This section describes the First Generation (experimental) GVI product, which was funded as a research effort by AgRISTARS.
3.1 DIGITAL PRODUCTS
There are four separate formats of digital GVI products:
A description and brief documentation of each format is given below.
The Daily GVI tape contains the daily files of mapped data from Channel 2 and Scaled Difference Vegetation Index (DVI) from which the composite products described above are computed. The tape contains up to seven sets of three files (Documentation, Channel 2, and Scaled DVI), or a maximum of 21 files. Less than 21 files will be present if one or more days of data are missing.
Each Documentation File contains a single record 4096 bytes in length, which is formatted as shown in Table 3.1.1-1.
The Data Files have the same format as the weekly composite GVI tapes described in Section 18.104.22.168.
Table 3.1.1-1. Format of the documentation file for First Generation GVI Products.
|5||2-digit year and day of year (YYDDD), in ASCII|
|6||1||Number of GAC data sets (N) processed, in binary|
|7-11||5||Date processed (YYDDD), in ASCII|
|12||1||Unused, blank filled|
|13 - x||36N||Data set name of those GAC data sets used to create this day's file: 33 bytes of ASCII information and 3 bytes blank fill for each N data sets, where N is usually 14, occasionally 15, and less than 14 if some data were missing.|
|X - 4096||n/a||Blank filled.|
The weekly composite GVI products consist of: 1) weekly composite GVI tapes, 2) stacked weekly composite GVI tapes, and 3) stacked weekly composite NDVI tapes. These products are described in the following sections.
The weekly composite tape contains seven-day composites of Channels 1 and 2, and the Scaled Difference and Normalized Difference Vegetation Indices (Scaled DVI and Scaled NDVI) in the Polar Stereographic projection. There are a total of five files on each tape:
All of the data files (files 2 through 5) include both the Northern and Southern hemispheres.
The original 10-bit AVHRR data values are truncated to 8-bit precision, and the Channel 1 and Channel 2 files contain mapped 8-bit values. The vegetation indices are computed from these 8-bit values. As outlined below, both DVI and NDVI values are arbitrarily scaled to permit them to be represented by 8-bit integers (i.e., on a range of 0 to 255) over the range of DVI and NDVI values representative of vegetation.
The Scaled Difference Vegetation Index (DVI) is computed as follows:
If the Scaled DVI is negative, it is set to 0.
The Normalized Difference Vegetation Index (NDVI) is computed as follows:
The NDVI is then scaled according to the following criteria:
The Scaled NDVI is present on the weekly composite tapes.
The Documentation File consists of one record with a length of 4096 bytes. It contains the starting and ending date and the number of days (maximum of seven) which were used to create the composites. The Documentation File record has the format shown in Table 22.214.171.124-1.
Table 126.96.36.199-1. Format of the Documentation file for the Weekly Composite GVI tapes.
|1||1||Number of days processed for the composite, in binary|
|2||1||Unused, blank filled.|
|3-8||6||First day processed (YYDDDb) b=blank, in ASCII|
|9-14||6||Second day processed, as above.|
|39-44||6||Seventh day processed, as above.|
The Data files (Channel 2, Scaled DVI, Scaled NDVI, and Channel 1) have identical formats. They consist of 512 records which each have a length of 4096 bytes. The first 256 records of each data file contain data for the Northern Hemisphere and the last 256 records contain data for the Southern Hemisphere. The data are mapped into a Polar Stereographic mosaic consisting of an array of 1024 lines of 1024 bytes each. The first sample from the first line represents the upper left corner of the Polar Stereographic mosaic. A set of FORTRAN subroutines which may be used to convert from latitude/longitude to position in the mapped array, or the reverse, are available in Appendix J.
Each 4096 byte record contains four mapped data lines of 1024 bytes; each byte contains a channel or GVI value. Appendix C lists the time periods for which these First Generation weekly composites were produced including all periods of missing data.
The stacked composite tapes contain the same information as the regular composite tapes (documentation file plus four data files) with the exception that four weeks worth of data (i.e. twenty files) have been "stacked" onto one IBM 3480 cartridge tape. Users seeking more than one week's worth of data can realize a savings by ordering a copy or file extraction from a stacked tape. A listing of the week-by-week composition of the stacked tapes is given in Appendix E .
Files 1 and 4 (documentation and Scaled NDVI) are also available in a stacked format. These tapes can contain as much as four months worth of NDVI composites. Appendix G contains a listing of the composition of these stacked tapes.
The Scaled NDVI, Scaled DVI, and Channel 1 and Channel 2 composites are also available in the form of hardcopy imagery. Each image contains one hemisphere in Polar Stereographic projection (or an enlargement of a user specified area.)
For Scaled NDVI imagery, the darker the shade of gray is on the image, the greener the area on the ground. Bodies of water, clouds, snow, and ice appear as white or light gray; deserts as light gray; forests and cultivated areas as medium to dark gray, depending on the greenness and density of the vegetation. Dark data and lines appearing in the image are bad data transmissions from the spacecraft. Persistent cloud cover is a problem in parts of the world at certain seasons. For example, tropical land areas often appear speckled because of daily buildup of convective activity which is near maximum at the afternoon overpass time of the satellite.
Contact NCDC for more specific information on the availability of the First Generation GVI hardcopy images (some but not all images during this time period were archived).
A user may request a 10" x 10" negative, glossy contact print, positive transparency, or an enlargement. If an enlargement print or positive transparency is desired, a specific area may be requested. Other photo products such as slides, 20" x 20" prints, etc. are also available upon request.