Calculate Maidenhead Grid Square Locator from Latitude/Longitude coordinate pairs.

MaidenHeadLocater.php

<?php

/**
 * Calculates a Maidenhead Grid Square Locator containing
 * the given latitude/longitude (WGS84) coordinate pair.
 *
 * @param  string|float  $input_lat
 * @param  string|float  $input_lng
 * @return string
 */
function latlng_to_locator(string|float $input_lat, string|float $input_lng): string
{
	$locator = '';

	$lat = floatval($input_lat);
	$lng = floatval($input_lng);

	if ($lng >= 180 || $lng <= -180) {
		throw new \Exception('Longitude value is out of bounds.');
	}

	if ($lat >= 180 || $lat <= -180) {
		throw new \Exception('Latitude value is out of bounds.');
	}

	// Maidenhead grid squares begin at 90, 180 so coordinates should be from 0 to 360
	$lng += 180;
	$lat += 90;

	$asciiBaseOne   = ord('A');
	$asciiBaseTwo   = ord('0');
	$asciiBaseThree = ord('a');


	/* === Fields === */
	// there are 18 fields that are 20 degrees across        (360 / 18 = 20)
	// there are 18 fields that are 10 degrees up            (180 / 18 = 20)
	$locator  = chr($asciiBaseOne + intval($lng / 20));
	$locator .= chr($asciiBaseOne + intval($lat / 10));


	/* === Squares === */
	// there are 10 squares that are 2 degrees across        (20 / 10 = 2)
	// there are 10 squares that are 1 degrees up            (10 / 10 = 1)
	$locator .= chr($asciiBaseTwo + intval(intval($lng) % 20 / 2));
	$locator .= chr($asciiBaseTwo + intval(intval($lat) % 10 / 1));


	/* === Subsquares === */
	// there are 24 subsquares that are 1/12 degree across   (2 / 24 = 1/12)
	// there are 24 subsquares that are 1/24 degrees up      (1 / 24 = 1/24)
	$locator .= chr($asciiBaseThree + intval(($lng - intval($lng / 2) * 2) * 12));
	$locator .= chr($asciiBaseThree + intval(($lat - intval($lat / 1) * 1) * 24));


	return $locator;
}

echo latlng_to_locator(41.71480231330993, "-72.72718595450274");           // outputs: FN31pr

// EOF

MaidenheadLocator.js

/**
 * Calculates a Maidenhead Grid Square Locator containing
 * the given latitude/longitude (WGS84) coordinate pair.
 */
const LatLngToLocator = (inputLat, inputLng) => {
	let locator = ""

	let lat = parseFloat(inputLat)
	let lng = parseFloat(inputLng)

	if (lng >= 180 || lng <= -180) throw new Error('Longitude value is out of bounds.')
	if (lat >= 90  || lat <= -90)  throw new Error('Latitude value is out of bounds.')

	// Maidenhead grid squares begin at 90, 180 so coordinates should be from 0 to 360
	lng += 180
	lat += 90


	/* === Fields === */
	// there are 18 fields that are 20 degrees across        (360 / 18 = 20)
	// there are 18 fields that are 10 degrees up            (180 / 18 = 20)
	let fieldLng = 'ABCDEFGHIJKLMNOPQRS' . charAt(Math.floor(lng / 20))
	let fieldLat = 'ABCDEFGHIJKLMNOPQRS' . charAt(Math.floor(lat / 10))

	locator += fieldLng + fieldLat

	lng %= 20
	lat %= 10


	/* === Squares === */
	// there are 10 squares that are 2 degrees across        (20 / 10 = 2)
	// there are 10 squares that are 1 degrees up            (10 / 10 = 1)
	let squareLng = '0123456789' . charAt(Math.floor(lng / 2))
	let squareLat = '0123456789' . charAt(Math.floor(lat / 1))

	locator += squareLng + squareLat

	lng %= 2
	lat %= 1


	/* === Subsquares === */
	// there are 24 subsquares that are 1/12 degree across   (2 / 24 = 1/12)
	// there are 24 subsquares that are 1/24 degrees up      (1 / 24 = 1/24)
	let subLng = 'abcdefghijklmnopqrstuvwx' . charAt(Math.floor(lng * 12))
	let subLat = 'abcdefghijklmnopqrstuvwx' . charAt(Math.floor(lat * 24))

	locator += subLng + subLat


	return locator
}

// Lat/Lng may be strings or floats
let locator = LatLngToLocator(41.71480231330993, "-72.72718595450274")

// outputs: FN31pr