Finding River Numbers

Finding Your New River Number

There are 7 million river segments modeled in the GEOGloWS V2 datasets. These numbers are unique to the TDX-Hydro dataset and are different than river numbers from GEOGloWS V1 and any other dataset.

TanDEM-X Hydro (TDX-Hydro) is a new dataset developed by the United States National Geospatial-Intelligence Agency (NGA) which includes global streams and basins derived from 12 m TanDEM-X data. These are delineated using the TauDEM software and further enhanced and conditioned with vegetation bias removal, adaptive smoothing, river enforcement, and depression handling.

GEOGloWS V2 is derived from a different source Digital Elevation Model (DEM) than GEOGloWS 1 and has significantly more modeled streams. Because of the higher resolution and changes in placements of river channels, we are using the TDX-Hydro stream numbering system. It is not possible to provide a mapping of all old Version 1 ID numbers to the new Version 2 ID. We have outlined some steps below to map an old ID to a GEOGloWS 2 ID.

Example Comparison of GEOGloWS 1 vs GEOGloWS 2 Streams

Left: GEOGloWS Version 1's stream network (similar to hydrosheds). Notice how the river centerlines do not line up exactly with the rivers in the satellite imagery. Right: GEOGloWS Version 2's stream network (derived from TDX Hydro). More streams are included and river centerlines are more aligned to the streams in the imagery. This river network is at a higher spatial resolution allowing for lines which more smoothly follow river paths compared to the more jagged or blocky looking lines on the left.

Step 1: Find the watershed group of your stream

For computation and data organization purposes, data are broken into groups of watersheds. These were referred to as "regions" in V1 and followed major watersheds and some political boundaries. There were 13 regions in V1. In V2, there are more of these groups and are identified with a 3 digit number. We call these "vector processing units" or VPU in order to share the same terminology used by other similar modeling projects. There are 125 divisions in V2. Below we have the GEOGloWS 1 regions mapped to all the applicable GEOGloWS 2 VPU numbers.

First, find the region your river is part of using the table and maps. Next, download all V2 VPUs or only the ones specifically applicable to the watersheds you need. Use the Dataset Reference page on this site to find links for dataset downloads. For example, if you had a river in the East Asia region, download the V2 VPUs 401 to 406.

GEOGloWS V1 Region Boundaries

GEOGloWS V2 VPU Boundaries

Step 2: Comparison with GIS

Load the old network into your GIS software of choice. Identify the stream in the old network, either by querying the attribute table of the network or by visually locating the stream. The image on the right shows the Central America region loaded in QGIS. A stream was selected in red using the Select tool, and zoomed in by right clicking the network name in the Layers panel and selecting "Zoom to Layer(s)"

Load the VPU(s) into the GIS. The new network will appear, and new prospective rivers should appear in the same area as the selected stream. If none appear in the area of the selected stream, it may be that you need to download another VPU region. Consult the VPU boundary map and download any additional regions. The image shows that the VPU region 718 has been loaded (orange).

In this example, it is fairly intuitive that stream should map to the new stream that is closest to it. The image to right shows the new stream selected in red that most closely matches the old stream.

By selecting the VPU region layer in the Layers panel, and then selecting the new stream using the "Identify Features Tool", we can obtain the new id. The image on the right shows the attributes of the stream. The new ID is labeled as the "TDXHydroLinkNo." Other useful attributes can be seen in this panel. 

While this process is usually straightforward, there are some areas where more work is required to correctly map an old stream to a new stream. Below on the left is the old network in blue and the new network in green, with the old stream we want to map highlighted in red. On the right, a potential new stream is highlighted in red. This appears to be a logic choice, but more inspection is warranted. By checking the attribute tables for both features, it is revealed that the drainage area upstream of the old stream is roughly 6000 sq. km, while the proposed new stream has a drainage area of about 200 sq. km, which indicates that our selection is probably incorrect.

Old stream selected

New proposed stream selected

Below, the map has been zoomed out. After carefully tracing the paths of the old stream and the new streams to the main trunk, it becomes clear that the new network routes the main river to a different outlet. This suggests that the stream selected in the right picture is the correct one. An inspection of the attribute table reveals that this new selection has a drainage area of about 5300 sq. km, which better matches our expectations. 

Old stream selected

New correct stream selected