Abstract: A data communication acquires a map image, determines high- and low-risk areas in the map, determines whether to transmit data related to the high- or low-risk areas, detects objects around the system, determines a position in the map image for each of the objects detected, determines whether the objects belongs to the high- or low-risk areas, determines a data compression ratio for each of the objects detected, compresses data related to each of the objects, compresses data related to each of the objects belonging to the high-risk area when data related to the high-risk area is determined to be transmitted, compresses data related to each of the objects belonging to the low-risk area when data related to the low-risk area is determined to be transmitted, receives reply data replied in association with the compression data transmitted, and makes a decision in accordance with the reply data.