The Weather-Huffman method of data compression of weather images
October 31, 1997
Providing an accurate picture of the weather conditions in the pilot's area of interest is a highly useful application for ground-to-air datalinks. The problem with using data links to transmit weather graphics is the large number of bits required to exactly specify the weather image. To make transmission of weather images practical, a means must be found to compress the data to a size compatible with a limited datalink capacity. The Weather-Huffman (WH) Algorithm developed in this report incorporates several subalgorithms in order to encode as faithfully as possible an input weather image within a specified datalink bit limitation. The main algorithm component is the encoding of a version of the input image via the Weather Huffman runlength code, a variant of the standard Huffman code tailored to the peculiarities of weather images. If possible, the input map itself is encoded. Generally, however, a resolution-reduced version of the map must be created prior to the encoding to meet the bit limitation. In that case, the output map will contain blocky regions, and higher weather level areas will tend to bloom in size. Two routines are included in WH to overcome these problems. The first is a Smoother Process, which corrects the blocky edges of weather regions. The second, more powerful routine, is the Extra Bit Algorithm (EBA). EBA utilizes all bits remaining in the message after the Huffman encoding to correct pixels set at too high a weather level. Both size and shape of weather regions are adjusted by this algorithim. Pictorial examples of the operation of this algorithm on several severe weather images derived from NEXRAD are presented.