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myrrh
Occasional Visitor
Joined: Jan 19, 2004
Posts: 41
Location: Dayton, Ohio, USA
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 Posted: Thu Jun 07, 2007 4:38 pm Post subject: How does a GPS determine where to display on map? |
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| I have a question about GPS maps. Once a gps calculates it's position on the earth how does it determine where it displays on the map? Is the map a database with every cell in the database pointing to segment of the picture of the whole map? |
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MaFt
Pocket GPS Staff
Joined: Aug 31, 2005
Posts: 15140
Location: Bradford, West Yorkshire
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| Posted: Thu Jun 07, 2007 4:43 pm Post subject: |
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my understanding is that the map as a whole has key points on it with set co-ordinates / reference points. once the gps chip gets the satellite fix it calculates it's current co-ordinates and then locates the relevent part on the map using the reference points.
it's probably more technical than that though ;)
MaFt |
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robertn
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Joined: Feb 06, 2005
Posts: 564
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| Posted: Thu Jun 07, 2007 9:35 pm Post subject: |
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Generally map are represented as flat surfaces and the earth is sort of spherical. The translation between these is known as Datums. GPS usually uses a datum known as WGS84, there are hundreds of others (ordinence survey is one).
For small distances requiring lower accuracy (Say 100km / 5 meters), at latitudes below about 60 degrees, a linear translation is all that is needed. - Tell the system where the lat/long of 2 points on the map are (3 if the map is not north/south orenited), and you can get away with using simple linear translations, and don't need to worry about mixing up datums. At higher latitudes, the model starts to get too much error too quickly for linear translations.
If you want to maintain say 1 meter accuracy over 100's or 1000's of miles (yes, it is done), then you need some very complex maths, and must ensure all the coordinates are in the correct datum. Most systems use someting in the middle, a few points and some no-linear, but not too complex calculations. |
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DennisN
Tired Old Man
Joined: Feb 27, 2006
Posts: 14892
Location: Keynsham
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| Posted: Thu Jun 07, 2007 10:29 pm Post subject: |
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Awww! And here's me thinking just plain old fashioned magic. 
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Dennis
If it tastes good - it's fattening.
Two of them are obesiting!! |
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Skippy
Pocket GPS Verifier
Joined: 24/06/2003 00:22:12
Posts: 2946
Location: Escaped to the Antipodies! 36.83°S 174.75°E
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| Posted: Thu Jun 07, 2007 10:57 pm Post subject: Re: How does a GPS determine where to display on map? |
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| myrrh wrote: | | Once a gps calculates it's position on the earth how does it determine where it displays on the map? |
Ok, so it knows your co-ordinates and it knows what map scale it's going to use (eg, the map may be 1 mile wide by 4 miles high). From that it can figure out the Lat/Lon co-ordinates of the edges of the map it's going to display to you.
In the map data, road is made up of a series of co-ordinates (dots) and it links them up like a join-the-dots drawing to create the roads on the map. Using the known location reported by the receiver and the scale of the map, it gets all the points it needs from it's map data and joins the dots to draw roads.
Because the maps aren't 100% accurate the GPS will look at where you are and then fiddle your location slightly so you appear on the nearest road rather than 50 feet away from it. Mostly it gets it right but sometimes it can lock you to the wrong road with strange results.
That's a basic description, in reality it's much more complex. To be able to calculate routes, the GPS unit also has information about the road type, restrictions, intersections and maybe even traffic conditions.
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PaulB2005
Pocket GPS Moderator
Joined: Jan 04, 2006
Posts: 9323
Location: Durham, UK
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| Posted: Fri Jun 08, 2007 9:15 am Post subject: |
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| Quote: | | Awww! And here's me thinking just plain old fashioned magic. |
That's just silly. Everyone knows it's the Genius Pixie System....
Interesting thread... |
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DennisN
Tired Old Man
Joined: Feb 27, 2006
Posts: 14892
Location: Keynsham
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| Posted: Fri Jun 08, 2007 6:34 pm Post subject: |
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| PaulB2005 wrote: | | Quote: | | Awww! And here's me thinking just plain old fashioned magic. |
That's just silly. Everyone knows it's the Genius Pixie System. |
Don't be cruel - you know I'm going back to my second childhood now, so magic is my limit.
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Dennis
If it tastes good - it's fattening.
Two of them are obesiting!! |
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