Spherical harmonic models of the shape of Mercury
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The data used to generate these spherical harmonic models is the global digital elevation model (DEM) of Mercury, produced by the U.S. Geological Survey (USGS). The DEM was derived from from stereo image pairs (stereo photogrammetry) captured by the Mercury Dual Imaging System (MDIS) narrow-angle camera (NAC) and multispectral wide-angle camera (WAC) on board the MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft. The global DEM was downloaded throught the Astropedia catalog in geoTIFF format and equirectangular projection. Using the rasterio package, the dataset was loaded in python, scaled to the local height and radius (described in the Astropedia documentation), and exported into .dat format. Then, the file was converted into a netcdf format and resampled into a gridline registration using the generic-mapping-tools as follows:gmt xyz2grd filename.dat -Gfilename.grd -R0/360/-90/90 -I0.015625/0.015625 -ZTLd -fg -rpgmt grdsample filename.grd -Gfilename_gridline.grd -T The resulting netcdf file was read into the pyshtools software with the SHGrid.from_netcdf() and expanded into spherical harmonics using the function SHGrid.expand(). The spherical harmonic functions were chosen to be "4pi" normalized and to exclude the Condon-Shortley phase factor of (-1)m. The units of the coefficients are meters. The four files in this archive are - Mercury_shape_5759.sh.gz- Mercury_shape_2879.sh.gz- Mercury_shape_1439.sh.gz- Mercury_shape_719.sh.gz The numbers 5759, 2879, 1439, and 719 in the filename refer to the maximum spherical harmonic degree of file, which corresponds to effective spatial resolutions of 64, 32, 16, and 8 pixels per degree, respectively. The files are stored in the binary "bshc" format as described in the pyshtools documentation and are furthermore compressed using gzip. The lower resolution models were generated by truncating the spherical harmonic coefficients of the highest resolution model. The spherical harmonic coefficients can be loaded with pyshtools as follows:SHCoeffs.from_file("filename.sh.gz", format='bshc')
用于生成这些球谐模型的原始数据，来自美国地质调查局（U.S. Geological Survey, USGS）制作的水星全球数字高程模型（Digital Elevation Model, DEM）。该DEM由“信使号”（MErcury Surface, Space ENvironment, GEochemistry, and Ranging, MESSENGER）探测器搭载的水星双成像系统（Mercury Dual Imaging System, MDIS）窄角相机（narrow-angle camera, NAC）与多光谱广角相机（multispectral wide-angle camera, WAC）拍摄的立体影像对（立体摄影测量）反演得到。 该全球DEM通过Astropedia目录下载，格式为geoTIFF，投影方式为等经纬度投影。我们使用rasterio库在Python环境中加载该数据集，参照Astropedia文档说明将其缩放至局部高程与天体半径尺度，并导出为.dat格式文件。随后将该文件转换为netCDF格式，并通过通用制图工具（generic-mapping-tools）按如下命令重采样为网格线配准格式： gmt xyz2grd filename.dat -Gfilename.grd -R0/360/-90/90 -I0.015625/0.015625 -ZTLd -fg -rp gmt grdsample filename.grd -Gfilename_gridline.grd -T 将得到的netCDF文件通过pyshtools软件的SHGrid.from_netcdf()函数读取，并使用SHGrid.expand()函数展开为球谐函数形式。本次选用的球谐函数采用“4pi”归一化方式，并排除(-1)^m的康登-肖特利（Condon-Shortley）相位因子。系数的单位为米。 本归档文件共包含四个文件： - Mercury_shape_5759.sh.gz - Mercury_shape_2879.sh.gz - Mercury_shape_1439.sh.gz - Mercury_shape_719.sh.gz 文件名中的数字5759、2879、1439与719分别对应文件的最大球谐阶数，其对应的有效空间分辨率分别为每度64、32、16与8个像素。文件以pyshtools文档中描述的二进制“bshc”格式存储，并通过gzip进行压缩。低分辨率模型均通过截断最高分辨率模型的球谐系数生成。 可通过pyshtools按如下方式加载球谐系数： SHCoeffs.from_file("filename.sh.gz", format='bshc')
