A system and method are disclosed for incrementally updating an existing digital map of a client device in a digital map update system. The digital map update system comprises, for each of one or more baseline digital maps, at least one digital map update repository comprising data which may be communicated to one or more remote client devices for use by the or each remote client device in incrementally updating an existing digital map of the device. Each repository includes one or more digital map forward update journals in respect of a respective given version of the baseline map to which the digital map update repository relates. The forward update journal comprises data indicative of a set of one or more incremental updates that may be used to update the given version of the baseline map to a later version. Each repository also includes one or more digital map reverse update journals for reversing the effects of the or a forward update journal and reverting the later version of the baseline map back to an earlier version.