diff --git a/src/main/kotlin/uk/co/wedgetech/geodesy/Dms.kt b/src/main/kotlin/uk/co/wedgetech/geodesy/Dms.kt
index 3d83358..a60bbc4 100644
--- a/src/main/kotlin/uk/co/wedgetech/geodesy/Dms.kt
+++ b/src/main/kotlin/uk/co/wedgetech/geodesy/Dms.kt
@@ -124,7 +124,7 @@ import kotlin.math.roundToInt
 
     internal fun toDegreesMinutes(degrees: Double, dp: Int, separator : String) : String {
         var d = Math.floor(degrees).toInt()                // get component deg
-        var m = ((degrees * 60.0) % 60).toFixed(dp)           // get component min & round/right-pad
+        var m = ((degrees * 60.0) % 60.0).toFixed(dp)           // get component min & round/right-pad
         if (m == 60.0) {
             m = 0.0
             d++
@@ -135,8 +135,8 @@ import kotlin.math.roundToInt
 
     internal fun toDegreesMinutesSeconds(degrees: Double, dp: Int, separator: String) : String {
         var d = Math.floor(degrees).toInt()                       // get component deg
-        var m = (Math.floor((degrees * 3600) / 60) % 60).toInt()  // get component min
-        var s = (degrees * 3600 % 60).toFixed(dp);           // get component sec & round/right-pad
+        var m = (Math.floor((degrees * 3600.0) / 60.0) % 60.0).toInt()  // get component min
+        var s = (degrees * 3600.0 % 60.0).toFixed(dp);           // get component sec & round/right-pad
         if (s == 60.0) {
             s = 0.0
             m++
@@ -165,10 +165,10 @@ import kotlin.math.roundToInt
      * @param   {number} [dp=0|2|4] - Number of decimal places to use – default 0 for dms, 2 for dm, 4 for d.
      * @returns {string} Degrees formatted as deg/min/secs according to specified format.
      */
-    fun Double.toLatitude(format: String, dp: Int = 0, separator: String = ""): String
+    fun Double.toLatitude(format: String, dp: Int? = null, separator: String = ""): String
     {
         val lat = this.toDMS(format, dp, separator)
-        return if (lat == null) "–" else lat+separator+(if (this<0.0) 'S' else 'N');  // knock off initial '0' for lat!
+        return if (lat == null) "–" else lat.substring(1)+separator+(if (this<0.0) 'S' else 'N');  // knock off initial '0' for lat!
     };
 
 
@@ -180,7 +180,7 @@ import kotlin.math.roundToInt
      * @param   {number} [dp=0|2|4] - Number of decimal places to use – default 0 for dms, 2 for dm, 4 for d.
      * @returns {string} Degrees formatted as deg/min/secs according to specified format.
      */
-    fun Double.toLongitude(format: String, dp: Int = 0, separator : String = ""): String
+    fun Double.toLongitude(format: String, dp: Int? = null, separator : String = ""): String
     {
         val lon = this.toDMS(format, dp, separator);
         return if (lon == null) "–" else lon+separator+(if (this<0.0) 'W' else 'E');
diff --git a/src/main/kotlin/uk/co/wedgetech/geodesy/LatLon-Vectors.kt b/src/main/kotlin/uk/co/wedgetech/geodesy/LatLon-Vectors.kt
index 27d40f6..99cd07a 100644
--- a/src/main/kotlin/uk/co/wedgetech/geodesy/LatLon-Vectors.kt
+++ b/src/main/kotlin/uk/co/wedgetech/geodesy/LatLon-Vectors.kt
@@ -2,6 +2,7 @@ import uk.co.wedgetech.geodesy.LatLon
 import uk.co.wedgetech.geodesy.Vector3d
 import uk.co.wedgetech.geodesy.toDegrees
 import uk.co.wedgetech.geodesy.toRadians
+import java.util.*
 
 /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
 /*  Vector-based spherical geodetic (latitude/longitude) functions    (c) Chris Veness 2011-2017  */
@@ -396,6 +397,7 @@ fun LatLon.intersection(path1start: LatLon, path1brngEnd: LatLon, path2start: La
  *   var p1 = new LatLon(53.3206, -1.7297), p2 = new LatLon(53.1887, 0.1334);
  *   var d = pCurrent.crossTrackDistanceTo(p1, p2);  // -307.5 m
  */
+/*
 fun LatLon.crossTrackDistanceTo(pathStart: LatLon, pathBrngEnd, radius: Double = 6371e3): Double {
 
     var p = this.toVector();
@@ -410,7 +412,7 @@ fun LatLon.crossTrackDistanceTo(pathStart: LatLon, pathBrngEnd, radius: Double =
 
     return d;
 };
-
+*/
 
 /**
  * Returns how far ‘this’ point is along a path from from start-point, heading on bearing or towards
@@ -429,6 +431,7 @@ fun LatLon.crossTrackDistanceTo(pathStart: LatLon, pathBrngEnd, radius: Double =
  *   var p2 = new LatLon(53.1887,  0.1334);
  *   var d = pCurrent.alongTrackDistanceTo(p1, p2);  // 62.331 km
  */
+/*
 fun LatLon.alongTrackDistanceTo(pathStart, pathBrngEnd, radius: Double = 6371e3) {
     if (!(pathStart instanceof LatLon)) throw new TypeError('pathStart is not LatLon object');
 
@@ -446,7 +449,7 @@ fun LatLon.alongTrackDistanceTo(pathStart, pathBrngEnd, radius: Double = 6371e3)
 
     return d;
 };
-
+*/
 
 /**
  * Returns closest point on great circle segment between point1 & point2 to ‘this’ point.
@@ -530,34 +533,33 @@ fun LatLon.isBetween(point1: LatLon, point2: LatLon):Boolean {
  *   var p = new LatLon(45.1, 1.1);
  *   var inside = p.enclosedBy(bounds); // true
  */
-fun LatLon.enclosedBy(polygon: Array<LatLon>): Boolean {
+fun LatLon.enclosedBy(polygon_: Array<LatLon>): Boolean {
     // this method uses angle summation test; on a plane, angles for an enclosed point will sum
     // to 360°, angles for an exterior point will sum to 0°. On a sphere, enclosed point angles
     // will sum to less than 360° (due to spherical excess), exterior point angles will be small
     // but non-zero. TODO: are any winding number optimisations applicable to spherical surface?
 
