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  At the present stage of developing HydroSHEDS, the final data quality has not been evaluated systematically. Yet preliminary comparisons with other global hydrographic data sets indicate the following:

• Generally, HydroSHEDS shows significantly better accuracy than HYDRO1k, a global hydrographic data set at 1-km resolution (USGS 2000), due to HydroSHEDS being based on a superior digital elevation model.

• Generally, HydroSHEDS shows significantly better accuracy than the river layer of ArcWorld (even in difficult areas) as ArcWorld has been incorporated in the conditioning process of HydroSHEDS.

• Generally, HydroSHEDS shows better accuracy than DCW. However, the accuracy of both data sets varies by location. In some regions where HydroSHEDS is particularly susceptible to errors, such as vegetated floodplains, the quality of DCW can be superior to HydroSHEDS.

• Generally, HydroSHEDS does not reach the accuracy of high-resolution local river networks as depicted in existing maps or remote sensing imagery. The user is thus encouraged to further improve HydroSHEDS through incorporating local information.

  Typically, river network products derived from digital elevation surfaces are susceptible to various errors, foremost in flat regions without well-defined relief. Additionally, the quality of HydroSHEDS depends on the characteristics of the SRTM-based elevation model. Being a radar product, SRTM elevation values are influenced by vegetation and other surface effects, such as roughness, wetness, low backscatter signal at open water surfaces and radar shadow (Freeman 1996). At this point, known sources of errors in HydroSHEDS include:

• Areas of low or not well-defined relief, including lake surfaces.

• Varying vegetation cover, particularly in areas of low relief, e.g. large river floodplains. The radar signal is, at least partly, reflected from atop and within the vegetation cover and the returned signal is thus a complex mix of land surface elevation and vegetation height.

• Low-relief coastal areas, in part due to the barrier effect of mangroves.

• Large-scale roads or clearings in vegetation of low-relief areas. The lack of vegetation causes artificial depressions in the elevation surface.

• Rivers less than 90 m wide enclosed by riparian vegetation. The vegetation effect can cause the river channel to appear slightly elevated.

• Braided rivers and deltas. The use of the single flow direction algorithm does not allow for depiction of river bifurcations.

• Narrow gorges. If the gorges are less than 90 m wide, they can appear closed on the elevation surface at 3 arc-second resolution.

• Inland sinks and depressions. These are often ambiguous or temporary in nature. Additionally, in karst areas flow paths are not necessarily terminated at sinks due to possible underground connectivity, and artificial depressions like large-scale mining may have flow bypasses.

• Elevated “barriers” in the elevation surface that in reality have no effect on flow continuity (e.g. bridges, high-density housing areas).

• Areas of no-data voids in the original SRTM data. The larger the void, the more uncertain is the filled surface (see 3.2).

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