transformAndPoint.js

latLonToOffsets.js

/**
 * @param {number} latitude in degrees
 * @param {number} longitude in degrees
 * @param {number} mapWidth in pixels
 * @param {number} mapHeight in pixels
 */
function latLonToOffsets(latitude, longitude, mapWidth, mapHeight) {
  const FE = 180; // false easting
  const radius = mapWidth / (2 * Math.PI);

  const latRad = degreesToRadians(latitude);
  const lonRad = degreesToRadians(longitude + FE);

  const x = lonRad * radius;

  const yFromEquator = radius * Math.log(Math.tan(Math.PI / 4 + latRad / 2));
  const y = mapHeight / 2 - yFromEquator;

  return { x, y };
}