     // close the polygon so that the last point equals the first point
-    var closed = polygon[0].equals(polygon[polygon.length-1]);
-    if (!closed) polygon.push(polygon[0]);
+    val polygon = if(polygon_[0] == polygon_[polygon_.size-1]) polygon_ else polygon_ + arrayOf(polygon_[0])
 
-    var nVertices = polygon.length - 1;
+    var nVertices = polygon.size - 1;
 
     var p = this.toVector();
 
     // get vectors from p to each vertex
-    var vectorToVertex = [];
-    for (var v=0; v<nVertices; v++) vectorToVertex[v] = p.minus(polygon[v].toVector());
-    vectorToVertex.push(vectorToVertex[0]);
+    var vectorToVertex = LinkedList<Vector3d>()
+    for (value in polygon) vectorToVertex.add(p - value.toVector())
+    vectorToVertex.add(vectorToVertex[0]);
 
     // sum subtended angles of each edge (using vector p to determine sign)
-    var Σθ = 0;
-    for (var v=0; v<nVertices; v++) {
-        Σθ += vectorToVertex[v].angleTo(vectorToVertex[v+1], p);
+    var Σθ = 0.0;
+    var prevValue : Vector3d? = null
+    for (value in vectorToVertex) {
+        if (prevValue!=null) Σθ += prevValue.angleTo(value, p);
+        prevValue = value
     }
 
-    var enclosed = Math.abs(Σθ) > Math.PI;
-
-    if (!closed) polygon.pop(); // restore polygon to pristine condition
+    val enclosed = Math.abs(Σθ) > Math.PI;
 
     return enclosed;
 };
@@ -575,37 +577,41 @@ fun LatLon.enclosedBy(polygon: Array<LatLon>): Boolean {
  *   var polygon = [ new LatLon(0,0), new LatLon(1,0), new LatLon(0,1) ];
  *   var area = LatLon.areaOf(polygon); // 6.18e9 m²
  */
-fun LatLon.areaOf(polygon: Array<LatLon>, radius: Double = 6371e3) {
+fun LatLon.areaOf(polygon_: Array<LatLon>, radius: Double = 6371e3):Double {
     // uses Girard’s theorem: A = [Σθᵢ − (n−2)·π]·R²
 
+    if (polygon_.size < 2) return 0.0
     // close the polygon so that the last point equals the first point
-    var closed = polygon[0].equals(polygon[polygon.length-1]);
-    if (!closed) polygon.push(polygon[0]);
+    val polygon = if(polygon_[0] == polygon_[polygon_.size-1]) polygon_ else polygon_ + arrayOf(polygon_[0])
 
-    var n = polygon.length - 1; // number of vertices
+    val n = polygon.size - 1; // number of vertices
 
     // get great-circle vector for each edge
-    var c = [];
-    for (var v=0; v<n; v++) {
-        var i = polygon[v].toVector();
-        var j = polygon[v+1].toVector();
-        c[v] = i.cross(j); // great circle for segment v..v+1
+    val c = LinkedList<Vector3d>()
+    var j : Vector3d? = null
+    for (v in polygon) {
+        val i = v.toVector();
+        if (j!=null) c.add(i.cross(j)); // great circle for segment v..v+1
+        j = i
     }
-    c.push(c[0]);
+    c.add(c[0]);
 
     // sum interior angles; depending on whether polygon is cw or ccw, angle between edges is
     // π−α or π+α, where α is angle between great-circle vectors; so sum α, then take n·π − |Σα|
     // (cannot use Σ(π−|α|) as concave polygons would fail); use vector to 1st point as plane
     // normal for sign of α
     var n1 = polygon[0].toVector();
-    var Σα = 0;
-    for (var v=0; v<n; v++) Σα += c[v].angleTo(c[v+1], n1);
+    var Σα = 0.0;
+    var prevValue : Vector3d? = null
+    for (value in c) {
+        if (prevValue!=null) Σα += value.angleTo(prevValue, n1);
+        prevValue = value
+    }
+
     var Σθ = n*Math.PI - Math.abs(Σα);
 
     var E = (Σθ - (n-2)*Math.PI); // spherical excess (in steradians)
-    var A = E * R*R;              // area in units of R²
-
-    if (!closed) polygon.pop(); // restore polygon to pristine condition
+    var A = E * radius*radius;              // area in units of R²
 
     return A;
 };
diff --git a/src/main/kotlin/uk/co/wedgetech/geodesy/LatLon.kt b/src/main/kotlin/uk/co/wedgetech/geodesy/LatLon.kt
index 12e1287..e193ad8 100644
--- a/src/main/kotlin/uk/co/wedgetech/geodesy/LatLon.kt
+++ b/src/main/kotlin/uk/co/wedgetech/geodesy/LatLon.kt
@@ -45,7 +45,7 @@ open class LatLon(val lat: Double, val lon :Double) {
      * @param   {number} [dp=0|2|4] - Number of decimal places to use - default 0 for dms, 2 for dm, 4 for d.
      * @returns {string} Comma-separated latitude/longitude.
      */
-    fun toString(format: String, dp: Int = 0) : String
+    fun toString(format: String, dp: Int? = null) : String
     {
         return "${this.lat.toLatitude(format, dp)}, ${this.lon.toLongitude(format, dp)}"
     }
diff --git a/src/main/kotlin/uk/co/wedgetech/geodesy/OsGridRef.kt b/src/main/kotlin/uk/co/wedgetech/geodesy/OsGridRef.kt
index cdcaaa5..44b7f29 100644
--- a/src/main/kotlin/uk/co/wedgetech/geodesy/OsGridRef.kt
+++ b/src/main/kotlin/uk/co/wedgetech/geodesy/OsGridRef.kt
@@ -39,165 +39,7 @@ import java.math.BigDecimal
  */
 data class OsGridRef(val easting : Double, val northing: Double) {
 
