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The browse images for AVHRR scenes in the Polar Gridded AVHRR Brightness Temperatures data set are intended to provide potential data set users with information about AVHRR scenes prior to making data requests. The browse images illustrate each scene's area of coverage, highlight leads within the scene, and provide initial estimates of geophysical parameters (e.g., albedo, brightness temperatures). Each browse image provides an easily-distributed scene derived from the archived data and requires minimal manipulation of the raw sensor values.
The browse images are derived from original data from 1989. All browse images are 450 x 560 pixels and 8-bit per pixel, and are in hierarchical data format (HDF). Except for the summer season when melt degrades the ability of the thermal channel to discriminate leads, the browse images are from channel 4 of the AVHRR. For the summer season, the images are from channel 2 (near-infrared). The "summer" set includes images from May, June, July, and August. Browse images from all other months are referred to as "winter" images. The archive consists of four subsets of the original data:
The browse images were created by subsampling every fifth line and sample from the original 10-bit, full resolution (2250 x 2800) data. This resulted in 450 x 560 pixel images. The original data values were in counts. These smaller 10-bit images were stretched over a range of counts that best shows the leads to 8-bit values (0-255) in order to create the browse images. Each subset had a consistent stretch applied to each corresponding image; however, stretches vary among the four subsets. The linear stretches applied to the images are as follows:
The 8-bit values or digital numbers (DN) in the browse images can be backed out of the browse images by using the following equations. First, DN is first converted to counts from DN. Counts are then converted to either percent albedo or brightness temperature in degrees Kelvin:
The stretches are applicable over the DN range 0 to 249. Thus, the actual value may be calculated directly from the browse image DN value according to the following equations:
Each image has a latitude/longitude grid and land overlaid mask. The DN value of the grid is 254 and the land mask is 255. The browse images are in HDF format with a specified palette and included annotation.