GPS application with static jpg map

Question

Hi all,
I am writing an gps application with a static map image (jpg) of a smal area. I would like to use this image to display users position within the area when he is in this area and let users zoom in-out.  I also need to specify specific positions within the area of coverage. How can I specify it.
 Is there a standard way (preferably with example) of specifying the coordinates that the image covers and positions and how to handle it?
is there a better or other standard way of displaying map in the application?
will appreciate.
regards,
rnv

Answer 1

The only thing you really need is a translation function to convert geographical coordinates and pixel coordinated. Assuming your map is covering a smll area you should be able to treat the earth as though it's flat to keep the equations simple.  I will also assume the edges of your map are parallete to the lines of latitude and longitude. You will need to find the latitudes and longitudes that the 4 corners of the map respresent.
Let's assume you've got your image and you've found the four coordinates to which your cordners map. I'll name these coordinates A, B, D, and D.
A          B
  +------+
  |      |
  |      |
  +------+
C         D
Under this system for small maps you can use linear interpolation to map real coordinates to pixel coordinates on your map.  Let's say the coordinates of the 4 points are as follows (I don't know what actually falls in these coordinates in real life, they have been chosen arbitrarily).
A = (30N, 110W)
B = (30N, 120W)
C = (20N, 110W)
D = (20N, 120W)
Let's also say your image is 1000 pixels wide by 500 pixels high. If your user's real coordinates were (24N, 117W) then the pixel coordinates on the map could be found as follows.
X = ((1000)/(110-120)) * (110-117) = 700
Y = ((500) / (20N-30N)) * (20N-24N) = 200
So the point on the above map that is representational of the user's position would be (700, 200).  Since you would allow the user to zoom (and presumably pan) you would also have adjust these pixel coordinates so that they go through the same transformation that the map does.