-
-    /**
-     * Converts latitude/longitude to Ordnance Survey grid reference easting/northing coordinate.
-     *
-     * Note formulation implemented here due to Thomas, Redfearn, etc is as published by OS, but is
-     * inferior to Krüger as used by e.g. Karney 2011.
-     *
-     * @param   {LatLon}    point - latitude/longitude.
-     * @returns {OsGridRef} OS Grid Reference easting/northing.
-     *
-     * @example
-     *   var p = new LatLon(52.65798, 1.71605);
-     *   var grid = OsGridRef.latLonToOsGrid(p); // grid.toString(): TG 51409 13177
-     *   // for conversion of (historical) OSGB36 latitude/longitude point:
-     *   var p = new LatLon(52.65757, 1.71791, LatLon.datum.OSGB36);
-     */
-    fun latLonToOsGrid(point: LatLon): OsGridRef {
-        // if necessary convert to OSGB36 first
-        val osgbPoint = if (point.datum != LatLon.Companion.OSGB36) point.convertDatum(LatLon.Companion.OSGB36)
-            else point
-
-        val φ = osgbPoint.lat.toRadians();
-        val λ = osgbPoint.lon.toRadians();
-
-        val a = 6377563.396
-        val b = 6356256.909;              // Airy 1830 major & minor semi-axes
-        val F0 = 0.9996012717;                             // NatGrid scale factor on central meridian
-        val φ0 = (49.0).toRadians()
-        val λ0 = (-2.0).toRadians();  // NatGrid true origin is 49°N,2°W
-        val N0 = -100000
-        val E0 = 400000;                     // northing & easting of true origin, metres
-        val e2 = 1 - (b * b) / (a * a);                          // eccentricity squared
-        val n = (a - b) / (a + b)
-        val n2 = n * n
-        val n3 = n * n * n;         // n, n², n³
-
-        val cosφ = Math.cos(φ)
-        val sinφ = Math.sin(φ);
-        val ν = a * F0 / Math.sqrt(1 - e2 * sinφ * sinφ);            // nu = transverse radius of curvature
-        val ρ = a * F0 * (1 - e2) / Math.pow(1 - e2 * sinφ * sinφ, 1.5); // rho = meridional radius of curvature
-        val η2 = ν / ρ - 1;                                    // eta = ?
-
-        val Ma = (1 + n + (5 / 4) * n2 + (5 / 4) * n3) * (φ - φ0);
-        val Mb = (3 * n + 3 * n * n + (21 / 8) * n3) * Math.sin(φ - φ0) * Math.cos(φ + φ0);
-        val Mc = ((15 / 8) * n2 + (15 / 8) * n3) * Math.sin(2 * (φ - φ0)) * Math.cos(2 * (φ + φ0));
-        val Md = (35 / 24) * n3 * Math.sin(3 * (φ - φ0)) * Math.cos(3 * (φ + φ0));
-        val M = b * F0 * (Ma - Mb + Mc - Md);              // meridional arc
-
-        val cos3φ = cosφ * cosφ * cosφ;
-        val cos5φ = cos3φ * cosφ * cosφ;
-        val tan2φ = Math.tan(φ) * Math.tan(φ);
-        val tan4φ = tan2φ * tan2φ;
-
-        val I = M + N0;
-        val II = (ν / 2) * sinφ * cosφ;
-        val III = (ν / 24) * sinφ * cos3φ * (5 - tan2φ + 9 * η2);
-        val IIIA = (ν / 720) * sinφ * cos5φ * (61 - 58 * tan2φ + tan4φ);
-        val IV = ν * cosφ;
-        val V = (ν / 6) * cos3φ * (ν / ρ - tan2φ);
-        val VI = (ν / 120) * cos5φ * (5 - 18 * tan2φ + tan4φ + 14 * η2 - 58 * tan2φ * η2);
-
-        val Δλ = λ - λ0;
-        val Δλ2 = Δλ * Δλ
-        val Δλ3 = Δλ2 * Δλ
-        val Δλ4 = Δλ3 * Δλ
-        val Δλ5 = Δλ4 * Δλ
-        val Δλ6 = Δλ5 * Δλ;
-
-        val N = I + II * Δλ2 + III * Δλ4 + IIIA * Δλ6;
-        val E = E0 + IV * Δλ + V * Δλ3 + VI * Δλ5;
-
-        return OsGridRef(E.toFixed(3), N.toFixed(3)); // gets truncated to SW corner of 1m grid square
-    };
-
-
-    /**
-     * Converts Ordnance Survey grid reference easting/northing coordinate to latitude/longitude
-     * (SW corner of grid square).
-     *
-     * Note formulation implemented here due to Thomas, Redfearn, etc is as published by OS, but is
-     * inferior to Krüger as used by e.g. Karney 2011.
-     *
-     * @param   {OsGridRef}    gridref - Grid ref E/N to be converted to lat/long (SW corner of grid square).
-     * @param   {LatLon.datum} [datum=WGS84] - Datum to convert grid reference into.
-     * @returns {LatLon}       Latitude/longitude of supplied grid reference.
-     *
-     * @example
-     *   var gridref = new OsGridRef(651409.903, 313177.270);
-     *   var pWgs84 = OsGridRef.osGridToLatLon(gridref);                     // 52°39′28.723″N, 001°42′57.787″E
-     *   // to obtain (historical) OSGB36 latitude/longitude point:
-     *   var pOsgb = OsGridRef.osGridToLatLon(gridref, LatLon.datum.OSGB36); // 52°39′27.253″N, 001°43′04.518″E
-     */
-    fun osGridToLatLon(gridref :OsGridRef, datum: LatLon.Datum = LatLon.WGS84): LatLon
-    {
-        val E = gridref.easting;
-        val N = gridref.northing;
-
-        val a = 6377563.396
-        val b = 6356256.909;              // Airy 1830 major & minor semi-axes
-        val F0 = 0.9996012717;                             // NatGrid scale factor on central meridian
-        val φ0 = (49.0).toRadians()
-        val λ0 = (-2.0).toRadians()  // NatGrid true origin is 49°N,2°W
-        val N0 = -100000.0
-        val E0 = 400000.0;                     // northing & easting of true origin, metres
-        val e2 = 1 - (b * b) / (a * a);                          // eccentricity squared
-        val n = (a - b) / (a + b)
-        val n2 = n*n
-        val n3 = n*n*n;         // n, n², n³
-
-        var φ = φ0
-        var M = 0.0;
-        do {
-            φ = (N - N0 - M) / (a * F0) + φ;
-
-            val Ma = (1 + n + (5 / 4) * n2 + (5 / 4) * n3) * (φ - φ0);
-            val Mb = (3 * n + 3 * n * n + (21 / 8) * n3) * Math.sin(φ - φ0) * Math.cos(φ + φ0);
-            val Mc = ((15 / 8) * n2 + (15 / 8) * n3) * Math.sin(2 * (φ - φ0)) * Math.cos(2 * (φ + φ0));
-            val Md = (35 / 24) * n3 * Math.sin(3 * (φ - φ0)) * Math.cos(3 * (φ + φ0));
-            M = b * F0 * (Ma - Mb + Mc - Md);              // meridional arc
-
-        } while ((N - N0 - M) >= 0.00001);  // ie until < 0.01mm
-
-        val cosφ = Math.cos(φ)
-        val sinφ = Math.sin(φ);
-        val ν = a * F0 / Math.sqrt(1 - (e2 * sinφ * sinφ));            // nu = transverse radius of curvature
-        val ρ = a * F0 * (1 - e2) / Math.pow(1 - (e2 * sinφ * sinφ), 1.5); // rho = meridional radius of curvature
-        val η2 = ν / ρ - 1;                                    // eta = ?
-
-        val tanφ = Math.tan(φ);
-        val tan2φ = tanφ * tanφ
-        val tan4φ = tan2φ*tan2φ
-        val tan6φ = tan4φ*tan2φ;
-        val secφ = 1 / cosφ;
-        val ν3 = ν * ν * ν
-        val ν5 = ν3*ν*ν
-        val ν7 = ν5*ν*ν;
-        val VII = tanφ / (2 * ρ * ν);
-        val VIII = tanφ / (24 * ρ * ν3) * (5 + 3 * tan2φ + η2 - 9 * tan2φ * η2);
-        val IX = tanφ / (720 * ρ * ν5) * (61 + 90 * tan2φ + 45 * tan4φ);
-        val X = secφ / ν;
-        val XI = secφ / (6 * ν3) * (ν / ρ + 2 * tan2φ);
-        val XII = secφ / (120 * ν5) * (5 + 28 * tan2φ + 24 * tan4φ);
-        val XIIA = secφ / (5040 * ν7) * (61 + 662 * tan2φ + 1320 * tan4φ + 720 * tan6φ);
-
-        val dE = (E - E0)
-        val dE2 = dE*dE
-        val dE3 = dE2*dE
-        val dE4 = dE2*dE2
-        val dE5 = dE3*dE2
-        val dE6 = dE4*dE2
-        val dE7 = dE5*dE2;
-        φ -= (VII * dE2) + (VIII * dE4) - (IX * dE6);
-        val λ = λ0 + (X * dE) - (XI * dE3) + (XII * dE5) - (XIIA * dE7);
-
-        var point = LatLon (φ.toDegrees(), λ.toDegrees(), LatLon.OSGB36);
-        if (datum != LatLon.OSGB36) point = point.convertDatum(datum);
-
-        return point;
-    };
+    override fun toString() :String = toString(10)
 
