SRTM Grid Cell Area (3" resolution) derived from 3" SRTM DEM-S

The SRTM grid cell area dataset has values of cell area in square metres.\n\nThe grid cell area product was derived from the Smoothed Digital Elevation Model (DEM-S; ANZCW0703014016), which was derived from the 1 second resolution SRTM data acquired by NASA in February 2000. \n\nThe calculation of grid cell area from the DEM-S accounted for the varying spacing between grid points in the geographic projection. \nLineage: Source data\n1.\t3 second SRTM-derived Smoothed Digital Elevation Model (DEM-S; ANZCW0703014016)\n\nArea calculation\nThe 3 second resolution data has a cell size of 0.0008333333333 decimal degrees and the 1 degree extent input tiles have 1200 rows and 1200 columns. Cell width changes with latitude (decreases as latitude increases); width is recalculated for each row in the grid based on the centre of the cell.\n\nRadius for calculating distance around a parallel (constant latitude, in radians):\n N = a / sqrt ( 1 - e^2 sin^2(lat) )\n e^2 is the square of the eccentricity, calculated from flattening (f)\n\nRadius for calculating distance along a meridian (constant longitude):\n M = a ( 1 - e^2 ) / ( 1 - e^2 sin^2(lat) ) ^ (3/2)\n The 1 - e^2 sin^2(lat) term is used in both M and N, so it is calculated as p\n\n a = 6 378 137 m (in WGS84)\n f = 1/298.257223563 (in WGS84)\n e2 = f * (2 - f)\n p = 1 - e2 * sqr(sin(lat))\n\n N = a / sqrt(p)\n M = a * (1 - e2) / (p * sqrt(p))\n\nThe length of 1 degree in longitude and latitude are:\n long_len = N * cos(lat) * PI / 180.0\n lat_len = M * PI / 180.0\n\nThe area of a 3 second grid cell is:\n lat_len * long_len / 1200^2

SRTM网格单元面积数据集的数值单位为平方米。

该网格单元面积产品源自经平滑处理的数字高程模型（Smoothed Digital Elevation Model, DEM-S; ANZCW0703014016），而此DEM-S数据由美国国家航空航天局（National Aeronautics and Space Administration, NASA）2000年2月获取的1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数据计算网格单元面积时，已充分考虑地理投影下网格点间距的非均一性。

数据谱系：源数据
1. 3秒分辨率SRTM衍生平滑数字高程模型（DEM-S; ANZCW0703014016）

面积计算
该3秒分辨率数据的单元尺寸为0.0008333333333十进制度，1度范围的输入瓦片包含1200行与1200列。网格单元的宽度随纬度变化（随纬度升高而减小），需基于单元中心为网格的每一行重新计算宽度。

用于计算纬线（恒定纬度）周边距离的半径（单位：弧度）：
N = a / √(1 - e² sin²(lat))
其中e²为偏心率平方，由扁率（f）计算得到。

用于计算经线（恒定经度）方向距离的半径：
M = a(1 - e²) / (1 - e² sin²(lat))^(3/2)
上述公式中的1 - e² sin²(lat)项在M和N的计算中均会用到，因此将其记为p。

参数定义：
a = 6378137 米（世界大地测量系统1984, World Geodetic System 1984, WGS84）
f = 1/298.257223563（世界大地测量系统1984, World Geodetic System 1984, WGS84）
e² = f*(2 - f)
p = 1 - e²·sin²(lat)

N = a / √p
M = a(1 - e²) / (p·√p)

经度方向1度的长度与纬度方向1度的长度分别为：
long_len = N·cos(lat)·π / 180.0
lat_len = M·π / 180.0

3秒分辨率网格单元的面积计算公式为：
lat_len · long_len / 1200²