VEGETATION - 2000

Lake Maggiore - Italy, 3-6 April 2000

A new vegetation map of Central Africa
Update of the JRC-TREES map of 1992 with SPOT-VEGETATION imagery of 1998

Herman Eerens, Bart Deronde & Jan Van Rensbergen
Centre of Expertise on Remote Sensing and Atmospheric Processes (Vito - TAP)
Boeretang 200, B-2400 Mol, Belgium.
Tel:(+32) 14 336844 Fax:(+32) 14 322795
Internet: http://www.vito.be E-mail: jan.vanrensbergen@vito.be

Paper (pdf file, 842 k)

In the frame of a short-term feasibility study (2.5 man-months), the land cover in the Central-African region was mapped with recent imagery of the 1km²-resolution sensor SPOT4-VEGETATION. The work was performed on behalf of METAFRO-InfoSys, the information system of the Belgian Royal Museum of Central Africa. The image classification was calibrated with information from the well-known TREES-map, which was established by the EU-Joint Research Centre on the base of (mainly) NOAA-AVHRR imagery of 1992. In this light, the actual map should not be considered as a new, stand-alone product, but rather as an update of this TREES-map.

The used VGT-imagery comprised the 36 decadal syntheses ("VGT-S10" products) ranging from April 1998 until March 1999. This yearly image set was pre-processed in the following way. First, for each decade, the Red and NIR reflectances were combined into a modified version of the "Soil Adjusted Vegetation Index" (SAVI), which is less sensitive to variations in the reflectance of the soil background than the classical NDVI. In order to remove the cloud perturbations, the SAVI time series were then submitted to a cleaning procedure and from the smoothed curves, monthly mean SAVI-values were computed. Finally, these monthly profiles were used to derive "phenological" images, which quantify the general shape of each pixel's growth curve by means of parameters such as: the annual SAVI-mean, -extremes and -range, a seasonality index (mean-weighted amplitude), the start and length of the growing season, etc.

The image classification was realized by means of a Maximum Likelihood algorithm, applied on a subset of these phenological images and supervised with training areas selected from the JRC-TREES map of 1992. The resulting land cover map was embellished with vector information (boundaries, rivers and roads) and plotted on scale 1:4.000.000. Statistical tables with the acreage distribution of the land cover classes were also derived for the national and regional levels, and for both years (1992: TREES vs. 1998: VGT-update).

Mutual comparison pointed out that both maps agree fairly well (89% of the concerned acreage), which implies that no dramatic changes have taken place in the course of the last six years. However, as the updated map was not checked on the field, it remains unknown to what extent the observed deviations (11% of the pixels) are due to misclassifications or to real changes. Although part of the observed deviations are certainly artefacts, a number of probably significant land cover changes were revealed which deserve further inspection, either by field controls or by the analysis of high resolution imagery.