 class Mgrs {

    /**
     * Creates an Mgrs grid reference object.
     *
     * @param  {number} zone - 6° longitudinal zone (1..60 covering 180°W..180°E).
     * @param  {string} band - 8° latitudinal band (C..X covering 80°S..84°N).
     * @param  {string} e100k - First letter (E) of 100km grid square.
     * @param  {string} n100k - Second letter (N) of 100km grid square.
     * @param  {number} easting - Easting in metres within 100km grid square.
     * @param  {number} northing - Northing in metres within 100km grid square.
     * @param  {LatLon.datums} [datum=WGS84] - Datum UTM coordinate is based on.
     * @throws {RangeError}  Invalid MGRS grid reference.
     *
     * @example
     *   import Mgrs from '/js/geodesy/mgrs.js';
     *   const mgrsRef = new Mgrs(31, 'U', 'D', 'Q', 48251, 11932); // 31U DQ 48251 11932
     */
    constructor(zone, band, e100k, n100k, easting, northing, datum=LatLonEllipsoidal.datums.WGS84) {
        if (!(1<=zone && zone<=60)) throw new RangeError(`invalid MGRS zone ‘${zone}’`);
        if (zone != parseInt(zone)) throw new RangeError(`invalid MGRS zone ‘${zone}’`);
        const errors = []; // check & report all other possible errors rather than reporting one-by-one
        if (band.length!=1 || latBands.indexOf(band) == -1) errors.push(`invalid MGRS band ‘${band}’`);
        if (e100k.length!=1 || e100kLetters[(zone-1)%3].indexOf(e100k) == -1) errors.push(`invalid MGRS 100km grid square column ‘${e100k}’ for zone ${zone}`);
        if (n100k.length!=1 || n100kLetters[0].indexOf(n100k) == -1) errors.push(`invalid MGRS 100km grid square row ‘${n100k}’`);
        if (isNaN(Number(easting))) errors.push(`invalid MGRS easting ‘${easting}’`);
        if (isNaN(Number(northing))) errors.push(`invalid MGRS northing ‘${northing}’`);
        if (!datum || datum.ellipsoid==undefined) errors.push(`unrecognised datum ‘${datum}’`);
        if (errors.length > 0) throw new RangeError(errors.join(', '));

        this.zone = Number(zone);
        this.band = band;
        this.e100k = e100k;
        this.n100k = n100k;
        this.easting = Number(easting);
        this.northing = Number(northing);
        this.datum = datum;
    }


    /**
     * Converts MGRS grid reference to UTM coordinate.
     *
     * Grid references refer to squares rather than points (with the size of the square indicated
     * by the precision of the reference); this conversion will return the UTM coordinate of the SW
     * corner of the grid reference square.
     *
     * @returns {Utm} UTM coordinate of SW corner of this MGRS grid reference.
     *
     * @example
     *   const mgrsRef = Mgrs.parse('31U DQ 48251 11932');
     *   const utmCoord = mgrsRef.toUtm(); // 31 N 448251 5411932
     */
    toUtm() {
        const hemisphere = this.band>='N' ? 'N' : 'S';

        // get easting specified by e100k (note +1 because eastings start at 166e3 due to 500km false origin)
        const col = e100kLetters[(this.zone-1)%3].indexOf(this.e100k) + 1;
        const e100kNum = col * 100e3; // e100k in metres

        // get northing specified by n100k
        const row = n100kLetters[(this.zone-1)%2].indexOf(this.n100k);
        const n100kNum = row * 100e3; // n100k in metres

        // get latitude of (bottom of) band
        const latBand = (latBands.indexOf(this.band)-10)*8;

        // get northing of bottom of band, extended to include entirety of bottom-most 100km square
        const nBand = Math.floor(new LatLonEllipsoidal(latBand, 3).toUtm().northing/100e3)*100e3;

        // 100km grid square row letters repeat every 2,000km north; add enough 2,000km blocks to
        // get into required band
        let n2M = 0; // northing of 2,000km block
        while (n2M + n100kNum + this.northing < nBand) n2M += 2000e3;

        return new Utm_Mgrs(this.zone, hemisphere, e100kNum+this.easting, n2M+n100kNum+this.northing, this.datum);
    }


