Convert Lat/Longs to X/Y Co-ordinates
I have the Lat/Long value of New York City, NY; 40.7560540,-73.9869510 and a flat image of the earth, 1000px × 446px.
I would like to be able to convert, using Javascript, the Lat/Long to an X,Y coordinate where the point would reflect the location.
So the X,Y coordinate form the Top-Left corner of the image would be; 289, 111
Things to note:
- don't worry about issues of what projection to use, make your own assumption or go with what you know might work
- X,Y can be form any corner of the image
- Bonus points for the same solution in PHP (but I really need the JS)
Answer
The projection you use is going to change everything, but this will work assuming a Mercator projection:
<html>
<head>
<script language="Javascript">
var dot_size = 3;
var longitude_shift = 55; // number of pixels your map's prime meridian is off-center.
var x_pos = 54;
var y_pos = 19;
var map_width = 430;
var map_height = 332;
var half_dot = Math.floor(dot_size / 2);
function draw_point(x, y) {
dot = '<div style="position:absolute;width:' + dot_size + 'px;height:' + dot_size + 'px;top:' + y + 'px;left:' + x + 'px;background:#00ff00"></div>';
document.body.innerHTML += dot;
}
function plot_point(lat, lng) {
// Mercator projection
// longitude: just scale and shift
x = (map_width * (180 + lng) / 360) % map_width + longitude_shift;
// latitude: using the Mercator projection
lat = lat * Math.PI / 180; // convert from degrees to radians
y = Math.log(Math.tan((lat/2) + (Math.PI/4))); // do the Mercator projection (w/ equator of 2pi units)
y = (map_height / 2) - (map_width * y / (2 * Math.PI)) + y_pos; // fit it to our map
x -= x_pos;
y -= y_pos;
draw_point(x - half_dot, y - half_dot);
}
</script>
</head>
<body onload="plot_point(40.756, -73.986)">
<!-- image found at http://www.math.ubc.ca/~israel/m103/mercator.png -->
<img src="mercator.png" style="position:absolute;top:0px;left:0px">
</body>
</html>