Increase latitude accuracy level (#102)

.changeset/giant-elephants-pull.md

@@ -0,0 +1,7 @@
+---
+"react-three-map": patch
+---
+
+- Improve accuracy at long distances for `coordsToVector3`.
+- Improve translation accuracy for `NearCoordinates`, but scale is still ignored.
+- Update `earthRadius` from `6378137` to `6371008.8` to match [Mapbox](https://github.com/maplibre/maplibre-gl-js/blob/8ea76118210dd18fa52fdb83f2cbdd1229807346/src/geo/lng_lat.ts#L8) and [Maplibre](https://github.com/maplibre/maplibre-gl-js/blob/main/src/geo/lng_lat.ts#L8).

README.md

@@ -198,11 +198,11 @@ import { Canvas, Coordinates } from 'react-three-map'
 
 [![](https://img.shields.io/badge/-demo-%23ff69b4)](https://rodrigohamuy.github.io/react-three-map/?story=multi-coordinates--default)
 
-Same as `Coordinates`, but with an error margin that increases the further you are from the origin.
+Same as `Coordinates`, but scale is ignored in exchange of being able to be rendered under the same scene.
 
-Recommended to use at city level distances, but margin errors will be noticeable at country level distances.
+Works well at city level distances, but since scale remains unchanged, is not recommended at country level distances.
 
-Check the story to see the difference between the two.
+Check the story to see the difference between the two or check #102 for more info.
 
 ### useMap
 
@@ -224,7 +224,7 @@ const Component = () => {
 
 This utility function converts geographic coordinates into a `Vector3Tuple`, which represents a 3D vector in meters.
 
-Similar to `NearCoordinates` it has a relatively good precision at city distances, but is not recommended if your distances are too big.
+Similar to `NearCoordinates`, remember that this only updates positions (translation) but that scale is not taken into account, which has an important factor at very long distances (country level).
 
 
 | Parameter        | Description                                                     |
@@ -240,7 +240,9 @@ Returns a `Vector3Tuple` representing the 3D position of the point relative to t
 
 This utility function converts a `Vector3Tuple`, which represents a 3D vector in meters, back into geographic coordinates.
 
-It is the inverse of `coordsToVector3` and maintains the same level of precision. It is not recommended for use with very large distances.
+It is the inverse of `coordsToVector3` but it does not have a good level of precision at long distances since we haven't reverse engineered #102 fix yet.
+
+Recommended to use at city level distances, but margin errors will be noticeable at country level distances.
 
 | Parameter                | Description                                                     |
 | ------------------------ | --------------------------------------------------------------- |

src/api/coords-to-vector-3.ts

@@ -2,14 +2,40 @@ import { MathUtils, Vector3Tuple } from 'three';
 import { Coords } from './coords';
 import { earthRadius } from "../core/earth-radius";
 
+
+const mercatorScaleLookup: { [key: number]: number } = {};
+
+function getMercatorScale(lat: number): number {
+  const index = Math.round(lat * 1000);
+  if (mercatorScaleLookup[index] === undefined) {
+    mercatorScaleLookup[index] = 1 / Math.cos(lat * MathUtils.DEG2RAD);
+  }
+  return mercatorScaleLookup[index];
+}
+
+export function averageMercatorScale(originLat: number, pointLat: number, steps = 10): number {
+  let totalScale = 0;
+  const latStep = (pointLat - originLat) / steps;
+  for (let i = 0; i <= steps; i++) {
+    const lat = originLat + latStep * i;
+    totalScale += getMercatorScale(lat);
+  }
+  return totalScale / (steps + 1);
+}
+
 export function coordsToVector3(point: Coords, origin: Coords): Vector3Tuple {
   const latitudeDiff = (point.latitude - origin.latitude) * MathUtils.DEG2RAD;
   const longitudeDiff = (point.longitude - origin.longitude) * MathUtils.DEG2RAD;
   const altitudeDiff = (point.altitude || 0) - (origin.altitude || 0);
 
   const x = longitudeDiff * earthRadius * Math.cos(origin.latitude * MathUtils.DEG2RAD);
   const y = altitudeDiff;
-  const z = -latitudeDiff * earthRadius;
 
+  // dynamic step size based on latitude difference. calculate the mercator unit scale at origin
+  // and the scale average along the line to the point for better accuracy far from origin
+  const steps = Math.ceil(Math.abs(point.latitude - origin.latitude)) * 100 + 1;
+  const avgScale = averageMercatorScale(origin.latitude, point.latitude, steps);
+
+  const z = ((-latitudeDiff * earthRadius) / getMercatorScale(origin.latitude)) * avgScale;
   return [x, y, z] as Vector3Tuple;
-}
+}

src/core/earth-radius.ts

@@ -1 +1 @@
-export const earthRadius = 6378137;
+export const earthRadius = 6371008.8;