A dual mode facsimile data compression technique which possesses the advantage of symbol recognition and extended run-length coding methods. In operation, a symbol blocking operator isolates digital raster data representing valid alphanumeric characters and document symbols. The first symbol encoungered is placed in a memory or library and as each new symbol is detected, it is compared with each entry of the library. If the comparison is within a predetermined tolerance, a library identification code representing the symbol is generated and transmitted along with the symbol location coordinates. Otherwise, the new symbol is placed in the library and its digital pattern and location is transmitted. A scoring system determines which elements of the library are to be replaced by new prototypes once the library is filled. Non-isolated symbols are coded by a two-dimensional run-length coding method for transmission.