Display coordinates on a map, not the right location is displayed.


For my study i need to make a map of iceland and display location on it with coordinates. I have edit the code of this discussion: http://forum.processing.org/one/topic/using-a-world-map-in-processing.html

The location seems displays the location mirrored, i hope someone can look at the code. And more importend explain what the error/bug is:).

Hopefully someone can help me with this question.

Thank you very much.

map of iceland: https://www.dropbox.com/s/mmltfgzfu2ftc8a/island.png?dl=0


// Declaring a variable of type PImage

PImage backgroundMap;

float mapGeoLeft   =  -66.50;        // Longitude 66 degrees west
float mapGeoRight  =  -63.50;          // Longitude 63 degrees east
float mapGeoTop    =   24.20;            // Latitude 24 degrees north.
float mapGeoBottom =   13.80;       // Latitude 13 degrees south.

float mapScreenWidth,mapScreenHeight;  // Dimension of map in pixels.

void setup()
    // Make a new instance of a PImage by loading an image file
  backgroundMap   = loadImage("island.png");
  mapScreenWidth  = width;
  mapScreenHeight = height;

void draw()


  PVector p = geoToPixel(new PVector(-64.08,21.56));  // reykjavik


// Converts screen coordinates into geographical coordinates. 
// Useful for interpreting mouse position.
public PVector pixelToGeo(PVector screenLocation)
    return new PVector(mapGeoLeft + (mapGeoRight-mapGeoLeft)*(screenLocation.x)/mapScreenWidth,
                       mapGeoTop - (mapGeoTop-mapGeoBottom)*(screenLocation.y)/mapScreenHeight);

// Converts geographical coordinates into screen coordinates.
// Useful for drawing geographically referenced items on screen.
public PVector geoToPixel(PVector geoLocation)
    return new PVector(mapScreenWidth*(geoLocation.x-mapGeoLeft)/(mapGeoRight-mapGeoLeft),
                       mapScreenHeight - mapScreenHeight*(geoLocation.y-mapGeoBottom)/(mapGeoTop-mapGeoBottom));


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  • Thank you for the feedback Chrisir, i have edit the post.

  • Can somebody help me with this problem?

  • I can't really help to fix this code. But some thoughts:

    The way it is now, it seams inverted, are you getting all coordinates right?

    Have you minded the projection used for this specific map? You know, the earth is a sphere and all maps need a calculation to represent it as a plane. This does meters.

    Have you heard of unfolding maps? It's a library that makes map plotting really easy.

    Once I've done this for the 'Kavrayskiy' projection. Here the post with the code:



  • edited September 2014

    I have the same hunch as _vk. The conversion methods from Jo Wood are for Equirectangular projection, but your Iceland map is in a different projection (easily recognizable due to the bended arctic circle line).

    You need to either select a different map of Iceland (in equi-rectangular projection for use of the conversion methods above, or Mercator for use in Unfolding), or you have to use a map library capable of converting screen coordinates to the one from your map's projection (then you still would need to figure out which one that is exactly).

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