I have a city map of Moscow. We modified a Google Maps image with some artistic elements, but the relation between GPS coordinates and pixels remains the same.
Problem: How do I convert GPS coordinates from various data points we have into pixel coordinates in the image?
The key to all of this is understanding map projections. As others have pointed out, the cause of the distortion is the fact that the spherical (or more accurately ellipsoidal) earth is projected onto a plane.
In order to achieve your goal, you first must know two things about your data:
I'm assuming your data is in these coordinate systems.
The spherical Mercator projection defines a coordinate pair in meters, for the surface of the earth. This means, for every lat/long coordinate there is a matching meter/meter coordinate. This enables you to do the conversion using the following procedure:
In order to go from a WGS84 point to a pixel on the image, the procedure is now:
You can use the proj4js library like this:
// include the library <script src="lib/proj4js-combined.js"></script> //adjust the path for your server //or else use the compressed version // creating source and destination Proj4js objects // once initialized, these may be re-used as often as needed var source = new Proj4js.Proj('EPSG:4326'); //source coordinates will be in Longitude/Latitude, WGS84 var dest = new Proj4js.Proj('EPSG:3785'); //destination coordinates in meters, global spherical mercators projection, see http://spatialreference.org/ref/epsg/3785/ // transforming point coordinates var p = new Proj4js.Point(-76.0,45.0); //any object will do as long as it has 'x' and 'y' properties Proj4js.transform(source, dest, p); //do the transformation. x and y are modified in place //p.x and p.y are now EPSG:3785 in meters