Part 1: GPS Telemetry Detection Rates (Positive Openness), GCS NAD83 (2015)

Openness is an angular measure of the relationship between surface relief and horizontal distance. For angles less than 90 degrees it is equivalent to the internal angle of a cone with its apex at a DEM location, and is constrained by neighboring elevations within a specified radial distance. 480 meter search radius was used for this calculation of positive openness. Openness incorporates the terrain line-of-sight or viewshed concept and is calculated from multiple zenith and nadir angles-here along eight azimuths. Positive openness measures openness above the surface, with high values for convex forms and low values for concave forms (Yokoyama et al 2002). We calculated positive openness using a custom python script, following the methods of Yokoyama et. al (2002)using the USGS National Elevation Dataset as input.

开阔度是表征地表起伏与水平距离间关系的角度量度指标。当角度小于90°时，该指标等价于以某数字高程模型（Digital Elevation Model，DEM）点位为顶点的圆锥内角，其取值受指定径向范围内相邻高程值约束。本次正开阔度计算采用480米的搜索半径。开阔度融合了地形视线通视或视域（viewshed）概念，基于多组天顶角与天底角计算得到，本次计算沿8个方位角开展。正开阔度用于衡量地表上方的开阔程度：凸状地貌对应较高的开阔度值，凹状地貌则对应较低的开阔度值（Yokoyama等，2002）。本次研究参照Yokoyama等（2002）提出的方法，通过自定义Python脚本完成正开阔度计算，输入数据为美国地质调查局（United States Geological Survey，USGS）国家高程数据集（National Elevation Dataset）。