Add equal earth projection

Gruntfile.js

@@ -32,7 +32,8 @@ var projs = [
   'robin',
   'geocent',
   'tpers',
-  'geos'
+  'geos',
+  'eqearth'
 ];
 module.exports = function (grunt) {
   grunt.initConfig({

index.js

@@ -0,0 +1,4 @@
+const proj4 = require('./dist/proj4-src.js');
+
+const coords = proj4("+proj=stere +lat_0=90 +lat_ts=60 +lon_0=10 +x_0=0 +y_0=0 +a=6370040 +b=6370040 +to_meter=1000 +no_defs", "WGS84", [-217.962000,-4331.145000]);
+console.log(coords);

lib/includedProjections.js

@@ -26,6 +26,7 @@ import robin from './projections/robin';
 import geocent from './projections/geocent';
 import tpers from './projections/tpers';
 import geos from './projections/geos';
+import eqearth from "./projections/eqearth";
 
 var projs = [
   tmerc,
@@ -56,6 +57,7 @@ var projs = [
   geocent,
   tpers,
   geos,
+  eqearth
 ];
 
 export default function (proj4) {

lib/projString.js

@@ -70,6 +70,9 @@ export default function(defData) {
     b: function(v) {
       self.b = parseFloat(v);
     },
+    r: function(v) {
+      self.a = self.b = parseFloat(v);
+    },
     r_a: function() {
       self.R_A = true;
     },

lib/projections/eqearth.js

@@ -0,0 +1,93 @@
+/**
+ * Copyright 2018 Bernie Jenny, Monash University, Melbourne, Australia.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ *
+ * Equal Earth is a projection inspired by the Robinson projection, but unlike
+ * the Robinson projection retains the relative size of areas. The projection
+ * was designed in 2018 by Bojan Savric, Tom Patterson and Bernhard Jenny.
+ *
+ * Publication:
+ * Bojan Savric, Tom Patterson & Bernhard Jenny (2018). The Equal Earth map
+ * projection, International Journal of Geographical Information Science,
+ * DOI: 10.1080/13658816.2018.1504949
+ *
+ * Code released August 2018
+ * Ported to JavaScript and adapted for mapshaper-proj by Matthew Bloch August 2018
+ * Modified for proj4js by Andreas Hocevar by Andreas Hocevar March 2024
+ */
+
+import adjust_lon from "../common/adjust_lon";
+
+var A1 = 1.340264,
+    A2 = -0.081106,
+    A3 = 0.000893,
+    A4 = 0.003796,
+    M = Math.sqrt(3) / 2.0;
+
+export function init() {
+  this.es = 0;
+  this.long0 = this.long0 !== undefined ? this.long0 : 0;
+}
+
+export function forward(p) {
+  var lam = adjust_lon(p.x - this.long0);
+  var phi = p.y;
+  var paramLat = Math.asin(M * Math.sin(phi)),
+  paramLatSq = paramLat * paramLat,
+  paramLatPow6 = paramLatSq * paramLatSq * paramLatSq;
+  p.x = lam * Math.cos(paramLat) /
+  (M * (A1 + 3 * A2 * paramLatSq + paramLatPow6 * (7 * A3 + 9 * A4 * paramLatSq)));
+  p.y = paramLat * (A1 + A2 * paramLatSq + paramLatPow6 * (A3 + A4 * paramLatSq));
+
+  p.x = this.a * p.x + this.x0;
+  p.y = this.a * p.y + this.y0;
+  return p;
+}
+
+export function inverse(p) {
+  p.x = (p.x - this.x0) / this.a;
+  p.y = (p.y - this.y0) / this.a;
+
+  var EPS = 1e-9,
+      NITER = 12,
+      paramLat = p.y,
+      paramLatSq, paramLatPow6, fy, fpy, dlat, i;
+
+  for (i = 0; i < NITER; ++i) {
+    paramLatSq = paramLat * paramLat;
+    paramLatPow6 = paramLatSq * paramLatSq * paramLatSq;
+    fy = paramLat * (A1 + A2 * paramLatSq + paramLatPow6 * (A3 + A4 * paramLatSq)) - p.y;
+    fpy = A1 + 3 * A2 * paramLatSq + paramLatPow6 * (7 * A3 + 9 * A4 * paramLatSq);
+    paramLat -= dlat = fy / fpy;
+    if (Math.abs(dlat) < EPS) {
+        break;
+    }
+  }
+  paramLatSq = paramLat * paramLat;
+  paramLatPow6 = paramLatSq * paramLatSq * paramLatSq;
+  p.x = M * p.x * (A1 + 3 * A2 * paramLatSq + paramLatPow6 * (7 * A3 + 9 * A4 * paramLatSq)) /
+          Math.cos(paramLat);
+  p.y = Math.asin(Math.sin(paramLat) / M);
+
+  p.x = adjust_lon(p.x + this.long0);
+  return p;
+}
+
+export var names = ["eqearth", "Equal Earth", "Equal_Earth"];
+export default {
+  init: init,
+  forward: forward,
+  inverse: inverse,
+  names: names
+};

projs.js

@@ -27,6 +27,7 @@ import robin from './lib/projections/robin';
 import geocent from './lib/projections/geocent';
 import tpers from './lib/projections/tpers';
 import geos from './lib/projections/geos';
+import eqearth from './lib/projections/eqearth';
 export default function(proj4){
   proj4.Proj.projections.add(tmerc);
   proj4.Proj.projections.add(etmerc);
@@ -57,4 +58,5 @@ export default function(proj4){
   proj4.Proj.projections.add(geocent);
   proj4.Proj.projections.add(tpers);
   proj4.Proj.projections.add(geos);
+  proj4.Proj.projections.add(eqearth);
 }

test/testData.js

@@ -856,6 +856,16 @@ var testPoints = [
     ll: [0, 0],
     xy: [0, 12367396.218459858],
   },
+  {
+    code: '+proj=eqearth +lon_0=0 +x_0=0 +y_0=0 +R=6371008.7714 +units=m +no_defs +type=crs',
+    ll: [16, 48],
+    xy: [1284600.7230114893, 5794915.366010354]
+  },
+  {
+    code: '+proj=eqearth +lon_0=150 +x_0=0 +y_0=0 +R=6371008.7714 +units=m +no_defs +type=crs',
+    ll: [16, 48],
+    xy: [-10758531.055221224, 5794915.366010354]
+  }
 ];
 if (typeof module !== 'undefined') {
   module.exports = testPoints;