Add DTG calculation (#60)

README.md

@@ -12,6 +12,7 @@ It currently calculates:
  * Depth Below Keel (based on depth.belowSurface and design.draft.maximum)
  * Depth Below Keel (based on depth.belowTransducer and depth.transducerToKeel)
  * Depth Below Surface (based on depth.belowKeel and design.draft.maximum)
+ * Distance To Go (based on courseGreatCircle.nextPoint.position)
  * Outside air dew point (based on humidity and temperature)
  * Fuel economy (based on speed over ground, fuel rate)
  * Propeller Slip calculation (requires defaults.json to include propulsion.\*.drive.propeller.pitch and propulsion.\*.transmission.gearRatio)  

calcs/distanceToGo.js

@@ -0,0 +1,34 @@
+module.exports = function(app) {
+    return {
+        group: 'dtg',
+        optionKey: 'dtg',
+        title: "Distance To Go (based on courseGreatCircle.nextPoint.position)",
+        derivedFrom: [ "navigation.courseGreatCircle.nextPoint.position",
+                        "navigation.position" ],
+        calculator: function (nextPointPosition, vesselPosition) {
+            let distance= (!nextPointPosition || !vesselPosition) ?
+                null : calcDistance(vesselPosition, nextPointPosition)
+
+            //** Calculate the great circle distance between two points in metres
+            function calcDistance(srcpt, destpt) {
+                let Rk= 6373 // mean radius of the earth (km) at 39 degrees from the equator
+                let lat1= degreesToRadians(srcpt.latitude)
+                let lon1= degreesToRadians(srcpt.longitude)
+                let lat2= degreesToRadians(destpt.latitude)
+                let lon2= degreesToRadians(destpt.longitude)
+                let dlat= lat2 - lat1
+                let dlon= lon2 - lon1
+                let a= Math.pow(Math.sin(dlat/2),2) + Math.cos(lat1) * Math.cos(lat2) * Math.pow(Math.sin(dlon/2),2)
+                let c= 2 * Math.atan2(Math.sqrt(a),Math.sqrt(1-a)) // great circle distance in radians
+                return (c * Rk) * 1000 // great circle distance in m
+            }
+
+            function degreesToRadians(val) {  return val * Math.PI/180 }
+
+            return [{ 
+                path: "navigation.courseGreatCircle.nextPoint.distance",
+                value: distance
+            }]
+        }
+    };
+}