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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:
856
For coordinates captured using a GPS, or by any means, longitude is the X value and latitude is the Y value. These are for a geographic coordinate system and have units of degrees.
Easy. X = Longitude, Y = Latitude.
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>A basic conversion function in js would be:
MAP_WIDTH = 1000;
MAP_HEIGHT = 446;
function convert(lat, lon){
var y = ((-1 * lat) + 90) * (MAP_HEIGHT / 180);
var x = (lon + 180) * (MAP_WIDTH / 360);
return {x:x,y:y};
}This will return the number of pixels from upper left. This function assumes the following:
25
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