     /**
      * Converts ‘this’ numeric grid reference to standard OS grid reference.
@@ -209,7 +51,7 @@ data class OsGridRef(val easting : Double, val northing: Double) {
      * @example
      *   var ref = new OsGridRef(651409, 313177).toString(); // TG 51409 13177
      */
-    fun toString(digits: Int = 10): String
+    fun toString(digits: Int): String
     {
         if (digits % 2 != 0 || digits > 16) throw IllegalArgumentException("Invalid precision ${digits}")
 
@@ -311,7 +153,7 @@ fun String.parseOsGridReference() : OsGridRef
     var en = gridref.substring(2).trim().split(Regex("\\s+"));
     // if e/n not whitespace separated, split half way
     if (en.size == 1) {
-        en = listOf(en[0].substring(IntRange(0, en[0].length / 2)), en[0].substring(en[0].length / 2))
+        en = listOf(en[0].substring(IntRange(0, (en[0].length / 2)-1)), en[0].substring(en[0].length / 2))
     };
 
     // validation
@@ -329,4 +171,161 @@ fun String.parseOsGridReference() : OsGridRef
     return OsGridRef (e.toDouble(), n.toDouble());
 };
 
+/**
+ * Converts latitude/longitude to Ordnance Survey grid reference easting/northing coordinate.
+ *
+ * Note formulation implemented here due to Thomas, Redfearn, etc is as published by OS, but is
+ * inferior to Krüger as used by e.g. Karney 2011.
+ *
+ * @param   {LatLon}    point - latitude/longitude.
+ * @returns {OsGridRef} OS Grid Reference easting/northing.
+ *
+ * @example
+ *   var p = new LatLon(52.65798, 1.71605);
+ *   var grid = OsGridRef.latLonToOsGrid(p); // grid.toString(): TG 51409 13177
+ *   // for conversion of (historical) OSGB36 latitude/longitude point:
+ *   var p = new LatLon(52.65757, 1.71791, LatLon.datum.OSGB36);
+ */
+fun LatLonDatum.toOsGrid(): OsGridRef {
+    // if necessary convert to OSGB36 first
+    val osgbPoint = if (this.datum != LatLonDatum.OSGB36) this.convertDatum(LatLonDatum.OSGB36)
+        else this
+
+    val φ = osgbPoint.lat.toRadians();
+    val λ = osgbPoint.lon.toRadians();
+
+    val a = 6377563.396
+    val b = 6356256.909;              // Airy 1830 major & minor semi-axes
+    val F0 = 0.9996012717             // NatGrid scale factor on central meridian
+    val φ0 = (49.0).toRadians()
+    val λ0 = (-2.0).toRadians();      // NatGrid true origin is 49°N,2°W
+    val N0 = -100000.0
+    val E0 = 400000.0                  // northing & easting of true origin, metres
+    val e2 = 1 - (b * b) / (a * a);   // eccentricity squared
+    val n = (a - b) / (a + b)
+    val n2 = n * n
+    val n3 = n * n * n;         // n, n², n³
+
+    val cosφ = Math.cos(φ)
+    val sinφ = Math.sin(φ);
+    val ν = a * F0 / Math.sqrt(1 - e2 * sinφ * sinφ);            // nu = transverse radius of curvature
+    val ρ = a * F0 * (1.0 - e2) / Math.pow(1.0 - e2 * sinφ * sinφ, 1.5); // rho = meridional radius of curvature
+    val η2 = ν / ρ - 1.0;                                    // eta = ?
+
+    val Ma = (1.0 + n + (5.0 / 4.0) * n2 + (5.0 / 4.0) * n3) * (φ - φ0);
+    val Mb = (3.0 * n + 3.0 * n * n + (21.0 / 8.0) * n3) * Math.sin(φ - φ0) * Math.cos(φ + φ0);
+    val Mc = ((15.0 / 8.0) * n2 + (15.0 / 8.0) * n3) * Math.sin(2.0 * (φ - φ0)) * Math.cos(2 * (φ + φ0));
+    val Md = (35.0 / 24.0) * n3 * Math.sin(3.0 * (φ - φ0)) * Math.cos(3.0 * (φ + φ0));
+    val M = b * F0 * (Ma - Mb + Mc - Md);              // meridional arc
+
+    val cos3φ = cosφ * cosφ * cosφ;
+    val cos5φ = cos3φ * cosφ * cosφ;
+    val tan2φ = Math.tan(φ) * Math.tan(φ);
+    val tan4φ = tan2φ * tan2φ;
+
+    val I = M + N0;
+    val II = (ν / 2.0) * sinφ * cosφ;
+    val III = (ν / 24.0) * sinφ * cos3φ * (5.0 - tan2φ + 9.0 * η2);
+    val IIIA = (ν / 720.0) * sinφ * cos5φ * (61.0 - 58.0 * tan2φ + tan4φ);
+    val IV = ν * cosφ;
+    val V = (ν / 6.0) * cos3φ * (ν / ρ - tan2φ);
+    val VI = (ν / 120.0) * cos5φ * (5.0 - 18.0 * tan2φ + tan4φ + 14.0 * η2 - 58.0 * tan2φ * η2);
+
+    val Δλ = λ - λ0;
+    val Δλ2 = Δλ * Δλ
+    val Δλ3 = Δλ2 * Δλ
+    val Δλ4 = Δλ3 * Δλ
+    val Δλ5 = Δλ4 * Δλ
+    val Δλ6 = Δλ5 * Δλ;
+
+    val N = I + II * Δλ2 + III * Δλ4 + IIIA * Δλ6;
+    val E = E0 + IV * Δλ + V * Δλ3 + VI * Δλ5;
+
+    return OsGridRef(E.toFixed(3), N.toFixed(3)); // gets truncated to SW corner of 1m grid square
+};
+
 
