Distance conversion tests (#687)

* adds distance conversion tests

* Change latitude to zero
Consider using cheap-ruler for calculations based on latitudes

packages/turf-helpers/package.json

@@ -29,6 +29,7 @@
   },
   "homepage": "https://github.com/Turfjs/turf",
   "devDependencies": {
+    "@turf/distance": "^4.1.0",
     "benchmark": "^2.1.3",
     "tape": "^3.5.0"
   },

packages/turf-helpers/test.js

@@ -1,5 +1,6 @@
 const test = require('tape');
 const helpers = require('.');
+const distance = require('@turf/distance');
 const {
     point,
     polygon,
@@ -11,8 +12,11 @@ const {
     multiPolygon,
     geometryCollection,
     radiansToDistance,
+    distanceToRadians,
+    distanceToDegrees,
     bearingToAngle
-} = helpers;
+}
+= helpers;
 
 test('point', t => {
     const ptArray = point([5, 10], {name: 'test point'});
@@ -306,6 +310,26 @@ test('radiansToDistance', t => {
     t.end();
 });
 
+// the higher/lower in latitude, the greater the distortion (curvature of the earth)
+const dx = distance(point([-120, 0]), point([-120.5, 0]));
+const dy = distance(point([-120, 0]), point([-120, 0.5]));
+
+test('distanceToRadians', t => {
+    t.equal(distanceToRadians(dx), distanceToRadians(dy), 'radiance conversion');
+    t.equal(distanceToRadians(1, 'radians'), 1);
+    t.equal(distanceToRadians(6373, 'kilometers'), 1);
+    t.equal(distanceToRadians(3960, 'miles'), 1);
+
+    t.end();
+});
+
+test('distanceToDegrees', t => {
+    t.equal(Number(distanceToDegrees(dx).toFixed(6)), 0.5, 'degrees conversion');
+    t.equal(Number(distanceToDegrees(dy).toFixed(6)), 0.5, 'degrees conversion');
+
+    t.end();
+});
+
 test('bearingToAngle', t => {
     t.equal(bearingToAngle(40), 40);
     t.equal(bearingToAngle(-105), 255);