The Weather-Huffman Method of Data Compression of Weather Images

This document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The United States Government assumes no liability for its contents or use thereof. Providing an accurate picture of the weather conditions in the pilot's area ofinterest is a highly useful application for ground-ta-air datalinks. The problem with using data links to transmit weather graphics is the large number of bits required to exactly specify the weatherimage. 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 ofa version ofthe input image via the Weather Huffman runlength code, a variant ofthe 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 flI'St 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 weatherleveL Both size and shape of weather regions are adjusted by this algorithm. Pictorial examples of the operation ofthis algorithm on several severe weather images derived from NEXRAD are presented.