by Caitlin Scopel, Cartographic Product Engineer, Esri
The Hydro Basemap is now the WORLD Hydro Basemap. The World Hydro Basemap uses the same design as its predecessor, but extends the coverage to the World at scales of 1:147 million to 1:2.3 million. The World Hydro Basemap includes the map tiles from the original Hydro Basemap, providing coverage of the United States down to the 1:18,000 scale.
The World Hydro Basemap is comprised of two map service layers: the World Terrain Base, and the World Hydro Reference Overlay. The World Terrain Base provides the elevation context necessary to understand why the world’s surface water flows where it does.
The World Hydro Reference Overlay provides the streams, waterbodies, drainage areas, and cultural reference information necessary to make the World Hydro Basemap useful for scientists, professionals and researchers in the fields of Hydrology, Geography, Climate, Soils, and other natural sciences related to water.
Following the ‘Map Sandwich’ design, The World Hydro Basemap contains the outer “bread” layers to which thematic water-related map services can easily be sandwiched in between, resulting in a beautiful and legible hydro map mash-up.
World Hydro Reference Overlay
The World Hydro Reference Overlay is a cached PNG32 web map service. By creating the cached overlay in PNG32, transparency can be used to allow layers to be displayed under the overlay. The overlay symbolizes streams by the relative annual flow of surface water, so that stream symbols of the same size and color represent roughly the same annual flow volume. For example, the map below shows us that most of the surface water flowing in streams is in South America.
Or, that there is more water flowing in the Amazon than there is in the Congo.
Using this technique to visualize our Earth’s surface water produces an interesting view that has never been seen before in a dynamic multi-scale map.
How We Did It
Thanks to the great work done by NASA, the World Wildlife Fund, and USGS, we had many of the building blocks necessary to start the map: synthetic stream lines, waterbodies, and drainage areas derived from digital elevation models from the Shuttle Radar Topography Mission (SRTM) and HydroSHEDS projects. However, these layers had no attributes associated with them, not even names. So, over the last 6 months, countless hours were spent manually adding names and alternate names from the GeoNames database so that every stream had a regionally correct name, along with an English version as an alternate name. A flow class attribute was calculated for each stream segment so that we could symbolize our streams in a common way.
Here are some more details:
- River Geometry: For all areas South of 60 degrees Latitude, we used synthetic stream networks derived from HydroSHEDS (Hydrological Data based on Shuttle Radar Topography Mission (SRTM) Elevation Derivatives). North of 60 degrees Latitude we used Hydro1K data, which is based on the GTOPO30 Global 30 arc-seconds Topographic Data.
- River Name Attribution: Names and Alternate Names are from the GeoNames database, retrieved on October 1, 2011.
- Waterbodies: A by-product of the data editing performed by National Geospatial Intelligence Agency (NGGA) to produce the finished SRTM Digital Terrain Elevation Data, the waterbodies were captured from othorectified images from the February 2000 SRTM flight. These data were supplemented with lakes, reservoirs and double line streams from the Global Lakes and Wetlands Database (GLWD) and on-screen digitization of water areas from ESRI’s imagery service. Where available, names from the geonames database were added as attribution.
- River Flow Analysis: To derive a flow-centric map, the long term average discharge values from the Major Rivers of the World dataset, created by the Global Runoff Data Center (GRDC), were associated with the river geometry. For the tributaries to these major rivers, derived flow values were calculated based on the GRDC global runoff and precipitation grids. The WWF Global 2000 Terrestrial Eco Regions combined with the Koppen climate classification were used to ascertain if the river network density obtained from the runoff calculations was appropriate for particular geographies.
All of our hard work has led to a beautiful and interesting map of the world’s surface water. We’ve created the foundational piece required to frame other map services, like global precipitation, evapotranspiration, available water, networks of stream sensors, wetlands, drought, floods, and much more. But, to create the best basemap possible, we need additional coverage at larger scales. Using the data we have, our team plans to extend the global coverage down to 1:1 million by the end of this summer. In addition, we will make available a set of templates that will walk you through several workflows allowing you to start with a Digital Elevation Model and end with cached map tiles of your area, which you can then contribute to the World Hydro Basemap to expand its coverage. We hope that one day the World Hydro Basemap will become a community map, built by countries around the world that contribute their local surface water geography and flow information.
Other web mapping services available:
Special thanks to Caitlin Scopel for providing this post. Questions for Caitlin: CScopel@esri.com