Making Your Own Changes to GPS Maps
But, do you want to make your own modifications to the maps on your GPS unit? This Bright Hub article, "Where to Find Free Maps for GPS: Garmin, TomTom, and Others" by Tricia Gross, has suggestions for websites that may assist in this endeavour.
What is it you want to change or add to your maps for GPS? Is it extra text ornotes, audio, factual or graphics alterations, changes between US and UK English? If you check your GPS unit manufacturer's website, you may find they offer options and resources for map modifications, e.g., TomTom's Mapshare community, which offers frequent modifable updates and authorized versions including userbase changes.
If you're looking to create your own hacks, then just inputting your GPS model manufacturer and the word 'hack' into a search engine will produce plenty of results. But, bear in mind, it may also result in damage to your unit and/or your legal recourse in such a case! A search around in newsgroups, Yahoo, and sites such as exploristGPS may also prove informational and fruitful.