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Answer 1

A:

If you have a picture of the whole earth, the projection does always matter. But maybe I just don't understand your question.

hanno 2009-06-19 21:36:08

You are correct, but in this case I would gladly use an appropriate image to match a working solution for a specific projection.

michael 2009-06-21 00:47:40

Answer 2

+3 A:

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:

That your image is properly aligned with the upper left corner (0,0) aligning with 90* North by 180* West.
That your coords are signed with N being -, S being +, W being - and E being +

Mike Clark 2009-06-19 21:54:18

Answer 3

+6 A:

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 + longitude_shift) % map_width;

    // 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

    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>

Paul A Jungwirth 2009-06-20 01:04:17

This also worked for me and is an excellent answer. I did have change 'lng' to 'parseInt(lng, 10)' on this line:x = (map_width * (180 + lng) / 360 + longitude_shift) % map_width;It would work when I manually call the function, put passing value from an array returned NaN for xThanks!

michael 2009-06-21 02:32:29

i thinkx = (map_width * (180 + lng) / 360 + longitude_shift) % map_width;should bex = ((map_width * (180 + lng) / 360) % map_width) + longitude_shift;otherwise japan and the east coast of australia (me included) get "wrapped" to the middle of the pacific ocean. :P

matpalm 2009-10-15 11:20:26

Answer 4

+2 A:

There's a good Javascript library, PROJ4JS, that allows you to do transformations between different projections.

andri 2009-06-20 01:59:30

Answer 5

A:

how can i draw the points if i am using US mercetor map. Actually i want points some cities in US map. can you help me

Dinesh 2010-05-26 10:04:18

Probably best to create your own Question since this one is old and doesn't directly cover this. Might mean better response and visibility.

michael 2010-05-26 19:10:53

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Convert Lat/Longs to X/Y Co-ordinates

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