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Map databases for car navigation systems loaded with hard disk drives are rewritable. Thus, the ability to update map databases in such systems is expected. Furthermore, because telematics services are available through cell phones with reasonable costs, it is also becoming possible to download new information into car navigation systems anywhere. Hence, drivers want to update map databases in their terminals through telematics services. However, the volume of the entire map database is too large to be distributed through cell phones. Therefore, only changed data of the map database are distributed in differential map updates. However, even considering the reduction, the data size is still too large to distribute through cell phones. A Locally Differential Map Update (LDMU) is effective to reduce the data size, because with this method, a map is only partially updated. However, LDMU causes disconnection of the road network. Therefore, we propose Connection Maintained Locally Differential Map Update (CM-LDMU). This involves partially updating a map while maintaining the road network. In experiments with this method, less than 250 KB of data was required to update 20 times 20 km2 as wide as a medium-sized city. Thus, the feasibility of telematics map updating services is demonstrated.