Aspect derived from 1" SRTM DEM-S

Aspect measures the direction in which a land surface slope faces. The direction is expressed in degrees from north.\n\nThe aspect products were derived from the Smoothed Digital Elevation Model (DEM-S; ANZCW0703014016), which was derived from the 1 arc-second resolution SRTM data acquired by NASA in February 2000. The calculation of aspect from DEM-S accounted for the varying spacing between grid points in the geographic projection. \n\nThe aspect data are available at 1 arc-second and 3 arc-second resolution\n\nThe 3” resolution version of the aspect product has been masked by the 3” water and ocean mask datasets.\nLineage: Source data\n1.\t1 arc-second SRTM-derived Smoothed Digital Elevation Model (DEM-S; ANZCW0703014016).\t\n2.\t1 arc-second aspect product\n3.\t3 arc-second resolution SRTM water body and ocean mask datasets\n\nAspect calculation\nAspect was calculated at 1 arc-0second resolution from DEM-S using the finite difference method (Gallant and Wilson, 2000). The different spacing in the E-W and N-S directions due to the geographic projection of the data was accounted for by using the actual spacing in metres of the grid points calculated from the latitude.\n\nThe 3 arc-second resolution version of the aspect product was derived from the 1 arc-second slope and aspect products by reconstructing the x and y components of the surface normal vector, averaging those components, then calculating aspect from the aggregated components. \n\nThe aspect calculation was performed on 1° x 1° tiles, with overlaps to ensure correct values at tile edges. The data were masked using the 1" or 3” resolution water and ocean mask datasets.\n\nReferences\nGallant, J.C. and Wilson, J.P. (2000) Primary topographic attributes, chapter 3 in Wilson, J.P. and Gallant, J.C. Terrain Analysis: Principles and Applications, John Wiley and Sons, New York.

坡向（Aspect）指的是地表坡面的朝向方位，该方位以正北为基准，以角度值进行表示。

本坡向数据集源自平滑数字高程模型（Smoothed Digital Elevation Model，简称DEM-S；编号ANZCW0703014016），而该平滑DEM则由美国国家航空航天局（National Aeronautics and Space Administration，NASA）于2000年2月获取的1弧秒分辨率航天飞机雷达地形测绘任务（Shuttle Radar Topography Mission，SRTM）数据生成。从DEM-S计算坡向时，已充分考虑地理投影下网格点间距的变化差异。

本次发布的坡向数据包含1弧秒与3弧秒两种分辨率版本。

3弧秒分辨率的坡向数据集已通过对应分辨率的水体与海洋掩膜数据集完成掩膜处理。

数据溯源：原始源数据
1. 基于1弧秒SRTM数据生成的平滑数字高程模型（DEM-S；ANZCW0703014016）
2. 1弧秒分辨率坡向数据集
3. 3弧秒分辨率SRTM水体与海洋掩膜数据集

坡向计算方法：以DEM-S为数据源，采用有限差分法（finite difference method，Gallant与Wilson，2000年）计算得到1弧秒分辨率的坡向数据。针对数据地理投影导致的东西向与南北向网格间距差异，计算时通过基于纬度推导得到的网格点实际米级间距进行了修正。

3弧秒分辨率的坡向数据集则由1弧秒分辨率的坡度与坡向数据集生成：先重构地表法向量（surface normal vector）的x、y分量，对分量取平均值后，再基于聚合后的分量计算得到坡向。

坡向计算以1°×1°的瓦片为单元进行，且设置了重叠区域以确保瓦片边缘的计算结果准确。数据处理过程中，分别采用1弧秒或3弧秒分辨率的水体与海洋掩膜数据集完成掩膜操作。

参考文献
Gallant, J.C. 与 Wilson, J.P.（2000年）《主要地形属性》，收录于Wilson, J.P. 与 Gallant, J.C. 所著《地形分析：原理与应用》第三章，John Wiley and Sons出版社，纽约。