Adjust coordinates for Google Maps via pixel offsets. Ideal for offsetting the center of your map based on a marker position.

googlemapadjust.php

/**
 * Class GoogleMapAdjust
 *
 * Adjusts latitude & longitude coördinates by pixel offsets.
 *
 * This class takes a coordinate, converts it to a pixel value, calculates the
 * offsetted value and converts back to the coordinate.
 *
 * Primary use case: offset the center of a Google Map canvas from a marker
 * position on your map. This is useful if you don't want markers to be shown
 * dead center because you want to display extra information in a (fixed)
 * position near the marker.
 *
 * @see
 *   https://developers.google.com/maps/documentation/staticmaps/
 *
 * @see
 *   http://stackoverflow.com/questions/1763917/google-static-maps-move-maps-with-a-finger
 */
class GoogleMapAdjust {

  const GOOGLEOFFSET = 268435456;

  protected $googleOffsetRadius;

  protected $mathPi180;

  protected $preLonToX1;

  public function __construct() {
    $this->googleOffsetRadius = GoogleMapAdjust::GOOGLEOFFSET / M_PI;
    $this->mathPi180 = M_PI / 180;
    $this->preLonToX1 = $this->googleOffsetRadius * (M_PI / 180);
  }

  /**
   * Convert a longitude coordinate to a pixel value
   */
  private function LonToX($lon) {
    return round(GoogleMapAdjust::GOOGLEOFFSET + $this->preLonToX1 * $lon);
  }

  /**
   * Convert a latitude coordinate to a pixel value
   */
  private function LatToY($lat) {
    return round(GoogleMapAdjust::GOOGLEOFFSET - $this->googleOffsetRadius * log((1 + sin($lat * $this->mathPi180)) / (1 - sin($lat * $this->mathPi180))) / 2);
  }

  /**
   * Converts a longitude pixel value to a coordinate
   */
  private function XToLon($x) {
    return ((round($x) - GoogleMapAdjust::GOOGLEOFFSET) / $this->googleOffsetRadius) * 180 / M_PI;
  }

  /**
   * Converts a latitude pixel value to a coordinate
   */
  private function YToLat($y) {
    return (M_PI / 2 - 2 * atan(exp((round($y) - GoogleMapAdjust::GOOGLEOFFSET) / $this->googleOffsetRadius))) * 180 / M_PI;
  }

  /**
   * Adjust the longitude of a coördinate on a map by a given number of pixels.
   *
   * @param $lon
   *   The longitude ie. 4.22345
   * @param $delta
   *   The offset in pixels. Can be positive or negative
   * @param $zoom
   *   The zoom level of the map between 0..21
   * @return float
   *   The re-calculated longitude coordinate
   */
  public function adjustLonByPixels($lon, $delta, $zoom) {
    $x = $this->LonToX($lon) + ($delta << (21 - $zoom));
    return $this->XToLon($x);
  }

  /**
   * Adjust the latitude of a coördinate on a map by a given number of pixels.
   *
   * @param $lat
   *   The latitude ie 52.2345
   * @param $delta
   *   The offset in pixels. Can be positive or negative
   * @param $zoom
   *   The zoom level of the map between 0..21
   * @return float
   *   The re-calculated latitude coordinate
   */
  public function adjustLatByPixels($lat, $delta, $zoom) {
    $y = $this->LatToY($lat) + ($delta << (21 - $zoom));
    return $this->YToLat($y);
  }
}