+/**
+ * Converts Ordnance Survey grid reference easting/northing coordinate to latitude/longitude
+ * (SW corner of grid square).
+ *
+ * Note formulation implemented here due to Thomas, Redfearn, etc is as published by OS, but is
+ * inferior to Krüger as used by e.g. Karney 2011.
+ *
+ * @param   {OsGridRef}    gridref - Grid ref E/N to be converted to lat/long (SW corner of grid square).
+ * @param   {LatLon.datum} [datum=WGS84] - Datum to convert grid reference into.
+ * @returns {LatLon}       Latitude/longitude of supplied grid reference.
+ *
+ * @example
+ *   var gridref = new OsGridRef(651409.903, 313177.270);
+ *   var pWgs84 = OsGridRef.osGridToLatLon(gridref);                     // 52°39′28.723″N, 001°42′57.787″E
+ *   // to obtain (historical) OSGB36 latitude/longitude point:
+ *   var pOsgb = OsGridRef.osGridToLatLon(gridref, LatLon.datum.OSGB36); // 52°39′27.253″N, 001°43′04.518″E
+ */
+fun OsGridRef.toLatLonDatum(datum: LatLonDatum.Datum = LatLonDatum.WGS84): LatLonDatum
+{
+    val E = this.easting;
+    val N = this.northing;
+
+    val a = 6377563.396
+    val b = 6356256.909;              // Airy 1830 major & minor semi-axes
+    val F0 = 0.9996012717;                             // NatGrid scale factor on central meridian
+    val φ0 = (49.0).toRadians()
+    val λ0 = (-2.0).toRadians()  // NatGrid true origin is 49°N,2°W
+    val N0 = -100000.0
+    val E0 = 400000.0;                     // northing & easting of true origin, metres
+    val e2 = 1.0 - (b * b) / (a * a);                          // eccentricity squared
+    val n = (a - b) / (a + b)
+    val n2 = n*n
+    val n3 = n*n*n;         // n, n², n³
+
+    var φ = φ0
+    var M = 0.0;
+    do {
+        φ = (N - N0 - M) / (a * F0) + φ;
+
+        val Ma = (1.0 + n + (5.0 / 4.0) * n2 + (5.0 / 4.0) * n3) * (φ - φ0);
+        val Mb = (3.0 * n + 3.0 * n * n + (21.0 / 8.0) * n3) * Math.sin(φ - φ0) * Math.cos(φ + φ0);
+        val Mc = ((15.0 / 8.0) * n2 + (15.0 / 8.0) * n3) * Math.sin(2.0 * (φ - φ0)) * Math.cos(2.0 * (φ + φ0));
+        val Md = (35.0 / 24.0) * n3 * Math.sin(3.0 * (φ - φ0)) * Math.cos(3.0 * (φ + φ0));
+        M = b * F0 * (Ma - Mb + Mc - Md);              // meridional arc
+
+    } while ((N - N0 - M) >= 0.00001);  // ie until < 0.01mm
+
+    val cosφ = Math.cos(φ)
+    val sinφ = Math.sin(φ);
+    val ν = a * F0 / Math.sqrt(1.0 - (e2 * sinφ * sinφ));            // nu = transverse radius of curvature
+    val ρ = a * F0 * (1.0 - e2) / Math.pow(1.0 - (e2 * sinφ * sinφ), 1.5); // rho = meridional radius of curvature
+    val η2 = ν / ρ - 1.0;                                    // eta = ?
+
+    val tanφ = Math.tan(φ);
+    val tan2φ = tanφ * tanφ
+    val tan4φ = tan2φ*tan2φ
+    val tan6φ = tan4φ*tan2φ;
+    val secφ = 1.0 / cosφ;
+    val ν3 = ν * ν * ν
+    val ν5 = ν3*ν*ν
+    val ν7 = ν5*ν*ν
+    val VII = tanφ / (2.0 * ρ * ν)
+    val VIII = tanφ / (24.0 * ρ * ν3) * (5.0 + 3.0 * tan2φ + η2 - 9.0 * tan2φ * η2)
+    val IX = tanφ / (720.0 * ρ * ν5) * (61.0 + 90.0 * tan2φ + 45.0 * tan4φ)
+    val X = secφ / ν
+    val XI = secφ / (6.0 * ν3) * (ν / ρ + 2.0 * tan2φ)
+    val XII = secφ / (120.0 * ν5) * (5.0 + 28.0 * tan2φ + 24.0 * tan4φ);
+    val XIIA = secφ / (5040.0 * ν7) * (61.0 + 662.0 * tan2φ + 1320.0 * tan4φ + 720.0 * tan6φ);
+
+    val dE = (E - E0)
+    val dE2 = dE*dE
+    val dE3 = dE2*dE
+    val dE4 = dE2*dE2
+    val dE5 = dE3*dE2
+    val dE6 = dE4*dE2
+    val dE7 = dE5*dE2;
+    φ = φ - VII * dE2 + VIII * dE4 - IX * dE6
+    val λ = λ0 + X * dE - XI * dE3 + XII * dE5 - XIIA * dE7
+
+    var point = LatLonDatum(φ.toDegrees(), λ.toDegrees(), LatLonDatum.OSGB36);
+    if (datum != LatLonDatum.OSGB36) point = point.convertDatum(datum);
+
+    return point;
+};
diff --git a/src/main/kotlin/uk/co/wedgetech/geodesy/Test.java b/src/main/kotlin/uk/co/wedgetech/geodesy/Test.java
deleted file mode 100644
index 6fe511e..0000000
--- a/src/main/kotlin/uk/co/wedgetech/geodesy/Test.java
+++ /dev/null
@@ -1,4 +0,0 @@
-package uk.co.wedgetech.geodesy;
-
-public class Test {
-}
diff --git a/src/main/kotlin/uk/co/wedgetech/geodesy/Vector3d.kt b/src/main/kotlin/uk/co/wedgetech/geodesy/Vector3d.kt
index f15cd9a..10e2462 100644
--- a/src/main/kotlin/uk/co/wedgetech/geodesy/Vector3d.kt
+++ b/src/main/kotlin/uk/co/wedgetech/geodesy/Vector3d.kt
@@ -104,7 +104,7 @@ data class Vector3d(val x: Double , val y : Double , val z : Double) {
      *
      * @returns {Vector3d} Negated vector.
      */
-    var negate = Vector3d (-this.x, -this.y, -this.z)
+    val negate by lazy({Vector3d (-this.x, -this.y, -this.z) })
 
