THE ALASKA ADVANCED VERY HIGH RESOLUTION RADIOMETER TWICE-MONTHLY COMPOSITES DATA SET CHARACTERISTICS The data set is composed of 14 bi-weekly maximum NDVI composites that were created from an average of 18 images per composite period. The 14 composite periods span a continuous period from April 1 to October 13. Each daily observation includes 9 bands of information: AVHRR channels 1-5, NDVI, satellite zenith, solar zenith, and relative azimuth. The daily observations were calibrated to reflectance, scaled to byte data, and geometrically registered to the Albers Equal-Area Conic map projection for Alaska. Pixels were resampled to 1-km square. Each composite includes 10 bands of information - the 9 described for a daily observation and a 10th band that identifies the date of the source daily observation scene. The data for each pixel in the composite are extracted from the daily observation scene based on the maximum NDVI compositing process. Users should be aware that a few bad data lines may exist in the AVHRR thermal channels for some of the NDVI composite periods. These bad data lines result from an interruption in the transmission of the in-flight calibration target data. Normalized Difference Vegetation Index The NDVI was calculated from calibrated data that were scaled to byte range and geometrically registered. The NDVI is the difference of near-infrared (AVHRR channel 2) and visible (AVHRR channel 1) reflectance values divided by total reflectance: IR(Band 2) - Visible(Band 1) NDVI = ---------------------------------------- IR(Band 2) + Visible(Band 1) The equation produces NDVI values in the range of -1.0 to 1.0, where negative values generally represent clouds, snow, water, and other nonvegetated surfaces and positive values represent vegetated surfaces. To scale the computed NDVI results to byte data range the NDVI data range of -1.0 to 1.0 was scaled to 0 to 200, where computed -1.0 equals 0, computed 0 equals 100, and computed 1.0 equals 200. As a result, NDVI values less than 100 represent clouds, snow, water, and other nonvegetated surfaces and values greater than 100 represent vegetated surfaces. list of the composite period dates: _________________________________________________________ Calendar date Julian Reporting days period # _________________________________________________________ Apr 1 - Apr 14 04/01 - 04/14 91/104 1 Apr 15 - Apr 28 04/14 - 04/28 105/118 2 Apr 29 - May 12 04/29 - 05/12 119/132 3 May 13 - May 26 05/13 - 05/26 133/146 4 May 27 - Jun 9 05/27 - 06/09 147/160 5 Jun 10 - Jun 23 06/10 - 06/23 161/174 6 Jun 24 - Jul 7 06/24 - 07/07 175/188 7 Jul 8 - Jul 21 07/08 - 07/21 188/202 8 Jul 22 - Aug 4 07/22 - 08/04 203/216 9 Aug 5 - Aug 18 08/05 - 08/18 217/230 10 Aug 19 - Sep 1 08/19 - 09/01 231/244 11 Sep 2 - Sep 15 09/02 - 09/15 245/258 12 Sep 16 - Sep 29 09/16 - 09/29 259/272 13 Sep 30 - Oct 13 09/30 - 10/13 273/286 14 _________________________________________________________ Map projection parameters for the Alaska AVHRR composite data sets _________________________________________________________________ Albers Equal-Area Conic First standard parallel 55 00 00 N Second standard parallel 65 00 00 N Longitude of central meridian -154 00 00 W Latitude of origin 50 00 00 N False easting 0 False northing 0 Units of measure Meters Pixel size 1,000 meters For Alaska Center of pixel (1,1) ( -977000, 2422000 ) Number of lines 1,992 Number of samples 2,512 Albers meters for minimum bounding rectangle: Lower Left ( -977000, 431000 ) Upper Left ( -977000, 2422000 ) Upper Right ( 1534000, 2422000 ) Lower Right ( 1534000, 431000 ) Geographic decimal degrees: Lower Left ( -168.5970, 52.9222 ) Upper Left ( -179.8476 70.0416 ) Upper Right ( -116.0057 67.6962 ) Lower Right ( -131.5953 51.5372 ) Geographic degrees, minutes, and seconds: Lower Left ( -168 35' 49" 52 55' 20" ) Upper Left ( -179 50' 51" 70 02' 30" ) Upper Right ( -116 00' 21" 67 41' 46" ) Lower Right ( -131 35' 43" 67 32' 14" ) _________________________________________________________________