How to draw real world coordinates rotated relative to the device around a center coordinate? - iphone

Question

I'm working on a simple location-aware game where the current location of the user is shown on a game map, as well as the locations of other players around him. It's not using MKMapView but a custom game map with no streets.
How can I translate the other lat/long coordinates of other players into CGPoint values to represent them in the world scale game map with a fixed scale like 50 meters = 50 points in screen, and orient all the points such that the user can see in which direction he would have to go to reach another player?
The key goal is to generate CGPoint values for lat/long coordinates for a flat top-down view, but orient the points around the users current location similar to the orient map feature (the arrow) of Google Maps so you know where is what.
Are there frameworks which do the calculations?

Answer 1

first you have to transform lon/lat to cartesian x,y in meters.
next is the direction in degrees to your other players. the direction is dy/dx where dy = player2.y to me.y, same for dx. normalize dy and dx by this value by dividing by distance between playerv2 and me.
you receive
ny = dy / sqrt(dx*dx + dy*dy)
nx = dx / sqrt(dx*dx + dy*dy)
multipl with 50. now you have a point 50 m in direction of the player2:
comp2x = 50 * nx;
comp2y = 50 * ny;
now center the map on me.x/me.y. and apply the screen to meter scale

Answer 2

You want MKMapPointForCoordinate from MapKit. This converts from latitude-longitude pairs to a flat surface defined by an x and y. Take a look at the documentation for MKMapPoint which describes the projection. You can then scale and rotate those x,y pairs into CGPoints as needed for your display. (You'll have to experiment to see what scaling factors work for your game.)
To center the points around your user, just subtract the value of their x and y position (in MKMapPoints) from the points of all other objects. Something like:
MKMapPoint userPoint = MKMapPointForCoordinate(userCoordinate);
MKMapPoint otherObjectPoint = MKMapPointForCoordinate(otherCoordinate);
otherObjectPoint.x -= userPoint.x; // center around your user
otherObjectPoint.y -= userPoint.y;
CGPoint otherObjectCenter = CGPointMake(otherObjectPoint.x * 0.001, otherObjectPoint.y * 0.001);
// Using (50, 50) as an example for where your user view is placed.
userView.center = CGPointMake(50, 50);
otherView.center = CGPointMake(50 + otherObjectCenter.x, 50 + otherObjectCenter.y);