Convert LongLat To Mercator
From Openmoko
Here is a little piece of code i have wrote to be able to convert any longitude / latitude to a x,y coordinate plotted on a map based on a Mercator Projection. For this to work, you will need 2 points you know on the map (Set up those two points in p1x, p1y, p1lat, p1long, p2x, p2y, p2lat, p2long). Then put the longitude / latitude in fakeLong / fakeLat and it should give you the x,y coordinate of this point. This is wrote with C++.
#include <math.h>
#include <iostream>
// This is the first reference point (Here it is a top left corner point on the example map i used).
int p1x = 0;
int p1y = 0;
float p1lat = 45.554141f;
float p1long = 0.633598f;
// The second reference point (Bottom right corner of my example map):
int p2x = 1500;
int p2y = 1500;
float p2lat = 45.510440f;
float p2long = 0.696285f;
int delta_x = abs(p1x - p2x);
float delta_longitude = fabs(p1long - p2long);
float x_factor = delta_x / delta_longitude;
float p1mercY = lat2UMT(p1lat);
float p2mercY = lat2UMT(p2lat);
int delta_y = abs(p1y-p2y);
float delta_mercator = fabs(p1mercY-p2mercY);
float y_factor = delta_y / delta_mercator;
// Here we input the point we want to convert.
float fakeLat = 45.530824f;
float fakeLong = 0.663729f;
int plotX, plotY;
plotX = p1x + (int)((fakeLong-p1long)*x_factor);
if (fakeLat < p1lat)
plotY = p1y + (int)((fabs(p1mercY-lat2UMT(fakeLat)))*y_factor);
else
plotY = p1y - (int)((fabs(p1mercY-lat2UMT(fakeLat)))*y_factor);
std::cout << plotX << "," << plotY << ".\n";
float rad (float angle) {
return angle*(M_PI/180.0f);
}
float lat2UMT(float latitude) {
return 0.5f*log((1.0f+sin(rad(latitude))) / (1.0f-sin(rad(latitude))));
}
