MERIT Hydro is a new global flow direction map at 3 arc-second resolution (~90 m at the equator) derived from the version 1.0.3 of the MERIT DEM elevation data and water body datasets (G1WBM, GSWO and OpenStreetMap).

MERIT Hydro contains the output of a new algorithm that extracts river networks near-automatically by separating actual inland basins from noise caused by the errors in input elevation data. After a minimum amount of hand-editing, the constructed hydrography map shows good agreement with existing quality-controlled river network datasets in terms of flow accumulation area and river basin shape (see Figure 9a in the paper). The location of river streamlines was realistically aligned with existing satellite-based global river channel data (see Figure 10a in the paper). Relative error in the drainage area was smaller than 0.05 for 90% of GRDC gauges, confirming the accuracy of the delineated global river networks. Discrepancies in flow accumulation area were found mostly in arid river basins containing depressions that are occasionally connected at high water levels and thus resulting in uncertain watershed boundaries.

MERIT Hydro improves on existing global hydrography datasets in terms of spatial coverage (between 90N and 60S) and representation of small streams, mainly due to increased availability of high-quality baseline geospatial datasets.

You can use this web app to visualize MERIT Hydro data. The app's source code is available.

Known Problems:

• River width (Update from GWD-LR v1): Width algorithm was updated to consider sub-pixel water fraction. Now, 30 m water map is used for width calculation at 90 m resolution. Currently river width is calculated for each channel separately in braided/anabranching sections. Merged river width should be calculated.
• Water body map: There are some inconsistencies between the DEM land sea mask and wate body data (such as new islands along the coast). The quality of OpenStreeetMap water body layer is not uniform in all areas.
• Channel bifurcations: Channel bifurcation is not well represented in the current version. Each pixel is assumed to have only one downstream direction. Secondary (or multiple) downstream direction should be considered, to represent complex river networks in the delta regions, floodplains, and braided rivers.
• Underground rivers/tunnels: Major underground rivers/tunnels should be implemented to improve large-scale water balance.
• River/lake separation: Rivers and lakes need to be separated better for some applications.
• Below-sea-level areas: The areas below sea level in coastal regions are not well represented in adjusted elevation data.
• Flow direction over glaciers: Flow direction over glaciers is not well represented, because the elevation of glacier centerline is higher than glacier edge.
• Supplementary data: It would be better to add location of GRDC gauging stations, water falls, reservoirs, etc.

Data Sources:

• U-Tokyo MERIT DEM
• U-Tokyo G1WBM water body data
• OpenStreetMap water body layer
• EC-JRC Global Surface Water Occurrence
• U-Maryland Landsat forest cover data

  MERIT Hydro是一个新的全球流向图，分辨率为3弧秒（赤道上约90米），来源于MERIT DEM高程数据和水体数据集（G1WBM、GSWO和OpenStreetMap）的1.0.3版本。

  MERIT Hydro包含一个新算法的输出，该算法通过将实际的内陆盆地从输入高程数据的误差造成的噪声中分离出来，近乎自动地提取河流网络。经过最低限度的手工编辑，构建的水文地理图在流量积聚区和流域形状方面与现有质量控制的河网数据集显示出良好的一致性（见论文中的图9a）。河流流线的位置与现有的基于卫星的全球河道数据真实一致（见论文中的图10a）。90%的GRDC测站的排水面积的相对误差小于0.05，证实了划定的全球河网的准确性。流域积聚面积的差异主要出现在干旱的河流流域，这些流域含有洼地，在高水位时偶尔会连接起来，从而导致流域边界不确定。

• MERIT Hydro在空间覆盖面（90N和60S之间）和小溪流的代表性方面改进了现有的全球水文地理数据集，这主要是由于高质量基线地理空间数据集的可用性增加。

  你可以使用这个网络应用程序来可视化MERIT Hydro数据。该应用程序的源代码是可用的。

  已知问题。

  河流宽度（从GWD-LR v1更新）。宽度算法已经更新，以考虑子像素的水分量。现在，在90米的分辨率下，30米的水图被用于计算宽度。目前，河道宽度是在辫状/anabranching河段分别计算的。应该计算合并后的河流宽度。
  水体地图。DEM陆海掩码和水体数据之间存在一些不一致的地方（如沿海的新岛屿）。OpenStreeetMap水体层的质量在所有地区都不一致。
  海峡分岔。海峡分叉在当前版本中没有得到很好的体现。每个像素被假定为只有一个下游方向。应考虑第二（或多个）下游方向，以表示三角洲地区、洪泛区和辫状河道的复杂河网。
  地下河/地道。应实施主要的地下河/地道，以改善大规模的水平衡。
  河流/湖泊分离。河流和湖泊在某些应用中需要更好地分离。
  海平面以下地区。沿海地区的海平面以下区域在调整后的高程数据中没有得到很好的体现。
  冰川上的流向。由于冰川中心线的高程高于冰川边缘，所以冰川上的流向没有得到很好的体现。
  补充数据。最好增加GRDC测量站、瀑布、水库等的位置。
  数据来源。

