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The World Atlas of Desertification was published by UNEP in 1992 as the result of a cooperative effort between UNEP's Desertification Control Programme Activity Centre (DC/PAC), the Global Environment Monitoring System (GEMS) and the Global Resource Information Database (GRID). GRID compiled and/or derived most of the global and regional databases, produced the maps and carried out the data analyses and tabulations for the Atlas, assisted by a Technical Advisory Group on Desertification Assessment and Mapping composed of various international experts. The Atlas includes information and many maps derived from the Global Assessment of Human-Induced Soil Degradation (GLASOD), as conducted in 1990 by the International Soil Reference and Information Centre (ISRIC) at Wageningen, The Netherlands, on behalf of UNEP.
Aside from GLASOD's data on soil degradation, and in order to capture the multi-dimensional nature of global desertification processes, other data layers relating to global climate and vegetation were compiled by GRID for inclusion in the 1992 World Atlas of Desertification. The NOAA/GVI data set described herein was created by GRID-Nairobi as a unique product for the Desertification Atlas, in order to represent baseline or "normal" conditions of global vegetation, and to be used in combination with the climate, GLASOD and land degradation data sets.
The NOAA/GVI (Global Vegetation Index; see reference pg. 3) Eight-Year Mean Maximum data set was developed in the following manner. First, eight years of NOAA/GVI Monthly Maximum data were obtained from GRID's Geneva archive of these data*. At GRID-Nairobi, an analyst then used these data files (12 per year) to calculate yearly mean maximum images, and the eight yearly mean images were averaged in their turn, in order to create a single eight-year mean maximum image. The original idea had been to produce an eight-year maximum value image, but this was abandoned due to the accretion of "noise" from spurious maximum-value pixels in the individual data files (UNEP/GRID, 1990).
* - GRID-Geneva has compiled an archive of NOAA/GVI Weekly data from the U.S. National Oceanic and Atmospheric Administration / National Environmental Satellite Data and Information Service / National Climate Data Center / Satellite Data Services Division (or the NOAA / NESDIS / NCDC / SDSD). This collection covers the period from April 1982 to present. At GRID-Geneva, the Weekly data are used to create Monthly, Seasonal and Annual Maximum images, in addition to the archived NOAA/GVI Weekly data.
The eight-year mean maximum value image was fitted to the Boundaries Template for the Desertification Atlas (180 degrees West to 180 degrees East longitude, and 72 degrees North to 57 degrees South latitude), and a water mask was applied to exclude non-land areas. The image resulting from the latter step was then stretched to a full eight-bit range of values; i.e., from 0 to 255, to provide the greatest possible contrast. Finally, in the last step of the processing, the data were stratified or divided into a series of discrete classes twice, once with five levels and again with ten levels, and these two raster images were vectorized for use with the climate, land degradation and GLASOD data sets and inclusion in the World Atlas of Desertification.
The NOAA/GVI Eight-Year Mean Maximum data set thus shows 1980s "mean" vegetation conditions at a global scale, integrated across all years and growing seasons, and categorized according to the relative health and quantity of green biomass present. Broadly speaking, NOAA/GVI data values relate to the capacity of plants to photosynthesize and the relationship of the plant canopy to evapotranspiration rates. Thus, they should not be confused with a classification of vegetation types, nor considered an absolute measurement of some physical vegetation parameter, but simply a status indicator of "greenness" and vitality. For the Desertification Atlas and in the context of land degradation analyses, this was deemed to be more appropriate and useful than a strict categorization of vegetation types (UNEP/GRID, 1990).
Data format ___________
The NOAA/GVI Eight-Year Mean Maximum data set produced by GRID-Nairobi is available as any one of four ARC/INFO coverages or 'EXPORT'-format (.E00) data files. While the Geographic Projection data having the latitude/longitude coordinate system are generally distributed, the Desertification Atlas and GLASOD project made use of the Mollweide equal-area projection for deriving statistical information, and the Van der Grinten projection for display and analog map purposes.
GVI05VDG NOAA/GVI in five categories; Van der Grinten Projection GVI05GEO NOAA/GVI in five categories; Geographic Projection GVI10MOL NOAA/GVI in ten categories; Mollweide Projection GVI10GEO NOAA/GVI in ten categories; Geographic Projection
These are normally delivered on a computer compatible tape (CCT) which is written using the ARC/INFO 'TAPEWRITE' command, with the following defaults: blocksize of 8000; logical record length of 80; blocking factor 100; 6250 bpi; and in an uncompressed 'EXPORT' format. They can be read from tape to disk with the 'TAPEREAD' command, and then re-'IMPORT'ed using the following series of commands:
IMPORT COVER GVI05VDG(.E00) cover_name IMPORT COVER GVI05GEO(.E00) cover_name IMPORT COVER GVI10MOL(.E00) cover_name IMPORT COVER GVI10GEO(.E00) cover_name
The most important variables in the polygon attribute table (PAT) of the NOAA/GVI Eight-Year Mean Maximum data set are described as follows:
Name of Item Description Possible values ____________ ___________ _______________
CLASS5 One of five 0 (No GVI recorded) GVI levels 101 (Highest GVI class) 102 103 (Medium GVI class) 104 105 (Lowest GVI class)
NDVI10CL One of ten 0 (No GVI recorded) GVI levels 1 (Highest GVI class) 2 3 4 5 (Medium GVI 6 classes) 7 8 9 10 (Lowest GVI class)
The source document for the NOAA/GVI Eight-Year Mean Maximum data set is Deichmann, Uwe and Lars Eklundh. July 1991. "Global digital data sets for land degradation studies: a GIS approach". GRID Case Study Series No. 4; UNEP/GEMS and GRID; Nairobi, Kenya; 103 pages (see pp. 37-47).
The best reference for the original NOAA/GVI Weekly data product is NOAA's "Global Vegetation Index Users' Guide (Oct. 1990 revision)", compiled and edited by Katherine B. Kidwell and available from the U.S. Department of Commerce, NOAA / NESDIS / NCDC / SDSD, Princeton Executive Square, Wash. D.C. 20233 USA. There is also a separate document available from GRID-Geneva which describes the in-house processing of these data, their availability and archive formats.
Additional references on the use of NOAA/GVI data to monitor vegetation can be found in the UNEP/GRID (1990) document on pages 46-47. Finally, the application of this NOAA/GVI Eight-Year Mean Maximum data set can be found on pages 22-23 (and Map 16) within UNEP's 1992 World Atlas of Desertification, published by Edward Arnold, London (UK), 69 pages.
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