How can I transform latitude and longitude to x,y in Java?

Question

Hello,

I am working on a geographic project in Java.

The input are coordinates : 24.4444 N etc Output: a PLAIN map (not round) showing the point of the coordinates.

I don't know the algorithm to transform from coordinates to x,y on a JComponent, can somebody help me?

The map looks like this: http://upload.wikimedia.org/wikipedia/commons/7/74/Mercator-projection.jpg

Thank you

Answer 1

Given your sparse example, the range of your inputs will be (90.0N - 90.0S) and (180W - 180E). It is easiest - and standard - if you convert South and West to negatives giving you latitudes of (90.0..-90.0) and longitudes of (180.0..-180.0).

Given the size of your canvas - let's say it is 140x120 pixels - you get:

x = (latitude * canvas_height / 180.0) + (canvas_height / 2)
y = (longitude * canvas_width / 360.0) + (canvas_width / 2)

or:

x = (longitude * 120.0 / 180.0) + (120/2)
y = (latitude  * 140.0 / 360.0) + (140/2)

where I have ordered the operations to minimize rounding error. This assumes the canvas has point (0,0) in the upper-left or, if not, that you are Australian.

Added: you just threw in the bit about Mercator projections making my simple answer incorrect (but possibly still usable by you if you don't actually care about projection)

Answer 2

MSW provided a good example. Ultimately the algorithm depends on the map projection used. Here are a couple of good resources I used in the past.

The following link is a good reference to a number of different map projections with enough math formulas to choke a horse.

http://www.remotesensing.org/geotiff/proj_list/

Here is a decent reference for doing this in PhP. While not Java, it should be straight forward enough to apply the principles outlined.

http://www.web-max.ca/PHP/article_1.php