Communication system, method and program for navigation comprising server apparatus and terminal apparatus

A server apparatus (SV) divides map data into a plurality of block data to add identification information to each block data, and sends each block data to a terminal apparatus (T). The terminal apparatus (T) identifies data for each block data sent from the server apparatus (SV) and, when uncompleted block data is detected, sends retransmission request information, which requests only the uncompleted block data to be sent to the terminal apparatus (T), to the server apparatus (SV).