Africa Soil Information Service - Map Tool
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.
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:
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
Topographic Wetness Index
Digital Elevation Model
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:
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.
For Linux Users:
Most Linux distributors support gzip compression.
tar xzf filename.tar.gzIf 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:
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:
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 (ï¬lled), 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 ï¬lling depressions in the DEM, ï¬‚owpaths can also be generated. This dataset was produced at the World Agroforestry Centre (ICRAF) in Nairobi, Kenya, for the Africa Soil Information Service.
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.
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.
Pre-Set Data Downloads
For more information about these maps, see the 'about' tab (above).
-Digital Elevation Model (DEM)
Select region for DEM Download
Select region for TWI Download
Select region for SCA Download
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.