How does Google Maps determine where on the map the user is?

Question

I would like to make a iPhone mapping application. I know that the iPhone can give detailed longitude and latitude coordinates of the user's specific location. How do Google Maps and other mapping applications translate the user's location into the location on the map? There seems to be lots of things to consider. For example, what if the map is rotated, and it seems to me that it there is a lot of scaling and stuff to consider. How does Google Maps translate the user's current location into a specific point on the map and have it be so accurate (within 2 to 3 meters). I would really appreciate an answer or any help. Thanks!

Answer 1

Google maps uses a few different methods to get the location of the user including the location of the mobile network tower the user is connected to (sometimes Google uses triangulation of 2 or more towers to make this more accurate http://techcrunch.com/2007/11/28/google-mobile-maps-pinpoints-your-location-without-gps/), the location of the ISP if the user is connected to WiFi and GPS.

The most accurate is GPS which provides Google with Latitude and Longitude coordinates accurate to within (If I remember correctly) 10 meters. All the images in the Google maps system have GPS coordinates stored against them so all Google has to do is retrieve the right map image and plot the user's position on the image.

I think this is what you're after but if you need more detail about something or if I've misunderstood please leave a comment and I'll modify this answer.

Edit

Please see this page which gives a good introduction to using Google Maps on the iPhone https://developers.google.com/maps/articles/tutorial-iphone

You don't need to code the algorithm for placing a marker on the map to show the users location, it's a simple matter of calling a function/method in the Google Maps API and it does all the work for you. I would suggest having a thorough look through the Google Maps API documentation to find what you need (also found at the previous link).

I think this might be the precise snippet of code you're after: https://developers.google.com/maps/documentation/javascript/overlays#Markers

Answer 2

Your heading/rotation is irrelevant when determining your location. Your location is based on data received from the GPS satellites. There's multiple satellites, and based on how far you are from each, your location is narrowed down to one point. This point has a very detailed pair of coordinates (think 10 digits past decimal point). Every point on Google's provided map is associated with a pair of coordinates. So when your phone figures out your coordinates, it then checks to see where on the map it corresponds to, and places you there on the map. All other locations are also based on coordinates.

Now, the map also has a setting where the map rotates based on your heading. That feature uses the built-in compass that the phone has. The iPhone has had a built-in compass since the 3GS, I believe.

In short, think of the maps provided by Google as a giant, extremely high-resolution image provided by Google, where every pixel (or less) on this image corresponds to a pair of coordinates. After your coordinates are determined, your placed on the corresponding point on that image, regardless of zoom level.

Answer 3

The Google Maps application knows:

• the way the Google Maps service has divided the map into tiles
• the size and location (lat/lon) of every map tile
• the user's selected zoom level
• the size of the device screen
• the position latitude and longitude reported by the device

With this information, the maps application can do some simple linear arithmetic to find out which tile(s) need to be displayed on the screen so that the user's location is shown in the middle of the screen (or wherever desired).