 
     /**
diff --git a/src/main/kotlin/uk/co/wedgetech/geodesy/mgrs.kt b/src/main/kotlin/uk/co/wedgetech/geodesy/mgrs.kt
index 1f76745..b433d6b 100644
--- a/src/main/kotlin/uk/co/wedgetech/geodesy/mgrs.kt
+++ b/src/main/kotlin/uk/co/wedgetech/geodesy/mgrs.kt
@@ -4,10 +4,12 @@
 /* www.movable-type.co.uk/scripts/latlong-utm-mgrs.html                                           */
 /* www.movable-type.co.uk/scripts/geodesy/docs/module-mgrs.html                                   */
 /* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
+/*
 
 'use strict';
 if (typeof module!='undefined' && module.exports) var Utm = require('./utm.js');                   // ≡ import Utm from 'utm.js'
 if (typeof module!='undefined' && module.exports) var LatLon = require('./latlon-ellipsoidal.js'); // ≡ import LatLon from 'latlon-ellipsoidal.js'
+*/
 
 
 /**
@@ -25,19 +27,19 @@ if (typeof module!='undefined' && module.exports) var LatLon = require('./latlon
 /*
  * Latitude bands C..X 8° each, covering 80°S to 84°N
  */
-Mgrs.latBands = 'CDEFGHJKLMNPQRSTUVWXX'; // X is repeated for 80-84°N
+//Mgrs.latBands = 'CDEFGHJKLMNPQRSTUVWXX'; // X is repeated for 80-84°N
 
 
 /*
  * 100km grid square column (‘e’) letters repeat every third zone
  */
-Mgrs.e100kLetters = [ 'ABCDEFGH', 'JKLMNPQR', 'STUVWXYZ' ];
+//Mgrs.e100kLetters = [ 'ABCDEFGH', 'JKLMNPQR', 'STUVWXYZ' ];
 
 
 /*
  * 100km grid square row (‘n’) letters repeat every other zone
  */
-Mgrs.n100kLetters = [ 'ABCDEFGHJKLMNPQRSTUV', 'FGHJKLMNPQRSTUVABCDE' ];
+//Mgrs.n100kLetters = [ 'ABCDEFGHJKLMNPQRSTUV', 'FGHJKLMNPQRSTUVABCDE' ];
 
 
 /**
@@ -56,7 +58,7 @@ Mgrs.n100kLetters = [ 'ABCDEFGHJKLMNPQRSTUV', 'FGHJKLMNPQRSTUVABCDE' ];
  * @example
  *   var mgrsRef = new Mgrs(31, 'U', 'D', 'Q', 48251, 11932); // 31U DQ 48251 11932
  */
-function Mgrs(zone, band, e100k, n100k, easting, northing, datum) {
+/*function Mgrs(zone, band, e100k, n100k, easting, northing, datum) {
     // allow instantiation without 'new'
     if (!(this instanceof Mgrs)) return new Mgrs(zone, band, e100k, n100k, easting, northing, datum);
 
@@ -75,7 +77,7 @@ function Mgrs(zone, band, e100k, n100k, easting, northing, datum) {
     this.easting = Number(easting);
     this.northing = Number(northing);
     this.datum = datum;
-}
+}*/
 
 
 /**
@@ -88,6 +90,7 @@ function Mgrs(zone, band, e100k, n100k, easting, northing, datum) {
  *   var utmCoord = new Utm(31, 'N', 448251, 5411932);
  *   var mgrsRef = utmCoord.toMgrs(); // 31U DQ 48251 11932
  */
+/*
 Utm.prototype.toMgrs = function() {
     if (isNaN(this.zone + this.easting + this.northing)) throw new Error('Invalid UTM coordinate ‘'+this.toString()+'’');
 
@@ -117,6 +120,7 @@ Utm.prototype.toMgrs = function() {
 
     return new Mgrs(zone, band, e100k, n100k, easting, northing);
 };
+*/
 
 
 /**
@@ -127,6 +131,7 @@ Utm.prototype.toMgrs = function() {
  * @example
  *   var utmCoord = Mgrs.parse('31U DQ 448251 11932').toUtm(); // 31 N 448251 5411932
  */
+/*
 Mgrs.prototype.toUtm = function() {
     var zone = this.zone;
     var band = this.band;
@@ -158,6 +163,7 @@ Mgrs.prototype.toUtm = function() {
 
     return new Utm(zone, hemisphere, e100kNum+easting, n2M+n100kNum+northing, this.datum);
 };
+*/
 
 
 /**
@@ -178,6 +184,7 @@ Mgrs.prototype.toUtm = function() {
  *   var mgrsRef = Mgrs.parse('31UDQ4825111932');
  *   //  mgrsRef: { zone:31, band:'U', e100k:'D', n100k:'Q', easting:48251, northing:11932 }
  */
+/*
 Mgrs.parse = function(mgrsGridRef) {
     mgrsGridRef = mgrsGridRef.trim();
 
