Some examples in Africa

Within the FAO AFRICOVER project proposal aiming to the production of new land cover maps of Africa, a study was done in collaboration with ESA for assessing the usefulness of ERS SAR for this activity. As a result ERS SAR was found beneficial as additional information source during the rainy season when no optical data can be acquired. During the dry season SAR data can complement optical data and may allow to define additional land use classes.
In the region of the tropical rain forest optical data is difficult to obtain because of the atmospheric conditions. SAR images have proofed to provide all necessary information to monitor the forest coverage and its changes.

The Fouta Djallon/Guinea area has been choosen as first study area, it is located at 11°42' N, 10°37' E.
There are three main land use classes in the area:
- Lateritic crust, locally called Bowals, an open herbaceous layer, used only for grazing, The crust
dries out rapidly after the rainy season.
- Natural vegetation: areas of high and of low shrub
- Permanent/semipermanent agricultural fields, or fields with a long fallow period.
The figure shows an ERS-1 SAR multitemporal image composed of
- orbit 15902 frame 3392 of 31 July 1994 displayed in red
- orbit 14556 frame 3392 of 28 April 1994 in green
- orbit 6752 frame 3393 of 30 October 1992 in blue
The SAR image gives a good impression of the topography of the area. The Bowals are shown in blue and are found on the higher plateaux in the upper part of the scene. Shrub is represented in green and the fields under cultivation in brown, red or yellow.
The combined analysis of optical and radar data allows to map not only the main land use but also to distinguish between fields permanently used and fields with long fallow periods. Moreover the burned area in the Bowals can be mapped.

This figure shows a LANDSAT TM (Path 201 row 53 of 24 April 1994 bands 4,3,2).
The false-colour composite shows the Bowals in green, the shrub in red and the fields in bright bluish. Only images from the dry season are available, when most fields are harvested. For this reason no further separation can not be done.

Click here compare both radar and optical images.
The second study area is Kananga/Zaire, its coordinates are approximatively 5°47'S, 22°12'E.
The area is located approximately in the Central part of Zaire. The main land form is an undulating landscape, with elevation not exceeding 200 - 300 m. Average rainfall is of 2,000 mm/year. The main land cover feature is the moist forest, formed by different strata of deciduous and evergreen trees. A natural savanna mixed with thin lines of gallery forest can also be recognized.
Agricultural fields can be subdivided in short or long fallow. Generally, short fallow fields are located in areas where all trees have been cut. Long fallow ones (shifting cultivations) are located in areas of closed arboreal and scrub savanna

LANDSAT MSS 177/64 of 5 June 1989, bands 4,3,2
The forested area is shown in red, open savanna in orange and bluish.

ERS-1 SAR multitemporal image of
- orbit 15229 frame 3717 of 14 June 1994 displayed in red
- orbit 16291 frame 3717 of 27 Aug. 1994 in green
- 1st principal component of both dates (artificial channel) displayed in blue
The topography is well outlined. With the radar illumination from the right it is clear that the brightly imaged forest is restricted to the valleys, the higher plateau appear dark (smooth in terms of surface roughness) and hence only vegetated with low shrub or herbs. With respect to the 5 years older LANDSAT TM data there seems little changes in the land cover. In the SAR image the river seems to disappear in certain points. It can be assued that this is caused by rapids which represent rough water and hence are imaged much brighter than calm flowing water.
The result of the interpretation of both images is that all land cover units mapped in optical images are also clearly visible in the Radar images, with exception of the differentiation between closed and open forest. However the degradation of a forest cover could be assessed by mapping the patches of cuttings, which can be distinguished easily.
The study also stated that for the land use classes considered by the AFRICOVER Project monotemporal SAR image seems to be adequate.

Click here to compare both radar and optical images.

(FAO-AGRT, Rome and ERS Data Utilization Section ESA/ESRIN)

Keywords: ESA European Space Agency - Agence spatiale europeenne, observation de la terre, earth observation, satellite remote sensing, teledetection, geophysique, altimetrie, radar, chimique atmospherique, geophysics, altimetry, radar, atmospheric chemistry