How to convert MGRS coordinates to WGS84 lat/lon?

[cawa-93]

I need to convert MGRS coordinates into WGS84 latitude/longitude using PHPCoord. For example, I would like to transform the MGRS string: 36U XB 29352 65662 into geographic coordinates approximately: (51.12780249, 34.84857004)

Is there a built‑in way in PHPCoord to handle MGRS → WGS84 conversion, or do I need to parse MGRS manually into UTM and then convert? If there is an existing class or method, could you please point me to the correct usage?

[dvdoug]

You're correct, there's no built-in support for MGRS references so you'll need do the first part yourself the hard way and convert to a UTMPoint, from there everything should be automagic back into geographic coordinates.

Not sure of the spec on how the grid references letters are calculated, you might be able to take inspiration from BritishNationalGridPoint to calculate them but that does assume a square grid and some of the charts I just saw looked more degree-based.

But MGRS->UTM is definitely the way to go. If you get it working, I'd be happy to consider a PR to add it to the library

[cawa-93]

I made this class to do what I need. It works on proj. It kinda works for me and my tasks, but I doenst have time to make propper PR. It do mgrs -> utm -> latlong. The mgrs -> utm part is in decode method. If it is useful, you or anyone can use it as you wish.

use proj4php\Point;
use proj4php\Proj;
use proj4php\Proj4php;

final class MGRSConverter
{
    private const int NUM_100K_SETS = 6;
    private const string SET_ORIGIN_COLUMN_LETTERS = 'AJSAJS';
    private const string SET_ORIGIN_ROW_LETTERS = 'AFAFAF';

    private const int A = 65;
    private const int I = 73;
    private const int O = 79;
    private const int V = 86;
    private const int Z = 90;

    private readonly Proj4php $proj4;

    public function __construct(?Proj4php $proj4 = null)
    {
        $this->proj4 = $proj4 ?? new Proj4php();
    }

    /**
     * Convert MGRS string to WGS84 lat/lon.
     *
     * @param string $mgrs Example: "36U XB 29352 65662"
     */
    public function toLatLon(string $mgrs): CoordinatesPoint
    {
        $utm = $this->decode($mgrs);

        $zone = $utm['zoneNumber'];
        $hemisphere = ($utm['zoneLetter'] < 'N') ? 'south' : 'north';

        $projUtmDef = "+proj=utm +zone={$zone} +datum=WGS84 +units=m +no_defs";
        if ($hemisphere === 'south') {
            $projUtmDef .= ' +south';
        }

        $projUTM = new Proj($projUtmDef, $this->proj4);
        $projWGS84 = new Proj('EPSG:4326', $this->proj4);

        $pointUTM = new Point($utm['easting'], $utm['northing'], $projUTM);
        $pointLatLon = $this->proj4->transform($projWGS84, $pointUTM);

        return new CoordinatesPoint(latitude: $pointLatLon->y, longitude: $pointLatLon->x);
    }

    /**
     * Decode the UTM parameters from a MGRS string.
     *
     * @return array{easting: float, northing: float, zoneLetter: string, zoneNumber: int, accuracy: ?float}
     */
    private function decode(string $mgrsString): array
    {
        $mgrsString = preg_replace('/\s+/', '', strtoupper($mgrsString));
        if ($mgrsString === null || $mgrsString === '') {
            throw new \TypeError('MGRSPoint coverting from nothing');
        }

        $length = strlen($mgrsString);
        $sb = '';
        $i = 0;
        $testChar = '';

        while ($i < $length && !ctype_alpha($testChar = $mgrsString[$i])) {
            if ($i >= 2) {
                throw new \RuntimeException("MGRSPoint bad conversion from: {$mgrsString}");
            }
            $sb .= $testChar;
            $i++;
        }

        $zoneNumber = intval($sb, 10);

        if ($i === 0 || $i + 3 > $length) {
            throw new \RuntimeException("MGRSPoint bad conversion from {$mgrsString}");
        }

