It has to do with the map source data, not the accuracy of the GPS itself.
There are essentially 4 sources of quality electronic map data in the US today (there is a 5th being developed, but it has a long way to go and I won't discuss it here).
They all start with the same base source. US Census Tiger Database. this is a combination of data supplied by state and local municipalities and put together by the census bureau to help census takers find homes. It was last updated 10 years ago (before the last census, not this year's).
It has most roads as of 10 years ago, but the position accuracy of the roads is not great, and there are a lot of errors, from roads and connections that were planned but never built, to missing roads that are over 50 years old.
Delorme software (not an Android issue currently, but they have WM, Palm, and PC software, so worth discussing) took this data, made modifications based on satellite and aerial photographs and about 15 years of customer submitted corrections. The data is pretty good, but directions they provide need to be checked against another source or sometimes just common sense (more times than I can count, I have been told to take an exit, only to then pull a u-turn to get back on the same highway in the same direction).
Next up is TeleAtlas. This is used buy TomTom, Co-Pilot, Mapquest, Bing, Magellen, and several other vendors. It uses Tiger database for rural roads, but has improved field checked data in most cities. It is pretty good data, and worth paying a reasonable price for. They charge a licensing fee, and is not typcally available in free software (except Bing, but MS is paying the licensing fee and absorbing the cost elsewhere in the business).
Then we have Navteq. Navteq is the indisputed leader in map accuracy in North America, and much of Europe as well. As with the others, they started with the Tiger database, but they have over a thousand employees who field check and update their maps. In addition to having excellent routing algorithms in general, they also include extra data about typical traffic load, unsafe neighborhoods, and shortcuts typically known only to locals. Their algorithms are smart enough to take slightly longer routes if it means that you can take right turns and arrive at your destination on the right so you don't need to wait for traffic to clear to make left turns. Software using Navteq map and routing data is used by UPS to optimize delivery routes and they have saved millions of gallons of gasoline.
They are also used by Garmin and Trimble GPS. They used to be used by Google Maps, but for reasons I will get into momentarily, they no longer are. The issue with Navteq is their licensing requirements. They charge a lot of money for their data, and have very restrictive licenses on how it can be used. Typically, a vendor can sell a product with their maps, and it can be installed on multiple machines and used for viewing the map data. However, they are very very restrictive with their routing data. They used to allow software that could generate a route but not be directly used by a GPS to be used for a relatively low cost, and licenses could be shared between at least a couple of GPS devices. In the last few years, they have required a seperate license for every device that could do live routing, thus the expense of upgrading multiple Garmin units, and why if you use Garmin's PC software it still needs to be used with a Garmin GPS because the software validates the device serial number is tied to the map license.
Now, on to Google. As I mentioned above, they used to use Navteq (and occasionally TeleAtlas) on Google Maps. However, when they added GPS connectivity for free on Google Earth, and started developing navigation for Android, they were forced to drop their license with Navteq or we would be paying about $60 extra per device for the Navteq data. So, they went back to the free source data, the US Census Tiger database. However, at the same time, they started the StreetView project, and started accepting customer corrections. I believe, but have not confirmed yet, that they are also collecting data when navigation is used on Android and people take detours and the software needs to re-route. Anywhere you travel that has street view data will likely be very accurate. Other areas will vary between terrible (raw Tiger data) and decent (Tiger data with user corrections). The big advantage Google Navigation has is that when they do get updates, everyone gets them at once since it is all online data, and they have millions of users so are getting a lot of updates.
I do expect that within a few years, Google will rival Navteq for accuracy, but currently when I travel I use Google nav, but I pack my Garmin in case I get hopelessly lost because I know I can trust it anywhere in the US and most of Canada as well. I will admit, I haven't paid for an update in two years, and am saving my money to see if they release an Android version of the software (they do have it, but it is only in the Garmin Nuviphone on T-Mobile right now). They used to have a Windows Mobile and Palm version, but it has not been updated in some time. They do have Blackberry software still, but I hate the Blackberry my employer makes me use, so I would rather carry the Garmin I already own then spend on Blackberry software.
Alan
