A data compression method applicable to position data in AIS (Automatic Identification System) like system is proposed, in which MSBs (Most Significant Bits, higher bits) of position data are dropped from data to be exchanged. The MSB dropping technique has had a problem which comes from the nature that the distance corresponding to a degree longitude becomes smaller in the high latitude region than on the equator. This problem is solved by splitting region according to latitudes and dropping LSBs (Least Significant Bits, lower bits) in high latitude regions instead of dropping MSBs. In addition, a sample protocol is designed according to real AIS data. With the designed protocol, the data size of position data in the AIS is decreased by 20%.