AfSIS Web Map Service

The Africa Soil Information Service (AfSIS) Web Map Service supports the Open Geospatial Consortium (OGC) OpenGIS Web Map Service (WMS) Implementation Specifications and dynamically produces maps of georeferenced data. Support of this international standard opens the AfSIS map collection to users who can access its contents via machine-to-machine interaction.

GetCapabilities Request

The AfSIS WMS uses version 1.1.0. The full GetCapabilities document, including additional layers from the Center for International Earth Science Information Network (CIESIN), is available here:
http://ciesin.columbia.edu/geoserver/ows?service=wms&version=1.1&request...

If you would like the GetCapabilities document for AfSIS layers only (Digital Elevation Model, Specific Catchment Area, and Topographic Wetness Index), it is available at http://africasoils.net/sites/default/files/ows.

Sample GetMap Requests

Specific Catchment Area
http://ciesin.columbia.edu/geoserver/wms?service=WMS&version=1.1.0&reque...

Topographic Wetness Index
http://ciesin.columbia.edu/geoserver/wms?service=WMS&version=1.1.0&reque...

Digital Elevation Model
http://ciesin.columbia.edu/geoserver/wms?service=WMS&version=1.1.0&reque...

As these links illustrate, the WMS GetMap request: (http://www.ciesin.columbia.edu/geoserver/wms?service=WMS&version=1.1.0&r...) is followed by a list of variables (layers, styles, bbox, width, height, srs, format). The values for each of these can be specified by the user.

Additional information on open map standards can be found here:

-Web Map Service (Wikipedia)
-OGC OpenGIS Web Map Server Cookbook

Documentation on AfSIS-supported Web Coverage Services coming soon.

How to Open and View Downloaded SRTM Derivatives

The SRTM-derived base layers are available for download in GeoTIFF format and can be opened using a GIS package. The downloaded folder, which is in .tar.gz format, also contains the metadata catalog record for the selected base layer and a brief description of the AfHySrTM, AfSIS_SCA, and AfSIS_TWI datasets. Once the compressed folder has been downloaded, users will need to:

Step 1. Decompress downloaded folder

For Windows Users:

A .tar.gz file is a compressed archive, similar to a zip file or a RAR file. 7-Zip is a free file archiver that runs on Windows and can open filename.tar.gz files.
Download URL: http://www.7-zip.org/download.html
Install 7-Zip, and you will be able to open up your filename.tar.gz file.

For Linux Users:

Most Linux distributors support gzip compression.
To extract the archive filename.tar.gz into the current directory, type the following command:

tar xzf filename.tar.gz

If this fails, the version of tar may not support gzip compression. In this case, you can use the traditional two-stage command:

gzip -dc filename.tar.gz | tar xf -

Step 2. Open .tif file using a GIS package

GIS software such as QGIS (http://www.qgis.org/wiki/Download) is required to work with these files, which use a greyscale to display the data. The legends for the data layers as they appear online can be accessed here:

AfHySRTM -- Digital Elevation Model:
http://africasoils.net/sites/default/files/images/dem_legend_processed.PNG

AfSIS_TWI -- Topographic Wetness Index:
http://africasoils.net/sites/default/files/images/twi_legend_processed.PNG

AfSIS_SCA -- Specific Catchment Area:
http://africasoils.net/sites/default/files/images/sca_legend_processed.PNG

For ArcGIS users:

On average, file sizes larger than 2 GB degrades the performance of ArcMap. To increase performance to make ArcMap start and run faster, refer to:
http://resources.arcgis.com/content/kbase?fa=articleShow&d=31672

Base Maps Documentation: Shuttle Radar Topography Mission (SRTM) Derivatives

Hydrologically Corrected / Adjusted SRTM Digital Elevation Model (AfrHySRTM)

AfrHySRTM is an adjusted elevation raster in which any depressions in the source Digital Elevation Model (DEM) have been eliminated (filled), but allowing for internal drainage since some landscapes contain natural depressions. These landscapes have their own internal drainage systems, which are not connected to adjacent watersheds. Null cells (drains) were placed in depressions exceeding a depth limit of 20 m and with no less than 1000 cells (pixels) during the DEM adjustment process. After filling depressions in the DEM, flowpaths can also be generated. This dataset was produced at the World Agroforestry Centre (ICRAF) in Nairobi, Kenya, for the Africa Soil Information Service.

Metadata for the AfHySRTM dataset are available here.

Geomorphologic/Hydrologic Characteristics

1. Specifc catchment area

The Africa Soil Information Service: Specific Catchment Area (SCA) is a 90m raster dataset showing local flow accumulation and flow direction using the formula:

SCA = A/I

where A is unit contributing area of land upslope of a length of contour I. Unit flow width was calculated from the DEM cell/pixel dimensions and varies depending on whether flow direction in horizontal vertical or diagonal through the cell. The specific catchment area contributing to flow at any given location can be used to determine relative saturation and water runoff and, together with other topographic factors, can be used to model soil erosion, sediment yield, and landslide risk. The digital elevation model used to construct this dataset was AfHydSRTM, based on the CGIAR-SRTM 90m Version 4. This dataset was produced at the World Agroforestry Centre (ICRAF) in Nairobi, Kenya, for the Africa Soil Information Service.

Metadata for the AfSIS SCA dataset are available here.

2. Topographic wetness index

The Africa Soil Information Service: Topographic Wetness Index (TWI) is a 90m raster dataset showing zones of increased soil moisture where the landscape area contributing runoff is large and slopes are low. The topographic wetness index, originally developed by Beven and Kirkby in 1979, provides a measure of wetness conditions at the catchment scale. Local upslope contributing area and slope are combined to determine the wetness index:

WI = ln (A _s / tan(b) )

where As is flow accumulation or effective drainage area and b is slope gradient. Methods of computing this index differ primarily in the way the upslope contributing area is calculated. The use of effective drainage area gives a quasi-dynamic index overcomes assumptions of of steady-state (i.e. uniform soil properties). This index predicts zones of increased soil moisture where the landscape area contributing runoff is large and slopes are low and is commonly used in soil landscape modeling and in the analysis of vegetation patterns. The digital elevation model used to construct this dataset was AfHydSRTM, based on the CGIAR-SRTM 90m Version 4. This dataset was produced at the World Agroforestry Centre (ICRAF) in Nairobi, Kenya, for the Africa Soil Information Service.

Metadata for the AfSIS TWI are available here.

Click here for further details on AfSIS SRTM processing (pdf).

Pre-Set Data Downloads

For more information about these maps, see the 'about' tab (above).

-Digital Elevation Model (DEM)
-Topographic Wetness Index (TWI)
-Specific Catchment Area (SCA)

Select region for DEM Download

Northeast Africa Northwest Africa Southeast Africa Southwest Africa Kenya Malawi Mali Tanzania

Select region for TWI Download

Northeast Africa Northwest Africa Southeast Africa Southwest Africa Malawi Mali Kenya Tanzania

Select region for SCA Download

Northeast Africa Northwest Africa Southeast Africa Southwest Africa Malawi Mali Kenya Tanzania

These images have been mosaicked and are available for download as single files for each layer.

To the extent possible under law, the Africa Soil Information Service has waived all copyright and related or neighboring rights to the SRTM derivatives AfHydSRTM, AfsisTWI, and AfsisSCA.