The mapped GAC products consist of mapped mosaics displayed on Polar Stereographic and Mercator map projections with both forms available on CCTs. The mapped mosaics are of daytime visible (VIS) and Infrared (IR), and nighttime IR imagery. The mapped GAC product in Polar Stereographic form is described in Section 5.3.1, while the mapped GAC product in Mercator form is described in Section 5.3.2.

NESDIS/IPD produces an operational mapped GAC product which is known as the Global Vegetation Index Product. This product provides a means of monitoring the density and vigor of green vegetation over the growing areas of the Earth. Plate Carrée, Polar Stereographic and Mercator mosaics of the Global Vegetation Index, derived from AVHRR Channels 1, 2, plus coincident channels 4 and 5, and supporting information are produced weekly. For more information, contact NCDC to obtain a copy of the NOAA Global Vegetation Index User's Guide.

Mapped GAC (Polar Stereographic) Product (1979 - October 26, 1994)

Mapped GAC Product (in Polar Stereographic projection) produced between 1979 and October 26, 1994 have the format described in this section. The mapped mosaics are displayed on a 1024 x 1024 Polar Stereographic grid that provides 14.8 km resolution at the Equator decreasing to 29.6 km near the poles. The mapped GAC (Polar Stereographic) product tapes have the file structure as shown in Table 5.3.1-1.

Files 2 and 4 contain IR and VIS data for both hemispheres. The 8-bit IR daytime and 8-bit VIS mapped quantities are combined into a single 16-bit quantity for each map position. The upper order 8 bits are the IR day data and the lower order 8 bits are the VIS data. This 16-bit combination gives the user a comparison of identical time views with IR and VIS data for each grid position. Each data record contains 4,096 (8-bit) bytes which holds two mapped rows. Temperature values range from 0 (warm) to 254 (cold) for IR data, and a brightness count from 0 (dark) to 254 (bright) for VIS data. A value of 255 represents no data. Figure 5.3.1-1 shows the arrangement of the data on the Polar Stereographic map base for IR day and VIS.

The format for Files 6 and 8 (which contain IR night data for both hemispheres) is similar to Files 2 and 4 except that there is only one type of data represented. Each record contains 4,096 bytes which correspond to four map rows. Each byte represents a mapped value ranging from 0 (warm) to 254 (cold). Figure 5.3.1-2 shows the arrangement of the data on the Polar Stereographic map base for IR night.

Files 1, 3, 5, and 7 contain documentation for the data file immediately following. These documentation files contain a single 4,096 word (32-bit) record. The documentation record of each map file contains a 32-word documentation group for each of the passes mapped for a single day's archive. The first word of the record indicates the number of passes processed. The format of the documentation record is contained in Table 5.3.1-2. The time information is contained in the rightmost six-bytes of the specified two-word group. The time consists of the year, Julian day, and time of day (UTC) in milliseconds. The year is contained in the first seven bits of the first two bytes, with the nine-bit Julian day right justified in the same two bytes and the 27-bit millisecond time of day right justified in the last four bytes. All other bits are set to zero.

The processing block ID is written in ASCII character code. The conversion table for hex to ASCII is included in Table 2.1.1-2.

It is helpful when working with Polar Stereographic maps to be able to convert from latitude and longitude to i and j coordinates or vice-versa. Two subroutines called IJTOLL and LLTOIJ perform these functions. The source code and documentation for both of these subroutines are contained in Appendix A. Note: If applying IJTOLL and LLTOIJ to the Mapped GAC Product, use a prime longitude of +80W.

Mapped GAC (Polar Stereographic) Product - October 26, 1994 to present

On October 26, 1994, NESDIS replaced the Polar Stereographic GAC mapping system with the NOAA-KLM master map system. This resulted in corresponding format changes. This section describes the format of the Polar Stereographic Mapped GAC product in use since October 26, 1994.

The data are organized as daytime and nighttime for the northern and southern hemispheres. The daytime data contain both visible (Channel 1) and IR (Channel 4) data while the nighttime data contain only the IR data. The data are reported in pairs of files. The first file of each file pair consists of a documentation record for that variable, immediately followed by an EOF and a second file containing the data records for the same variable. All records are 16,384 bytes in length. Both documentation and data records are in binary format and have the same length. A value of zero indicates missing data.

Every day a 3480 cartridge is created which contains one day of data in the Polar Stereographic projection. Table 5.3.1.1-1 contains the general file structure of the daily KLM Master map cartridge. Each cartridge contains 12 files arranged as shown in the table.

The variables reported in the documentation record are all INTEGER*2 with the exception of the satellite type in the first two bytes of the record, which is CHARACTER*2. Table 5.3.1.1-2 defines the basic documentation record format.

Each data record (16,384 bytes) consists of four mapped rows, each containing 4096 pixels of data. Each pixel is represented by one byte of data.

Mapped GAC (Mercator) Product

Beginning June 24, l985, SSB began archiving the mapped GAC Product in the Mercator form. These mapped mosaics are displayed on a Mercator map which extends from 40N to 40S with a 9.8 km resolution at the Equator, and 7.5 km resolution at 40N and 40S. The Mercator mapped GAC data reside in Files 9 through 11 of the same archive tape as the Polar Stereographic mapped GAC. The Mercator files have the file structure as shown in Table 5.3.2-1. The first record of each file contains documentation for that file and it has the same format as the documentation for the Polar Stereographic mapped GAC product described in Section 5.3.1.

The data are contained in records 2 through 985 of each Mercator file. Each record contains data for one mapped row and 984 records comprise the Mercator map. The 4,052 byte records hold 4,050 8-bit bytes of data with the rightmost two bytes zero filled. The temperature values for IR data range from 0 (warm) to 254 (cold) and the visible data count values range from 0 (dark) to 254 (bright). Figure 5.3.2-1 shows the arrangement of the Mercator mapped GAC data for one file.

Mapped GAC (Mercator) Product - Oct. 26, 1994 to present

On October 26, 1994, NESDIS replaced the Polar Stereographic GAC mapping system with the NOAA-KLM master map system. This resulted in corresponding format changes. This section describes the format of the Mercator Mapped GAC product in use since October 26, 1994.

The data reported are organized as visible (Channel 1), and daytime and nighttime IR (Channel 4). These data are reported in pairs of files, totaling six files. The first file of each file pair consists of a documentation record for that variable, immediately followed by an EOF and a second file containing the data records for the same variable. All records are 4,052 bytes in length. Both documentation and data records are in binary format and have the same length. A value of zero indicates missing data.

Every day a 3480 cartridge is created which contains a day of data in the Mercator Projection. Table 5.3.2.1-1 contains the general file structure of the daily Mercator Master map cartridge. Each cartridge contains 6 files arranged as shown in the table.

The format of the basic documentation record is the same as the Polar Stereographic KLM Master Map which is contained in Table 5.3.1.1-2.

Each data record (4,052 bytes) consists of one mapped row, containing 4052 pixels of data. Each pixel is represented by one byte of data.

Source: http://www.ncdc.noaa.gov