@@ -211,6 +218,7 @@ Mgrs.parse = function(mgrsGridRef) {
 
     return new Mgrs(zone, band, e100k, n100k, e, n);
 };
+*/
 
 
 /**
@@ -231,6 +239,7 @@ Mgrs.parse = function(mgrsGridRef) {
  * @example
  *   var mgrsStr = new Mgrs(31, 'U', 'D', 'Q', 48251, 11932).toString(); // '31U DQ 48251 11932'
  */
+/*
 Mgrs.prototype.toString = function(digits) {
     digits = (digits === undefined) ? 10 : Number(digits);
     if ([ 2,4,6,8,10 ].indexOf(digits) == -1) throw new Error('Invalid precision ‘'+digits+'’');
@@ -251,7 +260,5 @@ Mgrs.prototype.toString = function(digits) {
 
     return zone+band + ' ' + e100k+n100k + ' '  + easting + ' ' + northing;
 };
+*/
 
-
-/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
-if (typeof module != 'undefined' && module.exports) module.exports = Mgrs; // ≡ export default Mgrs
diff --git a/src/main/kotlin/uk/co/wedgetech/geodesy/utm.kt b/src/main/kotlin/uk/co/wedgetech/geodesy/utm.kt
index 9c19362..e903280 100644
--- a/src/main/kotlin/uk/co/wedgetech/geodesy/utm.kt
+++ b/src/main/kotlin/uk/co/wedgetech/geodesy/utm.kt
@@ -33,7 +33,7 @@
  * @example
  *   var utmCoord = new Utm(31, 'N', 448251, 5411932);
  */
-function Utm(zone, hemisphere, easting, northing, datum, convergence, scale) {
+/*function Utm(zone, hemisphere, easting, northing, datum, convergence, scale) {
     if (!(this instanceof Utm)) { // allow instantiation without 'new'
         return new Utm(zone, hemisphere, easting, northing, datum, convergence, scale);
     }
@@ -55,7 +55,7 @@ function Utm(zone, hemisphere, easting, northing, datum, convergence, scale) {
     this.datum = datum;
     this.convergence = convergence===null ? null : Number(convergence);
     this.scale = scale===null ? null : Number(scale);
-}
+}*/
 
 
 /**
@@ -71,6 +71,7 @@ function Utm(zone, hemisphere, easting, northing, datum, convergence, scale) {
  *   var latlong = new LatLon(48.8582, 2.2945);
  *   var utmCoord = latlong.toUtm(); // utmCoord.toString(): '31 N 448252 5411933'
  */
+/*
 LatLon.prototype.toUtm = function() {
     if (isNaN(this.lat) || isNaN(this.lon)) throw new Error('Invalid point');
     if (!(-80<=this.lat && this.lat<=84)) throw new Error('Outside UTM limits');
@@ -174,6 +175,7 @@ LatLon.prototype.toUtm = function() {
     return new Utm(zone, h, x, y, this.datum, convergence, scale);
 };
 
+*/
 
 /**
  * Converts UTM zone/easting/northing coordinate to latitude/longitude
@@ -185,6 +187,7 @@ LatLon.prototype.toUtm = function() {
  *   var grid = new Utm(31, 'N', 448251.795, 5411932.678);
  *   var latlong = grid.toLatLonE(); // latlong.toString(): 48°51′29.52″N, 002°17′40.20″E
  */
+/*
 Utm.prototype.toLatLonE = function() {
     var z = this.zone;
     var h = this.hemisphere;
@@ -286,6 +289,7 @@ Utm.prototype.toLatLonE = function() {
 
     return latLong;
 };
+*/
 
 
 /**
@@ -306,6 +310,8 @@ Utm.prototype.toLatLonE = function() {
  *   var utmCoord = Utm.parse('31 N 448251 5411932');
  *   // utmCoord: {zone: 31, hemisphere: 'N', easting: 448251, northing: 5411932 }
  */
+/*
+
 Utm.parse = function(utmCoord, datum) {
     if (datum === undefined) datum = LatLon.datum.WGS84; // default if not supplied
 
@@ -318,6 +324,7 @@ Utm.parse = function(utmCoord, datum) {
 
     return new Utm(zone, hemisphere, easting, northing, datum);
 };
+*/
 
 
 /**
@@ -334,6 +341,7 @@ Utm.parse = function(utmCoord, datum) {
  * @example
  *   var utm = Utm.parse('31 N 448251 5411932').toString(4);  // 31 N 448251.0000 5411932.0000
  */
+/*
 Utm.prototype.toString = function(digits) {
     digits = Number(digits||0); // default 0 if not supplied
 
@@ -346,43 +354,4 @@ Utm.prototype.toString = function(digits) {
     return z+' '+h+' '+e.toFixed(digits)+' '+n.toFixed(digits);
 };
 
-
-/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
-
-/** Polyfill Math.sinh for old browsers / IE */
-if (Math.sinh === undefined) {
-    Math.sinh = function(x) {
-        return (Math.exp(x) - Math.exp(-x)) / 2;
-    };
-}
-
-/** Polyfill Math.cosh for old browsers / IE */
-if (Math.cosh === undefined) {
-    Math.cosh = function(x) {
-        return (Math.exp(x) + Math.exp(-x)) / 2;
-    };
-}
-
-/** Polyfill Math.tanh for old browsers / IE */
-if (Math.tanh === undefined) {
-    Math.tanh = function(x) {
-        return (Math.exp(x) - Math.exp(-x)) / (Math.exp(x) + Math.exp(-x));
-    };
-}
-
-/** Polyfill Math.asinh for old browsers / IE */
-if (Math.asinh === undefined) {
-    Math.asinh = function(x) {
-        return Math.log(x + Math.sqrt(1 + x*x));
-    };
-}
-
-/** Polyfill Math.atanh for old browsers / IE */
-if (Math.atanh === undefined) {
-    Math.atanh = function(x) {
-        return Math.log((1+x) / (1-x)) / 2;
-    };
-}
-
-/* - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -  */
-if (typeof module != 'undefined' && module.exports) module.exports = Utm; // ≡ export default Utm
+*/
diff --git a/src/test/kotlin/uk/co/wedgetech/geodesy/DmsTest.kt b/src/test/kotlin/uk/co/wedgetech/geodesy/DmsTest.kt
index 267f55f..4d8b34b 100644
--- a/src/test/kotlin/uk/co/wedgetech/geodesy/DmsTest.kt
+++ b/src/test/kotlin/uk/co/wedgetech/geodesy/DmsTest.kt
@@ -63,4 +63,26 @@ class DmsTest {
         assertEquals("037°X00′X00″", uk.co.wedgetech.geodesy.toDegreesMinutesSeconds(36.99999, 0, "X"))
 
