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Hi folks - newbie question, sorry if it has been answered before...

I'm on a road trip, and continue to be AMAZED at the level of detail contained in the maps built into my automotive GPS (Magellan Maestro 3250 I believe). I'm an engineer and can understand some fairly technical stuff; searches I've done on the web have done a good job explaining how a GPS receiver uses the signals from satellites to calculate its position - interesting stuff!

But what I can't wrap my mind around is how the receiver stores and makes use of the incredible amount of data needed to describe so many roads, points of interest, etc. It seems like an insurmountable job to collect all that data, keep it relatively up to date, and how the receiver uses it to figure routes. Any pointers to explanations of how this is done?

Thanks!
Rick

Your question has gone unanswered long enough. I'll post, even though I can't answer all of it.

It starts with an incredible amount of memory to store the data.

We throw the word gigabyte around these days like it's nothing, but think back to the days of those plastic floppy disks all your programs came on. Those were less than 1 and a half megabytes. It takes more than 700 of those to hold a gigabyte. And your GPS probably has at least 2 gigabytes of storage for its built-in map data. So think about those floppies, and think about buying a box of ten of them. Now think about buying 150 of those boxes.

That's how it holds all that stuff.

The rest of your question: I don't have a clue. The map is a database, and streets, intersections, and so on have attributes that can be searched. What those attributes are, and how they get searched and selected, I don't understand, either. But like most consumers, it works and I don't really care how.

Thanks! I appreciate the reply

Yes, memory HAS gotten a lot cheaper over the years. I remember when my first home PC had dual quater MB floppy drives - no HDD at all! Now I typically work with CAD files around 100MB each.

Anyway, back to GPS... Each road seems to be stored in pretty good detail. Plus the outline of bodies of water, etc. It continues to boggle my mind the amount of points contained, let alone the algorithms used to calculate routes.

As Arthur C. Clarke once said, and I have to paraphrase, "any well-functioning technology appears as magic"

Cheers!
Rick

Not that I know anything specific the the formats used to store map data, but think of it this way:

Even ignoring anything clever such as compression, stripping redundant data, etc (all of which I would be pretty sure is part of the commercial file formats!), the map detail and road layouts are stored in very efficient vector format. Assume there is on average one set of coordinates per 50 metres of typical road / river / feature (probably about the right order of magnitude for urban environments, conservative for rural areas, where you can often see the "jagginess" on the SatNav due to successive points being located further apart), and assume the coordinates are saved as a pair of double precision (4-byte) numbers, that gives about 8 bytes per 50 metres of road, or say 160 bytes per kilometre, or more than 6,000 km of road (or river, lake outline, etc) per megabyte.

Just doing some very basic maths, and realising that RAM on such devices is now measured in hundreds of MB or several GB, and you can see that there is plenty of room for some fairly highly detailed mapping!

Cheers!

This is a subject that I am very curious about. How did all of the information get gathered and put into a map program. For example, when I'm on our local highway US 270 and the speed limit is 55 my GPS receiver displays 55 mph. When I pass a sign that changes the speed limit to 45 within a few yards the GPS receiver displays 45 mph. There must be billions of little bits of data like this; again, how did it all get collected and put into the map?

Keebler Elves?

It boggles my mind too. I have seen articles describing special cars enabled with GPS (obviously) and roof-mounted cameras gathering details of city freeways, but I'm also amazed at the detail "out in the boonies".

Regards,
Rick

I don't believe the information comes from someone driving each road as Google does for their street photos; especially for the roads in the "boonies." I have friends and relatives that live in places where the only way you can find them is with dogs yet their dirt roads are on the GPS. I would guess that road departments for the county and state have detailed electronic maps that they give to people like Garmin. Companies like Garmin and TomTom wouldn't each send out teams to get the data. They must get the data from the same source. It's just that the level of detail amazes me.

I had seen something on this before so googled "where do GPS map databases come from?" and the best hit so far is this one:

http://blogs.howstuffworks.com/2010/08/ ... p-to-date/

As you suspected, there are two big companies that develop the databases, Navteq and Tele Atlas. This page has a couple of videos showing their vans running around, with cameras snapping images linked to GPS lat/long. That helps for the bigger cities, but doesn't help out in the sticks. The article also mentions use of government maps, which I assume is what they use out in the boonies. I've noticed that even the edges of parks and bodies of water are indicated, even if not right near the road - these are not from driving around.

On my drive to/from work there is a farmer's long driveway, and he has posted a hand-made sign reading "NOT Texas Street" with an arrow below it with "7 miles" added. Sure enough, our older GPS tags his driveway as Texas Street - someone entering and/or tagging the info from a satellite photo or older map got it wrong. I'm sure he got tired of people showing up at his door looking for one of the businesses located on Texas Street!

I'm still very impressed with HOW MUCH data there is in a GPS if it's holding virtually ALL the roads in the continental US!