Still playing with the reticle...

src/components/map/Reticle.tsx

@@ -1,26 +1,78 @@
+import _ from 'lodash';
 import React from 'react';
 import { useSelector } from 'react-redux';
 import { Point } from '../../lib/geo';
 import { MapState } from '../../store/map';
+import '../../theme/map.css';
 
 const Reticle: React.FC<{}> = () => {
-
   const windowState = useSelector(
     (state: { map: MapState }) => state.map.window
   );
+  const scope = useSelector((state: { map: MapState }) => state.map.scope);
+  const radius = Math.min(windowState.width, windowState.height) / 4;
+  const center: Point = {
+    x: windowState.width / 2,
+    y: windowState.height / 2,
+  };
 
-  const center: Point = { x: windowState.width / 2, y: windowState.height / 2 };
-  const radius = Math.min(windowState.width, windowState.height) / 6;
+  /**
+   * The horizontal distance represented by each square tile, measured along the parallel at a given latitude, is given by:
+   *     Stile = C ∙ cos(latitude) / 2 zoomlevel
+   * As tiles are 256-pixels wide, the horizontal distance represented by one pixel is:
+   *     Spixel = Stile / 256 = C ∙ cos(latitude) / 2 (zoomlevel + 8)
+   * where C means the equatorial circumference of the Earth (40 075 016.686 m ≈ 2π ∙ 6 378 137.000 m for the reference geoid used by OpenStreetMap).
+   *     see https://wiki.openstreetmap.org/wiki/Zoom_levels
+   */
+
+  const sTile =
+    (40075016.686 * Math.cos((scope.center.lat * Math.PI) / 180)) /
+    2 ** scope.zoom;
+
+  const radiusMeters = (sTile * radius) / 256;
+  const radiusOrder = Math.floor(Math.log10(radiusMeters));
+  const radiusRoughUnit = radiusMeters / 10 ** radiusOrder;
+  var radiusBaseUnit, radiusBaseSubUnit;
+  if (radiusRoughUnit <= 2) {
+    radiusBaseUnit = 1;
+    radiusBaseSubUnit = 0.2;
+  } else if (radiusRoughUnit <= 5) {
+    radiusBaseUnit = 2;
+    radiusBaseSubUnit = 0.5;
+  } else {
+    radiusBaseUnit = 5;
+    radiusBaseSubUnit = 1;
+  }
+
+  const nbSubUnits = Math.floor(
+    radiusMeters / (radiusBaseSubUnit * 10 ** radiusOrder)
+  );
+  const nbUnits = Math.floor(
+    radiusMeters / (radiusBaseUnit * 10 ** radiusOrder)
+  );
+
+  const radiusUnit = radiusOrder < 3 ? 'm' : 'km';
+  const radiusUnitValue =
+    radiusOrder < 3
+      ? radiusBaseUnit * 10 ** radiusOrder
+      : radiusBaseUnit * 10 ** (radiusOrder - 3);
+
+  const subUnitPxStep = ((radiusBaseSubUnit * 10 ** radiusOrder) / sTile) * 256;
+  const unitPxStep = ((radiusBaseUnit * 10 ** radiusOrder) / sTile) * 256;
+
+  console.log(
+    `radiusMeters: ${radiusMeters}, radiusOrder: ${radiusOrder}, radiusUnit: ${radiusBaseUnit}, radiusSubUnit: ${radiusBaseSubUnit}, nbSubUnits: ${nbSubUnits}`
+  );
 
   return (
-    <g>
+    <g className='reticle'>
       <circle
         cx={center.x}
         cy={center.y}
         r={radius}
         fill='transparent'
         stroke='black'
-        strokeWidth={2}
+        strokeWidth={1}
       />
       <path
         d={`M ${center.x - radius},${center.y} 
@@ -31,9 +83,39 @@ const Reticle: React.FC<{}> = () => {
             v ${radius - 3}
             m 0,${6}
             v ${radius - 3}`}
-        strokeWidth={1}
-        stroke='red'
+        strokeWidth={0.5}
       />
+      <g id='reticle-east'>
+        <path stroke='red' className='reticle'
+          strokeWidth={0.5}
+          d={_.range(1, nbSubUnits + 1)
+            .map(
+              (i) => `M ${center.x + i * subUnitPxStep},${center.y - 10} v 20`
+            )
+            .join(' ')}
+        />
+        <path stroke='red' className='reticle'
+          d={_.range(1, nbUnits + 1)
+            .map((i) => `M ${center.x + i * unitPxStep},${center.y - 20} v 40`)
+            .join(' ')}
+          strokeWidth={0.5}
+        />
+      </g>
+      <use stroke='red' className='reticle'
+        href='#reticle-east'
+        transform={`rotate(90, ${center.x}, ${center.y})`}
+      />
+      <use className='reticle'
+        href='#reticle-east'
+        transform={`rotate(180, ${center.x}, ${center.y})`}
+      />
+      <use className='reticle'
+        href='#reticle-east'
+        transform={`rotate(-90, ${center.x}, ${center.y})`}
+      />
+      <text x={center.x + unitPxStep - 15} y={center.y - 20}>
+        {`${radiusUnitValue} ${radiusUnit}`}
+      </text>
     </g>
   );
 };

src/theme/map.css

@@ -43,6 +43,17 @@ path.current {
   stroke: rgba(2, 71, 20, 0.8);
 }
 
+.reticle path,
+path.reticle,
+use.reticle,
+.reticle use {
+  stroke: red;
+}
+
+.reticle text {
+  font-size: 12px;
+}
+
 ion-modal {
   --height: 50%;
   --border-radius: 16px;
@@ -60,7 +71,7 @@ ion-modal.full-height {
 } */
 
 ion-modal ion-toolbar {
-  --background: rgba(14,116,144, .7);
+  --background: rgba(14, 116, 144, 0.7);
   --color: white;
 }
 
@@ -68,15 +79,13 @@ ion-modal.full-height ion-toolbar {
   --background: rgba(14, 116, 144, 1);
 }
 
-
 ion-modal ion-toolbar.secondary {
   --background: inherit;
   --color: inherit;
 }
 
 ion-modal ion-content {
-  background-color: rgba(255,255,255,.7)
-  ;
+  background-color: rgba(255, 255, 255, 0.7);
 }
 
 .hidden {