Abstract:
UNEP/GRID Documentation Summary for Data Set:
West Africa Tropical Forest/Non-forest Digital Map
This digital data set is one of the principal results of the West
Africa "Methodology Development Project for Tropical Forest Cover
Assessment" carried out at GRID-Geneva between December 1988 and
January 1990 by the University of Joensuu, Finland/FINNIDA and
... GRID.
The major purpose of this project was to test the utility of
one-kilometer resolution Local Area Coverage (LAC) data from the
NOAA/AVHRR* satellite sensor for mapping the tropical forest cover.
The secondary aim was to define and map the total area of tropical
closed forest remaining in West Africa.
The digital data set showing the distribution of tropical forest in
West Africa was derived entirely from NOAA(-9 & -11)/AVHRR-LAC
one-kilometer resolution data, and covers the area from the West
African coast to 12 degrees North latitude, and from Sierra Leone in
the west to 10 degrees East longitude (western Cameroon). Three
sub-areas, each approximately 35,000 sq. km., were selected within the
overall West African study area for intensive analysis. These
sub-areas, one each in southwestern Nigeria, central Ghana and eastern
Liberia/western Ivory Coast cover both dryer and wetter rain forest
areas. These three areas were also the subject of high-resolution
data collection and field visits for ground truth collection in August
1988 and January 1989.
Several visual and digital image processing techniques were used to
analyze the NOAA/AVHRR-LAC data sets. Among these were the following:
- visual analysis of single- and multi-channel images;
- multi-channel density slicing;
- unsupervised training with different sets of channels,
followed by maximum likelihood classification;
- supervised training with maximum likelihood classification;
- principal component analysis followed by unsupervised or
supervised training and maximum likelihood classification; and
- calculation of normalized difference vegetation index (NDVI)
as (near-infrared - red) / (near-infrared + red) channels (2 &
1).
The forest/non-forest map derived from the NOAA/AVHRR-LAC data was
tested by: 1) comparing it visually with other maps available; 2)
overlaying it with the classification maps obtained from
high-resolution satellite data (Landsat MSS and TM, and SPOT
multispectral data sets); and 3) comparing the area of the closed
forest with the statistics presented by the 1980 FAO/UNEP Tropical
Forest Resources Assessment Project report (FAO, 1981).
The map includes six land cover categories as follows: 1) Tropical
Forest (closed canopy); 2) Mangrove (coastal) forest; 3) Fallow (mixed
agriculture and open forest); 4) Savanna; 5) Urban; 6) Cloud and 7)
Water. Most of the map was compiled using December 1986 - January
1987 AVHRR-LAC digital scenes, with much of western West Africa being
revised/updated using a 9 January 1989 data set at the University of
Joensuu, Finland.
The source of the data set is UNEP/GRID-Geneva and the University of
Joensuu/FINNIDA. The proper reference to the data set is "Paivinen,
Risto, Pitkanen, J. and R. G. Witt, 1992. Mapping closed tropical
forest cover in West Africa using NOAA/AVHRR-LAC data :hp2.in :ehp2.
Silva Carelica 21 - Assessment of Tropical Forests Using Satellite
Data, editor Timo Pukkala, University of Joensuu, Faculty of Forestry,
Joensuu, Finland, pp. 27-52. A more lengthy technical document is:
"Paivinen, Risto T. M. and R. G. Witt, 1989, The methodology
development project for tropical forest cover assessment in West
Africa" (an internal report to UNEP/GEMS-PAC/GRID), Nairobi, Kenya, 51
pages.
The West Africa Tropical Forest/Non-forest data set is available on a
source approval basis to researchers only, as a single digital raster
data file. Technical specifications for the data file are as follows:
Bits per pixel: eight
Spatial resolution: 32 seconds lat./long.
Number of rows: 2739 (also lines, records of data)
Number of columns: 940 (also elements/pixels/samples)
Origin in DD/MM/SS: 11 36' 48" North latitude
(northwest or -14 27' 12" West longitude
upper-left corner)
Terminus in DD/MM/SS: 03 15' 28" North latitude
(southeast or 09 53' 36" East longitude
lower-right corner)
* - the U.S. National Oceanic and Atmospheric Administration (NOAA)
satellite's / Advanced Very High Resolution Radiometer (AVHRR)
sensor and - "Local Area Coverage" (LAC) data (NOAA/AVHRR-LAC). 
