栅格计算器],计算输出最高像元(区); ③[栅格转面],栅格高值像元转面型矢量; ④[要素转点],提取高值像元面几何中心点矢量(面内); ⑤[值提取至点],计算矢量点位置的DEM 数值; ⑥编辑整理,删除虚假最高点(山峰点)
垭口点
①编辑分水线,使得每个洼地分水岭只有一个线矢量记录,即一个洼地的分水线只是一条线,修改矢量线为最北点为起点,顺时针矢量方向; ②[沿线生成分水线洼地等深线点],以一个像素为间隔提取分水线上等间隔顺序点; ③[值提取至点],提取顺序点位置的 DEM 高程值; ④导出包含位置和高程的顺序点数据,应用 EXCEL 对顺序点高程值进行比较判断,即可得到分水线上垭口点(分水线上局部低值)、山峰点(分水线上局部高值); ⑤分类导入,获得分水线上的各个垭口点
分水线
①[流向],计算 DEM 范围每个像元最大的坡降方位(流向); ②[汇],计算和编号每个没有流向的像元或像元区,也即连续地形中的山峰、垭口、最低点区; ③[集水区],应用流向数据和汇数据,获得最大范围的集水区; ④[分区统计],通过集水区和原始 DEM 数据,选取统计类型 MINIMUM 或 MAXIMUM,即可得到集水区最小 DEM 值或最大高程值作为高程值的栅格; ⑤[栅格转面],将最值栅格转为矢量面; ⑥[消除],设定最小的流域面积和归并方式,消除小多边形; ⑦[要素转线],将流域面多边形转为矢量线; ⑧编辑整理,消除锯齿,删除虚假和不合理矢量线,即可得到完整流域的分水线
洼地等深线
①[填洼],将一定区域内的 DEM 值修改为最低高程值,使得只有高于这个高程的水才会流出洼地; ②[栅格计算器],计算填洼栅格和原始 DEM 栅格,得到各栅格点的填洼深度; ③[等值线],插值填洼深度值栅格表面,可获得填洼深度等值线,也即最低垭口以下等深度线; ④编辑整理,消除锯齿,删除虚假和不合理等深线
Tab.4 地形参数模块化提取主要工具和步骤
Fig.5 大窝凼周区主要洼地地形参数图
Fig.6 大窝凼洼地分水线高值点、鞍部点分布图
Fig.7 大窝凼洼地分水线垭口(低值)点与山峰(高值)点分布图
Fig.8 某洼地地形-球冠拟合填挖分布图
Fig.9 某洼地地形-球冠叠合剖面图
Fig.10 某洼地反射面-支撑塔-地形剖面三维显示图
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