  U-Tokyo MERIT DEM
  U-Tokyo G1WBM水体数据
  OpenStreetMap水体层
  EC-JRC全球地表水出现情况
  U-Maryland Landsat森林覆盖数据

Dataset Availability

1987-01-01T00:00:00 - 2017-01-01T00:00:00

Dataset Provider

Dai Yamazaki (University of Tokyo)

Collection Snippet

ee.Image("MERIT/Hydro/v1_0_1")

Resolution

92.77 meters

Bands Table

Name	Description	Units
elv	Elevation	meters
dir	Flow Direction (Local Drainage Direction) * 1: east * 2: southeast * 4: south * 8: southwest * 16: west * 32: northwest * 64: north * 128: northeast * 0: river mouth * -1: inland depression
wth	River channel width at the channel centerlines. River channel width is calculated by the method described in [Yamazaki et al. 2012, WRR], with some improvements/changes on the algorithm.
wat	Land and permanent water * 0: Land * 1: permanent water
upa	Upstream drainage area (flow accumulation area)	km^2
upg	Upstream drainage pixel (flow accumulation grid).	Number of upstream pixels
hnd	Hydrologically adjusted elevations, also know as "hand" (height above the nearest drainage). The elevations are adjusted to satisfy the condition "downstream is not higher than its upstream" while minimizing the required modifications from the original DEM. The elevation above EGM96 geoid is represented in meters, and the vertical increment is set to 10cm. For detailed method, see [Yamazaki et al., 2012, WRR].	meters
viswth	Visualization of the river channel width.

  使用说明：

'Citation to the paper is adequate if you simply use MERIT DEM. If you asked for help for additional handling/editing of the dataset, or if your research outcome highly depends on the product, the developer would request co-authorship.

MERIT DEM is licensed under a Creative Commons "CC-BY-NC 4.0" or Open Data Commons "Open Database License (ODbL 1.0)". With a dual license, you can choose an appropriate license for you.

To view a copy of these license, please visit:

• CC-BY-NC 4.0 license: Non-Commercial Use with less restriction.
• ODbL 1.0 license: Commercial Use is OK, but the derived data based on MERIT DEM should be made publicly available under the same ODbL license. For example, if you create a flood hazard map using MERIT DEM and you would like to provide a COMMERCIAL service based on that, you have to make the hazard map PUBLICLY AVAILABLE under OdBL license.

Note that the above license terms are applied to the "derived data" based on MERIT DEM, while they are not applied to "produced work / artwork" created with MERIT DEM (such as figures in a journal paper). The users may have a copyright of the artwork and may assign any license, when the produced work is not considered as "derived data".

By downloading and using the data the user agrees to the terms and conditions of one of these licenses. Notwithstanding this free license, we ask users to refrain from redistributing the data in whole in its original format on other websites without the explicit written permission from the authors.

The copyright of MERIT DEM is held by the developers, 2018, all rights reserved.'

'如果你只是使用MERIT DEM，引用该论文就足够了。如果你要求帮助对数据集进行额外的处理/编辑，或者你的研究成果高度依赖于该产品，开发者会要求共同作者。

MERIT DEM采用知识共享 "CC-BY-NC 4.0 "或开放数据共享 "开放数据库许可（ODbL 1.0）"进行许可。通过双重许可证，你可以选择适合你的许可证。

要查看这些许可证的副本，请访问。

CC-BY-NC 4.0许可证。非商业使用，限制较少。
ODbL 1.0许可证。商业使用是可以的，但基于MERIT DEM的衍生数据应在相同的ODbL许可证下公开提供。例如，如果你用MERIT DEM创建了一个洪水灾害地图，并且你想在此基础上提供商业服务，你必须在ODBL许可证下公开提供该灾害地图。
请注意，上述许可条款适用于基于MERIT DEM的 "衍生数据"，而不适用于用MERIT DEM创作的 "作品/艺术作品"（如期刊论文中的数字）。当制作的作品不被视为 "派生数据 "时，用户可以拥有艺术品的版权，并可以转让任何许可。

通过下载和使用数据，用户同意这些许可证之一的条款和条件。尽管有这个免费许可，我们要求用户在没有得到作者明确的书面许可的情况下，不要在其他网站上以原始格式重新发布整个数据。

MERIT DEM的版权由开发者持有，2018年，所有权利保留。

引用：

Yamazaki D., D. Ikeshima, R. Tawatari, T. Yamaguchi, F. O'Loughlin, J.C. Neal, C.C. Sampson, S. Kanae & P.D. Bates. A high accuracy map of global terrain elevations. Geophysical Research Letters, vol.44, pp.5844-5853, 2017. doi:10.1002/2017GL072874