How to convert latitude and longitude into pixel coordinates using PHP

With the continuous development of Web technology, more and more websites now present information through maps, and showing a suitable marking point on the map is a very critical technical issue. When using maps to display data, we sometimes need to convert the longitude and latitude coordinates on the map into pixel coordinates, such as automatically locating marker points on the map. Therefore, in this article, we will introduce how to convert latitude and longitude into pixel coordinates using PHP.

What are longitude, latitude and pixel coordinates

Before we start to explain how to convert longitude and latitude into pixel coordinates, we need to understand the basic concepts of longitude, latitude and pixel coordinates.

The coordinates obtained from the map are longitude and latitude, where longitude refers to the position of a point to the west of the earth, and latitude refers to the position of a point to the south or north of the equator.

The pixel coordinates are the coordinates on the screen, which are composed of pixels on the screen.

When displaying data on a map, we need to convert the latitude and longitude into pixel coordinates in order to locate the specific location.

Calculate latitude and longitude based on pixels

Next, we will learn how to calculate latitude and longitude based on pixel coordinates. In this process, we need to use some basic mathematics. The following is the code for this process:

function pixel2LatLng($x, $y, $zoom) {
    $numTiles = 1 << $zoom; // 2 ^ zoom
    $tileSize = 256;
    $x /= $tileSize;
    $y /= $tileSize;
    $x = min($x, $numTiles - 1);
    $y = min($y, $numTiles - 1);
    $lng = ($x / $numTiles) * 360.0 - 180.0;
    $lat = rad2deg(atan(sinh(pi() * (1 - 2 * $y / $numTiles))));
    return [&#39;lat&#39; => $lat, 'lng' => $lng];
}

This function receives 3 parameters, which are the $x, $y coordinates of the pixel and the $zoom parameter, which describes the zoom level. Next, we create two variables, $numTiles and $tileSize, which represent the number of standard 2D map tiles and the size of each tile, which is 256 pixels.

Note that when converting pixel coordinates, we first need to divide the coordinates by $tileSize to ensure that they are within the standard range. Then limit them to the maximum value in each dimension to avoid them exceeding the map display area.

Next, use simple mathematical formulas to calculate latitude and longitude. First, we calculate $lng, which is the longitude corresponding to the pixel coordinate $x. To do this we use the subtraction calculation (([$x / $numTiles) * 360.0 - 180.0]) which gives us the degree scale of the pixel and then subtract 180 degrees from it. The reason you need to subtract 180 degrees is because longitude can be equal to -180 to 180 degrees. Therefore, we need to limit the value to this range.

Finally, we calculate the latitude value and return an associative array containing the $lat and $lng coordinates.

Calculate pixel coordinates based on longitude and latitude

Inverse of the above process, we can also calculate a pixel coordinate point based on longitude and latitude. Here is the code for this process:

function latLng2Pixel($latitude, $longitude, $zoom) {
    $numTiles = 1 << $zoom; // 2 ^ zoom
    $tileSize = 256;
    $x = round(($longitude + 180.0) / 360.0 * $numTiles);
    $y = round((1.0 - log(tan(deg2rad($latitude)) + 1.0 / cos(deg2rad($latitude))) / pi()) / 2.0 * $numTiles);
    return [&#39;x&#39; => $x * $tileSize, 'y' => $y * $tileSize];
}

This function is very similar to the one above, except that it calculates pixel coordinates based on longitude and latitude instead of the other way around. We use the same variables as before, $numTiles and $tileSize, and introduce the $lat and $lng variables to represent the latitude and longitude values ​​passed to the function.

Then, we first do a simple calculation to get the pixel coordinate of $x. The idea of ​​this formula is that when moving on the map, because the longitude can change from -180 degrees to 180 degrees, we first add 180 degrees to the specified longitude, so that after determining 0 degrees, we can ensure that the calculated result is Positive value. Next, we divide this value by 360 degrees to get the x-coordinate of the decimal point location, and multiply it by the power-of-2 number of the map, $numTiles, to get the pixel coordinates.

Next, we calculate the pixel coordinate of $y. When traveling on the map, latitude can change from -90 degrees to 90 degrees. The idea of ​​this formula is that, starting from a reference point (the equator), we use the tangent and arctangent functions to calculate the distance to and around the point we want to find. Finally, we divide by $tileSize to convert the pixel coordinates to pixels instead of coordinates.

Finally, an associative array containing the $x and $y coordinates is returned.

Summary

The above is the PHP implementation method of converting longitude and latitude into pixel coordinates and converting pixel coordinates into longitude and latitude. These two transformation techniques can be very useful when displaying data using maps. Hope this article helps you!

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