     }
-}
\ No newline at end of file
+
+    @Test
+    fun test_compassPoint() {
+        //TODO clean up
+        assertEquals("1 -> N ","N",       compassPoint(1.0) )
+        assertEquals("0 -> N ","N",       compassPoint(0.0) )
+        assertEquals("-1 -> N " ,"N",      compassPoint(-1.0) )
+        assertEquals("359 -> N " ,"N",     compassPoint(359.0) )
+        assertEquals("24 -> NNE ","NNE",    compassPoint(24.0) )
+        assertEquals("24:1 -> N ",    compassPoint(24.0, 1) ,"N")
+        assertEquals("24:2 -> NE ",   compassPoint(24.0, 2) ,"NE")
+        assertEquals("24:3 -> NNE ","NNE",  compassPoint(24.0, 3) )
+        assertEquals("226 -> SW ",    compassPoint(226.0) ,"SW")
+        assertEquals("226:1 -> W ",   compassPoint(226.0, 1) ,"W")
+        assertEquals("226:2 -> SW ",  compassPoint(226.0, 2) ,"SW")
+        assertEquals("226:3 -> SW ",  compassPoint(226.0, 3) ,"SW")
+        assertEquals("237 -> WSW ",   compassPoint(237.0) ,"WSW")
+        assertEquals("237:1 -> W ",   compassPoint(237.0, 1) ,"W")
+        assertEquals("237:2 -> SW ",  compassPoint(237.0, 2) ,"SW")
+        assertEquals("237:3 -> WSW ", compassPoint(237.0, 3) ,"WSW")
+    }
+
+    }
\ No newline at end of file
diff --git a/src/test/kotlin/uk/co/wedgetech/geodesy/OsGridRefTest.kt b/src/test/kotlin/uk/co/wedgetech/geodesy/OsGridRefTest.kt
index 017cd7b..70bcd42 100644
--- a/src/test/kotlin/uk/co/wedgetech/geodesy/OsGridRefTest.kt
+++ b/src/test/kotlin/uk/co/wedgetech/geodesy/OsGridRefTest.kt
@@ -1,5 +1,6 @@
 package uk.co.wedgetech.geodesy
 
+import jdk.nashorn.internal.objects.NativeFunction.function
 import org.junit.Assert.assertEquals
 import org.junit.Test
 
@@ -47,5 +48,52 @@ class OsGridRefTest {
         assertEquals( "000000,1200000", OsGridRef(0.0, 1200000.0 ).toString(0) )
     }
 
+    @Test fun fromJSTest() {
+        var osgb = LatLonDatum("52°39′27.2531″N".parseDegreesMinutesSeconds(), "1°43′4.5177″E".parseDegreesMinutesSeconds(), LatLonDatum.OSGB36)
+        var gridref = osgb.toOsGrid()
+        assertEquals( "C1 E", 651409.903, gridref.easting.toFixed(3), 0.001)
+        assertEquals( "C1 N", 313177.270, gridref.northing.toFixed(3), 0.001)
+
+        val osgb2 = gridref.toLatLonDatum(LatLonDatum.OSGB36);
+        assertEquals( "C1 round-trip",  "52°39′27.2531″N, 001°43′04.5177″E", osgb2.toString("dms", 4))
+
+        gridref = OsGridRef(651409.903, 313177.270);
+        osgb = gridref.toLatLonDatum(LatLonDatum.OSGB36);
+        assertEquals( "C2",  "52°39′27.2531″N, 001°43′04.5177″E", osgb2.toString("dms", 4))
+        val gridref2 = osgb2.toOsGrid()
+
+        assertEquals("parse 100km origin", "SU 00000 00000", "SU00".parseOsGridReference().toString())
+        assertEquals("parse 100km origin", "SU 00000 00000", "SU 0 0".parseOsGridReference().toString())
+        assertEquals("parse  no whitespace", "SU 38700 14800", "SU387148".parseOsGridReference().toString())
+        assertEquals("parse  6-digit", "SU 38700 14800", "SU 387 148".parseOsGridReference().toString())
+        assertEquals("parse  10-digit", "SU 38700 14800", "SU 38700 14800".parseOsGridReference().toString())
+        assertEquals("parse  numeric", "SU 38700 14800", "438700,114800".parseOsGridReference().toString())
+
+        val greenwichWGS84 = LatLonDatum(51.4778, -0.0016); // default WGS84
+        val greenwichOSGB36 = greenwichWGS84.convertDatum(LatLonDatum.OSGB36);
+        assertEquals("convert WGS84 -> OSGB36", "51.4773°N, 000.0000°E" , greenwichOSGB36.toString("d"))
+        assertEquals("convert OSGB36 -> WGS84", "51.4778°N, 000.0016°W", greenwichOSGB36.convertDatum(LatLonDatum.WGS84).toString("d"))
+
+        // limits
+        assertEquals("SW regular", "SV 00000 00000", OsGridRef( 0.0, 0.0).toString());
+        assertEquals("NE regular", "JM 99999 99999", OsGridRef( 699999.0, 1299999.0).toString());
+        assertEquals("SW numeric", "000000,000000", OsGridRef( 0.0, 0.0).toString(0));
+        assertEquals("NW numeric", "699999,1299999", OsGridRef( 699999.0, 1299999.0).toString(0));
+
+        // DG round-trip
+
+        val dgGridRef: OsGridRef = "TQ 44359 80653".parseOsGridReference()
+
+        // round-tripping OSGB36 works perfectly
+        val dgOsgb: LatLonDatum = dgGridRef.toLatLonDatum(LatLonDatum.OSGB36)
+        assertEquals("DG round-trip OSGB36",         dgGridRef.toString(), dgOsgb.toOsGrid().toString())
+        assertEquals("DG round-trip OSGB36 numeric", "544359,180653", dgOsgb.toOsGrid().toString(0));
+
+        // reversing Helmert transform (OSGB->WGS->OSGB) introduces small error (≈ 3mm in UK), so WGS84
+        // round-trip is not quite perfect: test needs to incorporate 3mm error to pass
+        val dgWgs = dgGridRef.toLatLonDatum()
+        assertEquals("DG round-trip WGS84 numeric",  "544358.997,180653", dgWgs.toOsGrid().toString(0));
+    }
+
 
 }
\ No newline at end of file