Why does the accuracy of my longitude latitude translate appear to diminish below the equator?

Question

I received some help with a previous question on how to go about translating my map to given longitude and latitude values, here: How can I use SVG translate to center a d3.js projection to given latitude and longitude values? I wrote a method which pans to the given location at the given scale:

zoomTo: function(point, scale) {
    var lat = -SFMap.projection(point)[0] * scale;
    var lon = -SFMap.projection(point)[1] * scale;
    var x = (SFMap.config.width / 2) + lat;
    var y = (SFMap.config.height / 2) + lon;

    SFMap.g
        .transition()
        .duration(500)
        .attr("transform", "translate("+ x +","+ y +") scale("+ scale +")");
}

I can 'zoom' around Europe just fine, but when I move to Jakarta, Indonesia, the center-point lays clearly over the ocean.

London - SFMap.zoomTo([0.1062, 51.5171], 1300) London

Jakarta - SFMap.zoomTo([106.7500, 6.1333], 1300); Jakarta

And, this problem is emphasised if I try Australia - I can't even see it.

I should note that I am using d3 to render a Mercator projection and retrieving longitude and latitude values from Google Search.

Please could someone suggest why this is happening? I am aware that there is a lot of math behind the scenes, but I'd hoped d3 would take care of this.

Edit:

After reading Pablo's comment, I decided to remove my zoomTo() method from the equation and simply test whether d3 could center([106.7500, 6.1333]) correctly on Jarkata at a fixed scale(1000), but it did not; it still dumps you in the ocean.

Are you returning the group to the projection scale between the transformations? When you apply a projection, you are using the scale of the projection as a starting point, not the current scale of the figure. Are the errors consistent? If you go from A, B and then to C, you have the same error than going from B, A and then C? — Pablo Navarro, May 30 '13 at 14:34
@PabloNavarro, you have an interesting point. I am not passing scale adjustments back to `SFMap.projection()` because I figured I didn't need to if I was calculating the affect scale had on the translate within my transform method, `zoomTo()` - shown above. I will explore passing `scale` to the projection, before using its values, when I get home from work. I will respond accordingly. :) Many thanks. — Check12, May 30 '13 at 15:16

Check12 · Accepted Answer · 2013-05-31T13:50:16.067

Embarrassingly, I discovered a silly mistake with my use of longitude and latitude coordinates.

Although I was passing coordinates to projection([106.7500, 6.1333]) on the inverse after retrieving them from Google, I had not noticed the significance of the orientation (or cardinal direction) that Google was also giving.

6.1333° S, 106.7500° E means Jakarta is 6.1333 degrees South of the equator.

So, my problem occurred because I wasn't passing projection() a negative value for the Southern coordinate - meaning, my projection was always going to be 6.1333 degrees above the equator, when it should have been below.

Why does the accuracy of my longitude latitude translate appear to diminish below the equator?

1 Answers1