Convert lat/long to pixel X&Y co-ordinates

Question

This type of question seems to have been asked numerous times but none of the solutions posted get me anywhere near the answer I need.

I have this map of Northland,

Answer 1

in Javascript....

var lat=-35.3989854471;
var long=173.504676819;

var imageNorthLat=???;
var imageSouthLat=???;

var imageWestLong=???;
var imageEastLong=???;

var imageLongPixels=400;
var imageLatPixels=400;

var pixelsPerLat=imageLatPixels/(imageNorthLat-imageSouthLat);
var pixelsPerLong=imageLongPixels/(imageEastLong-imageWestLong);

var xPixelPosition=(long-imageWestLong)*pixelsPerLong;
var yPixelPosition=Math.abs(lat-imageNorthLat)*pixelsPerLat;

I didn't try to run this, so there's probably a bit of debugging necessary, and the x and y pixel positions would have to have added to them the x and y positions of the top left or your map. But you can see the idea. And, of course, I may not have understood what you're really asking.