diff --git a/src/components/map/map.tsx b/src/components/map/map.tsx
index afb75c1..37296f9 100644
--- a/src/components/map/map.tsx
+++ b/src/components/map/map.tsx
@@ -20,6 +20,7 @@ const Map: react.FC<{}> = (props: {}) => {
 
   useEffect(() => {
     window.addEventListener('resize', debouncedResizeHandler);
+//    dispatch(mapActions.shift({ x: -50, y: 0 }));
   }, []);
 
   return (
diff --git a/src/setupTests.ts b/src/setupTests.ts
new file mode 100644
index 0000000..87988d6
--- /dev/null
+++ b/src/setupTests.ts
@@ -0,0 +1,14 @@
+// jest-dom adds custom jest matchers for asserting on DOM nodes.
+// allows you to do things like:
+// expect(element).toHaveTextContent(/react/i)
+// learn more: https://github.com/testing-library/jest-dom
+import '@testing-library/jest-dom/extend-expect';
+
+// Mock matchmedia
+window.matchMedia = window.matchMedia || function() {
+  return {
+      matches: false,
+      addListener: function() {},
+      removeListener: function() {}
+  };
+};
diff --git a/src/store/map.test.ts b/src/store/map.test.ts
new file mode 100644
index 0000000..e99c0f0
--- /dev/null
+++ b/src/store/map.test.ts
@@ -0,0 +1,69 @@
+import _ from 'lodash';
+import { initialMapState, reevaluateState } from './map';
+
+//interface CustomMatchers<R = unknown> {
+//  isDeepEqual(received: any, target: any): R;
+//}
+
+//declare global {
+//  namespace jest {
+//    interface Expect extends CustomMatchers {}
+//    interface Matchers<R> extends CustomMatchers<R> {}
+//    interface InverseAsymmetricMatchers extends CustomMatchers {}
+//  }
+//}
+
+expect.extend({
+  isDeepEqual: (received: any, target: any) => {
+    const pass = _.isEqual(received, target);
+    if (pass) {
+      return { message: () => '', pass: true };
+    }
+    return {
+      message: () =>
+        `${JSON.stringify(received)} instead of\n ${JSON.stringify(target)}`,
+      pass: false,
+    };
+  },
+  isAlmostDeepEqual: (received: any, target: any, delta: number) => {
+    const pass = _.isEqualWith(received, target, (r: any, t: any) => {
+      if (typeof r !== 'number' || typeof t !== 'number') {
+        return undefined;
+      }
+      console.log(`r: ${r}, t:${t}, ${Math.abs(r - t) <= delta}`);
+      return Math.abs(r - t) <= delta;
+    });
+    if (pass) {
+      return { message: () => '', pass: true };
+    }
+    return {
+      message: () =>
+        `${JSON.stringify(received)} instead of\n ${JSON.stringify(target)}`,
+      pass: false,
+    };
+  },
+});
+
+describe('Our isAlmostDeepEqual matcher', () => {
+  test('compares correctly two numbers that are almost equals', () => {
+    expect({ x: 1.001 }).isAlmostDeepEqual({ x: 1 }, 1e-2);
+  });
+});
+
+describe('Map store methods', () => {
+  test('initialize its state', () => {
+    const state = _.cloneDeep(initialMapState);
+    expect(state.tiles.nb.x).not.toBe(0);
+  });
+  test('reevaluateState keeps the same values', () => {
+    const state = _.cloneDeep(initialMapState);
+    reevaluateState(state);
+    expect(state).isAlmostDeepEqual(initialMapState, 1e-7);
+  });
+  test('reevaluateState computes the right longitude after a shift 50 pixels left', () => {
+    const state = _.cloneDeep(initialMapState);
+    state.slippy.translation.x = state.slippy.translation.x - 50;
+    reevaluateState(state);
+    expect(state.scope.center.lon).not.toBe(77.5539501);
+  });
+});
diff --git a/src/store/map.ts b/src/store/map.ts
index e622d10..4b7b428 100644
--- a/src/store/map.ts
+++ b/src/store/map.ts
@@ -1,7 +1,14 @@
 import { createSlice } from '@reduxjs/toolkit';
 import _ from 'lodash';
 import { tileSize } from '../components/map/tiled-map';
-import { geoPoint, Point, lon2tile, lat2tile } from '../lib/geo';
+import {
+  geoPoint,
+  Point,
+  lon2tile,
+  lat2tile,
+  tile2lat,
+  tile2long,
+} from '../lib/geo';
 
 // Top level properties (the other properties can be derived from them)
 
@@ -12,7 +19,7 @@ export interface MapScope {
 }
 const initialMapScope: MapScope = {
   center: { lat: -37.8403508, lon: 77.5539501 },
-  zoom: 13.49,
+  zoom: 13,
 };
 
 // Derived properties
@@ -60,7 +67,7 @@ const nbTilesY = _.ceil(mapState.viewport.height / tileSize) + 4;
 
 */
 
-let initialMapState: MapState = {
+export var initialMapState: MapState = {
   scope: initialMapScope,
   slippy: {
     scale: 1,
@@ -82,7 +89,8 @@ let initialMapState: MapState = {
   },
 };
 
-const evaluateStateFromScope = (state: MapState) => {
+export const evaluateStateFromScope = (state: MapState) => {
+  console.log('<<<<<<<<<<<< evaluateStateFromScope');
   state.tiles.zoom = _.round(state.scope.zoom);
   const softZoom = state.scope.zoom - state.tiles.zoom;
   state.slippy.scale = 2 ** softZoom;
@@ -115,9 +123,37 @@ const evaluateStateFromScope = (state: MapState) => {
 
 evaluateStateFromScope(initialMapState);
 
-const reevaluateState = (state: MapState) => {};
-
-
+export const reevaluateState = (state: MapState) => {
+  // Update the scope (center and zoom level)
+  const centerPX = {
+    x: window.innerWidth / 2,
+    y: window.innerHeight / 2,
+  };
+  const visibleTileSize = tileSize / state.slippy.scale;
+  const centerTiles = {
+    x:
+      state.tiles.first.x +
+      (centerPX.x - state.slippy.translation.x) / visibleTileSize,
+    y:
+      state.tiles.first.y +
+      (centerPX.y - state.slippy.translation.y) / visibleTileSize,
+  };
+  state.scope.center = {
+    lat: tile2lat(centerTiles.y, state.tiles.zoom),
+    lon: tile2long(centerTiles.x, state.tiles.zoom),
+  };
+  // TODO: zoom level
+  if (
+    -state.slippy.translation.x < visibleTileSize ||
+    -state.slippy.translation.y < visibleTileSize ||
+    -state.slippy.translation.x >
+      (state.tiles.nb.x - 1) * visibleTileSize - window.innerWidth ||
+    -state.slippy.translation.y >
+      (state.tiles.nb.y - 1) * visibleTileSize - window.innerHeight
+  ) {
+    evaluateStateFromScope(state);
+  }
+};
 
 const mapSlice = createSlice({
   name: 'map',
@@ -131,6 +167,7 @@ const mapSlice = createSlice({
         x: state.slippy.translation.x + action.payload.x,
         y: state.slippy.translation.y + action.payload.y,
       };
+      reevaluateState(state);
     },
     scale: (state, action) => {
       state.slippy.scale = state.slippy.scale * action.payload.factor;
@@ -144,6 +181,7 @@ const mapSlice = createSlice({
           (state.slippy.translation.y - action.payload.center.y) *
             (action.payload.factor - 1),
       };
+      reevaluateState(state);
     },
   },
 });