Global 30 Arc-Second Elevation Data Set (GTOPO30)

GTOPO30 is a global digital elevation model (DEM) with a horizontal grid spacing of 30-arc seconds (0.008333333333333 degrees or approximately 1 kilometer), resulting in a DEM having dimensions of 21,600 rows and 43,200 columns. The horizontal coordinate system is decimal degrees of latitude and longitude referenced to World Geodetic System 84 (WGS84). The vertical units represent elevation in meters above mean sea level. The elevation values range from -407 to 8,752 meters. In the DEM, ocean areas have been masked as no data and have been assigned a value of -9999. Lowland coastal areas have an elevation of at least 1 meter (so in the event that a user reassigns the ocean value from -9999 to 0 the land boundary portrayal will be maintained). Small islands in the ocean less than approximately 1 square kilometer are not represented.

GTOPO30 was derived from several raster and vector sources of topographic information. These sources include: Digital Terrain Elevation Data, Digital Chart of the World, USGS 1-degree Digital Elevation Models, Army Map Service 1:1,000,000-scale Maps, International 1:1,000,000-scale Map of the World, Peru 1:1,000,000-scale Map, New Zealand DEM, and Antarctic digital Database.

GTOPO30 was developed to meet the needs of the geospatial data user community for regional and continental scale topographic data. The data are suitable for many regional and continental applications, such as climate modeling, continental-scale land cover mapping, extraction ofdrainage features for hydrologic modeling and geometric and atmospheric correction of medium and coarse resolution satellite image data.

An example of a recent application derived from GTOPO30 is HYDRO1k, a geographic database (at a resolution of 1 km) developed to provide comprehensive and consistent global coverage of topographically derived data sets, including streams, drainage basins, and ancillary layers . HYDRO1k provides a suite of geo-referenced data sets, both raster and vector, which will be of value for all users who need to organize, evaluate, or process hydrologic information on a continental scale. The raster data sets are the hydrologically correct DEM, derived flow directions, flow accumulations, slope, aspect, and a compound topographic (wetness) index. The derived streamlines and basins are distributed as vector data sets.

GTOPO30 was developed through a collaborative effort led by staff at the U.S. Geological Survey's EROS EDC. The following organizations participated by contributing funding or source data: the National Aeronautics and Space Administration (NASA), the United Nations Environment Programme/Global Resource Information Database (UNEP/GRID), the U.S. Agency for International Development (USAID), the Instituto Nacional de Estadistica Geografica e Informatica (INEGI) of Mexico, the Geographical Survey Institute (GSI) of Japan, Manaaki Whenua Landcare Research of New Zealand, and the Scientific Committee on Antarctic Research (SCAR).

GTOPO30是一款全球数字高程模型（digital elevation model, DEM），水平网格间距为30角秒（即0.008333333333333度，约1千米），对应数据集的维度为21600行与43200列。其水平坐标系采用以世界大地测量系统84（World Geodetic System 84, WGS84）为基准的经纬度十进制度数，垂直单位为平均海平面以上的海拔高度（米），高程值范围介于-407至8752米之间。该DEM中，海洋区域被标记为无数据，并被赋值为-9999；沿海低地的海拔至少为1米（因此若用户将海洋区域的赋值从-9999改为0，仍可维持陆地边界的正确显示）。面积小于约1平方千米的海洋中小岛未被纳入该数据集。

GTOPO30源自多套栅格与矢量地形数据源，具体包括：数字地形高程数据、世界数字海图、美国地质调查局1度分辨率数字高程模型、陆军地图服务1:100万比例尺地图、国际1:100万世界地图、秘鲁1:100万比例尺地图、新西兰数字高程模型以及南极数字数据库。

GTOPO30的开发旨在满足地理空间数据用户社区对区域及大陆尺度地形数据的需求。该数据集适用于诸多区域与大陆级应用，例如气候建模、大陆尺度土地覆盖制图、水文建模所需的水系特征提取，以及中低分辨率卫星影像数据的几何与大气校正。

近期基于GTOPO30开发的典型应用为HYDRO1k——一款分辨率为1千米的地理数据库，旨在提供覆盖全球的、一致的地形衍生数据集，包括水系、流域及辅助图层。HYDRO1k提供一套地理参考数据集（涵盖栅格与矢量格式），可助力所有需要在大陆尺度上组织、评估或处理水文信息的用户。其中栅格数据集包括水文校正后的DEM、流向提取结果、流量累积量、坡度、坡向以及复合地形（湿度）指数；提取得到的水系与流域以矢量数据集形式分发。

GTOPO30由美国地质调查局地球资源观测与科学中心（EROS EDC）的工作人员牵头，通过合作开发完成。以下机构通过提供资金或源数据参与了该项目：美国国家航空航天局（National Aeronautics and Space Administration, NASA）、联合国环境规划署/全球资源信息数据库（UNEP/GRID）、美国国际开发署（U.S. Agency for International Development, USAID）、墨西哥国家地理统计与信息研究所（Instituto Nacional de Estadistica Geografica e Informatica, INEGI）、日本国土地理院（Geographical Survey Institute, GSI）、新西兰Manaaki Whenua Landcare Research机构，以及南极研究科学委员会（Scientific Committee on Antarctic Research, SCAR）。