This section provides only a brief overview of the data sources, development and quality of the HydroSHEDS database.
For more information please refer to the Technical Documentation.
HydroSHEDS is derived primarily from elevation data of the Shuttle Radar Topography Mission (SRTM) at 3 arc-second resolution. SRTM data were used in different versions and processing stages, including SRTM-3 unfinished data; DTED-1 finished data; and CGIAR void-filled SRTMv2 data. A variety of auxiliary datasets were used for reference and quality control, including the SRTM Water Body Data (SWBD); the river network of the Digital Chart of the World (also known as VMAP0); the Global Lakes and Wetlands Database (GLWD); and several other global and regional digital river maps.
The original SRTM data were mosaicked and merged and were then hydrologically conditioned using a sequence of automated procedures. Existing methods of data improvement and newly developed algorithms were applied, including: void filling; sink identification; sink filling; filtering (removal of outliers and smoothing of elevation surface); stream burning; and carving, weeding, and molding techniques to remove artifacts (barriers), e.g. along valley courses and in coastal areas. Manual corrections were applied in an iterative process by comparing the results to existing river maps and remote sensing imagery. In a final step, the results were upscaled from the native 3 arc-second resolution to a variety of coarser resolutions through an automated, newly developed upscaling algorithm.
Preliminary quality assessments indicate that the accuracy of HydroSHEDS significantly exceeds that of existing global watershed and river maps. However, users must be aware of certain characteristics of the data, such as resolution, accuracy, method of production, and resulting artifacts, in order to be able to judge its suitability for a specific application. As a global product, HydroSHEDS does not reach the accuracy of high-resolution local river networks (e.g. national 1:50,000 hydrographic maps).The quality of HydroSHEDS mostly depends on the characteristics of the SRTM-based elevation model. Being a radar product, SRTM elevation values are influenced by vegetation and other surface features which affect hydrographic derivatives. Known regions prone to errors in HydroSHEDS include:
- Areas of low or not well-defined relief (including lake surfaces)
- Areas with varying vegetation cover and low-relief topography (e.g. large river floodplains)
- Low-relief coastal areas (in part due to the effect of mangroves)
- Large-scale roads or clearings in the vegetation cover of low-relief areas (causing artificial depressions in the elevation data)
- 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 a single flow direction per pixel does not allow to properly represent river bifurcations)
- Narrow gorges (if a gorge is less than 90 m wide, it can appear closed on the elevation surface)
- Inland sinks, depressions, and karst features such as sinkholes (these are often ambiguous or temporary in nature)
- Elevated “barriers” in the elevation surface that in reality have no effect on flow connectivity (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)