        $zoneLetter = $mgrsString[$i++];
        if ($zoneLetter <= 'A' || $zoneLetter === 'B' || $zoneLetter === 'Y' || $zoneLetter >= 'Z' || $zoneLetter === 'I' || $zoneLetter === 'O') {
            throw new \RuntimeException("MGRSPoint zone letter {$zoneLetter} not handled: {$mgrsString}");
        }

        $hunK = substr($mgrsString, $i, 2);
        $i += 2;

        $set = $this->get100kSetForZone($zoneNumber);

        $east100k = $this->getEastingFromChar($hunK[0], $set);
        $north100k = $this->getNorthingFromChar($hunK[1], $set);

        while ($north100k < $this->getMinNorthing($zoneLetter)) {
            $north100k += 2000000;
        }

        $remainder = $length - $i;
        if ($remainder % 2 !== 0) {
            throw new \RuntimeException(
                "MGRSPoint has to have an even number of digits after the zone letter and two 100km letters - {$mgrsString}"
            );
        }

        $sep = intdiv($remainder, 2);
        $sepEasting = 0.0;
        $sepNorthing = 0.0;
        $accuracyBonus = null;

        if ($sep > 0) {
            $accuracyBonus = 100000 / pow(10, $sep);
            $sepEastingString = substr($mgrsString, $i, $sep);
            $sepNorthingString = substr($mgrsString, $i + $sep);
            $sepEasting = (float) $sepEastingString * $accuracyBonus;
            $sepNorthing = (float) $sepNorthingString * $accuracyBonus;
        }

        return [
            'easting' => $sepEasting + $east100k,
            'northing' => $sepNorthing + $north100k,
            'zoneLetter' => $zoneLetter,
            'zoneNumber' => $zoneNumber,
            'accuracy' => $accuracyBonus,
        ];
    }

    private function get100kSetForZone(int $zoneNumber): int
    {
        return (($zoneNumber - 1) % self::NUM_100K_SETS) + 1;
    }

    private function getEastingFromChar(string $letter, int $set): int
    {
        $curCol = ord(self::SET_ORIGIN_COLUMN_LETTERS[$set - 1]);
        $eastingValue = 100000;
        $rewindMarker = false;
        $target = ord($letter);

        while ($curCol !== $target) {
            $curCol++;
            if ($curCol === self::I) {
                $curCol++;
            }
            if ($curCol === self::O) {
                $curCol++;
            }
            if ($curCol > self::Z) {
                if ($rewindMarker) {
                    throw new \RuntimeException("Bad character: {$letter}");
                }
                $curCol = self::A;
                $rewindMarker = true;
            }
            $eastingValue += 100000;
        }

        return $eastingValue;
    }

    private function getNorthingFromChar(string $letter, int $set): int
    {
        if ($letter > 'V') {
            throw new \TypeError("MGRSPoint given invalid Northing {$letter}");
        }

        $curRow = ord(self::SET_ORIGIN_ROW_LETTERS[$set - 1]);
        $northingValue = 0;
        $rewindMarker = false;
        $target = ord($letter);

        while ($curRow !== $target) {
            $curRow++;
            if ($curRow === self::I) {
                $curRow++;
            }
            if ($curRow === self::O) {
                $curRow++;
            }
            if ($curRow > self::V) {
                if ($rewindMarker) {
                    throw new \RuntimeException("Bad character: {$letter}");
                }
                $curRow = self::A;
                $rewindMarker = true;
            }
            $northingValue += 100000;
        }

        return $northingValue;
    }

    private function getMinNorthing(string $zoneLetter): int
    {
        return match ($zoneLetter) {
            'C' => 1100000,
            'D' => 2000000,
            'E' => 2800000,
            'F' => 3700000,
            'G' => 4600000,
            'H' => 5500000,
            'J' => 6400000,
            'K' => 7300000,
            'L' => 8200000,
            'M' => 9100000,
            'N' => 0,
            'P' => 800000,
            'Q' => 1700000,
            'R' => 2600000,
            'S' => 3500000,
            'T' => 4400000,
            'U' => 5300000,
            'V' => 6200000,
            'W' => 7000000,
            'X' => 7900000,
            default => throw new \TypeError("Invalid zone letter: {$zoneLetter}"),
        };
    }
}