Data Biosphere GNV20

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1) Nature of the Data Set :

The NOAA Global Vegetation Index (hereafter, NOAA/GVI) archive held by GRID is a series of weekly, monthly, seasonal and annual data files as produced by GRID-Geneva from the original NOAA/GVI data set, the source of which is the U.S. NOAA/NESDIS/NCDC/SDSD.* The time period covered by these data as held by GRID is 12 April 1982 through 11 September 1994.

2) Background and Uses of the Data :

- Global Vegetation Index: The GVI is derived from NOAA/ Advanced Very High Resolution Radiometer (AVHRR) Global Area Coverage (GAC) data which are obtained on a daily basis. For any given week, the highest daily value of the so-called Normalized Difference Vegetation Index (NDVI) is sampled to produce a Weekly GVI data file. The NDVI is calculated from channels one (visible red light) and two (near-infrared light) values imaged by the AVHRR sensor, as follows:

NDVI = (ch. 2 - ch. 1)/(ch. 2 + ch. 1)

The NDVI has been correlated with such physical measurements as total standing biomass, green leaf-area index (LAI) and per cent vegetation cover, but is probably best described as a relative measure of vegetation vigor and photosynthetic activity. It is most often used among other applications as a tool for monitoring temporal changes in vegetation.

-Calibrated Vegetation Index: An additional variable has been provided by NOAA since the beginning of July 1990, the calibrated vegetation index or CVI, and is based on the albedo rather than raw channel reflectance values, as follows:

CVI = (A2 - A1)/(A2 + A1);

where A1 and A2 are albedoes calculated from weekly composites of channels 1 and 2.

3) Data format :

The NOAA/GVI data files held by GRID-Geneva cover both the so-called First Generation and Second Generation NOAA products. They are all held in the Plate Carree or Simple Cylindrical Projection used as a standard by GRID, which is a particular form of the Equirectangular. As NOAA only offers pre-April 1985 weekly GVI data in the Polar Stereographic projection (two hemispherical halves), all April 1982 through March 1985 data files have been converted to Plate Carree using a modified algorithm from Singh et al. (University of Reading, U.K.). Also, monthly, seasonal and annual maximum vegetation index values have been calculated for all months/seasons/years of data which are archived (i.e., April 1982 through September 1994), and their equivalent data files are stored and distributed for NDVI and CVI values.

The NOAA/GVI data files are all raster arrays consisting of 904 lines by 2500 elements. Their origin point (northwest or upper-left corner) is 75 degrees North latitude and 180 degrees West longitude, and they extend to 55 degrees South latitude and 180 degrees East longitude at their terminal point (southeast or lower-right corner). The nominal spatial resolution of the data (pixel size) is 16 km. at the equator, and each data file comprises 2.26 Mb.

The following terminology has been used for the seasonal data files:

Season: Extension: Related months: --------- ------------ ------------------- 1982-1992 Winter ".hiv" = December, January, February Spring ".prn" = March, April, May Summer ".ete" = June, July, August Autumn ".aut" = September, October, November

1993-1994 Winter ".win" = January, February, March Spring ".spr" = April, May, June Summer ".sum" = July, August, September Autumn ".aut" = October, November, December

Because of different NOAA satellite and AVHRR sensor failures, GRID-Geneva does not hold all weeks within the entire 1982-1994 period of time. For example, week 26 of 1993 (June 28 - July 4) is missing, and all related data files (July, summer, annual) for 1993 were calculated without taking this week in account. Also, data for year 1994 extend from week 1 to week 36 (January 1-September 11) only, and thus monthly vegetation index data files are available for January to August, and the Summer period was calculated using July, August and weeks 35 and 36 (beginning of September 1994).

Section 5 shows the complete list of weeks missing from the NOAA/GVI data archive (and thus at GRID-Geneva) since April 1982

4) Reference :

The best reference for these products is the NOAA "Global Vegetation Index Users' Guide (rev. Dec. 1994)", compiled and edited by Katherine B. Kidwell and available from the U.S. Department of Commerce, NOAA/ NESDIS/NCDC/Satellite Data Services Division, Princeton Executive Square, Wash. D.C. 20233 USA. There also exists at GRID-Geneva an internal document which describes GRID's processing of these data sets, their availability and archive formats.

5) Weeks not available (list from GVI Users Guide and/or GRID-Geneva Meta-database):

Year: Weeks missing: ------- -------------------------------------------------------- 1982 (Beginning of First Generation product 12 April, week 15)

1983 Week 26 (27 June - 3 July) | Week 31 (1-7 August) | Week 42 (17-23 October) 1984 Week 7 (13-19 February) | Week 8 (20-26 February) | Week 13 (26 March - 1 April) | Week 14 (2-8 April) | Week 49 (3-9 December) | Week 51 (17-23 December) 1985 Week 3 (14-20 January) | Week 5 (28 January - 3 February)

(End of First Generation product 7 April 1985. Beginning of Second Generation Weekly Composite 9 April 1985.)

1992 Week 11 (9-15 March) 1993 Week 26 (28 June - 4 July) 1994 From 12 September on, weeks are not available (weeks 37 ...).

* - U.S. National Oceanic and Atmospheric Administration / National Environmental Satellite Data and Information Service / National Climate Data Center / Satellite Data Services Division

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