    /**
     * Parses string representation of MGRS grid reference.
     *
     * An MGRS grid reference comprises (space-separated)
     *  - grid zone designator (GZD)
     *  - 100km grid square letter-pair
     *  - easting
     *  - northing.
     *
     * @param   {string} mgrsGridRef - String representation of MGRS grid reference.
     * @returns {Mgrs}   Mgrs grid reference object.
     * @throws  {Error}  Invalid MGRS grid reference.
     *
     * @example
     *   const mgrsRef = Mgrs.parse('31U DQ 48251 11932');
     *   const mgrsRef = Mgrs.parse('31UDQ4825111932');
     *   //  mgrsRef: { zone:31, band:'U', e100k:'D', n100k:'Q', easting:48251, northing:11932 }
     */
    static parse(mgrsGridRef) {
        if (!mgrsGridRef) throw new Error(`invalid MGRS grid reference ‘${mgrsGridRef}’`);

        // check for military-style grid reference with no separators
        if (!mgrsGridRef.trim().match(/\s/)) {
            if (!Number(mgrsGridRef.slice(0, 2))) throw new Error(`invalid MGRS grid reference ‘${mgrsGridRef}’`);
            let en = mgrsGridRef.trim().slice(5); // get easting/northing following zone/band/100ksq
            en = en.slice(0, en.length/2)+' '+en.slice(-en.length/2); // separate easting/northing
            mgrsGridRef = mgrsGridRef.slice(0, 3)+' '+mgrsGridRef.slice(3, 5)+' '+en; // insert spaces
        }

        // match separate elements (separated by whitespace)
        const ref = mgrsGridRef.match(/\S+/g);

        if (ref==null || ref.length!=4) throw new Error(`invalid MGRS grid reference ‘${mgrsGridRef}’`);

        // split gzd into zone/band
        const gzd = ref[0];
        const zone = gzd.slice(0, 2);
        const band = gzd.slice(2, 3);

        // split 100km letter-pair into e/n
        const en100k = ref[1];
        const e100k = en100k.slice(0, 1);
        const n100k = en100k.slice(1, 2);

        let e = ref[2], n = ref[3];

        // standardise to 10-digit refs - ie metres) (but only if < 10-digit refs, to allow decimals)
        e = e.length>=5 ?  e : (e+'00000').slice(0, 5);
        n = n.length>=5 ?  n : (n+'00000').slice(0, 5);

        return new Mgrs(zone, band, e100k, n100k, e, n);
    }


    /**
     * Returns a string representation of an MGRS grid reference.
     *
     * To distinguish from civilian UTM coordinate representations, no space is included within the
     * zone/band grid zone designator.
     *
     * Components are separated by spaces: for a military-style unseparated string, use
     *   Mgrs.toString().replace(/ /g, '');
     *
     * Note that MGRS grid references get truncated, not rounded (unlike UTM coordinates); grid
     * references indicate a bounding square, rather than a point, with the size of the square
     * indicated by the precision - a precision of 10 indicates a 1-metre square, a precision of 4
     * indicates a 1,000-metre square (hence 31U DQ 48 11 indicates a 1km square with SW corner at
     * 31 N 448000 5411000, which would include the 1m square 31U DQ 48251 11932).
     *
     * @param   {number}     [digits=10] - Precision of returned grid reference (eg 4 = km, 10 = m).
     * @returns {string}     This grid reference in standard format.
     * @throws  {RangeError} Invalid precision.
     *
     * @example
     *   const mgrsStr = new Mgrs(31, 'U', 'D', 'Q', 48251, 11932).toString(); // 31U DQ 48251 11932
     */
    toString(digits=10) {
        if (![ 2, 4, 6, 8, 10 ].includes(Number(digits))) throw new RangeError(`invalid precision ‘${digits}’`);

        const { zone, band, e100k, n100k, easting, northing } = this;

        // truncate to required precision
        const eRounded = Math.floor(easting/Math.pow(10, 5-digits/2));
        const nRounded = Math.floor(northing/Math.pow(10, 5-digits/2));

        // ensure leading zeros
        const zPadded = zone.toString().padStart(2, '0');
        const ePadded = eRounded.toString().padStart(digits/2, '0');
        const nPadded = nRounded.toString().padStart(digits/2, '0');

        return `${zPadded}${band} ${e100k}${n100k} ${ePadded} ${nPadded}`;
    }
}


constructor(31U, DQ, 48251, 11932, easting, northing, datum=LatLonEllipsoidal.datums.WGS84)

created 2 years ago

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