diff --git a/index.html b/index.html
index b89e682a..ab9a073a 100644
--- a/index.html
+++ b/index.html
@@ -4456,13 +4456,20 @@
                 type="range"
                 min="1"
                 max="13"
-                value="8"
-                data-cells="50000"
+                value="4"
+                data-cells="10000"
                 oninput="document.getElementById('submapPointsOutput').value=cellsDensityConstants[+this.value]/1000 + 'K'; event.stopPropagation()"
               />
             </td>
             <td>
-              <output id="submapPointsOutput" style="color: #053305">50K</output>
+              <output id="submapPointsOutput" style="color: #053305">10K</output>
+            </td>
+          </tr>
+          <tr>
+            <td>Shift</td>
+            <td colspan="2">
+              X: <input id="submapShiftX" autocomplete="off" min="0" size="4" value="0"/>
+              Y: <input id="submapShiftY" autocomplete="off" min="0" size="4" value="0"/>
             </td>
           </tr>
           <tr>
@@ -4483,10 +4490,13 @@
             </td>
           </tr>
           <tr>
-            <td>Shift</td>
+            <td>Mirror</td>
             <td colspan="2">
-              X: <input id="submapShiftX" autocomplete="off" min="0" size="4" value="0"/>
-              Y: <input id="submapShiftY" autocomplete="off" min="0" size="4" value="0"/>
+              <input type="checkbox" class="checkbox" id="submapMirrorH" />
+              <label for="submapMirrorH" class="checkbox-label" >Horizontally</label>
+              &nbsp;
+              <input type="checkbox" class="checkbox" id="submapMirrorV" />
+              <label for="submapMirrorV" class="checkbox-label">Vertically</label>
             </td>
           </tr>
         </table>
diff --git a/main.js b/main.js
index 7f6b451d..93b14fea 100644
--- a/main.js
+++ b/main.js
@@ -1413,6 +1413,7 @@ function reMarkFeatures() {
     cells.harbor[i] = water.length;
   };
 
+  if (!cells.i.length) return; // no cells -> there is nothing to do
   for (let i = 1, queue = [0]; queue[0] !== -1; i++) {
     const start = queue[0]; // first cell
     cells.f[start] = i; // assign feature number
diff --git a/modules/ui/submap.js b/modules/ui/submap.js
index 1b5382cd..15906e52 100644
--- a/modules/ui/submap.js
+++ b/modules/ui/submap.js
@@ -25,6 +25,12 @@ window.UISubmap = (function () {
 
   function openRemapOptions() {
     resetZoom(0);
+    document.getElementById("submapRotationAngle").value = 0;
+    document.getElementById("submapRotationAngleOutput").value = "0°";
+    document.getElementById("submapShiftX").value = 0;
+    document.getElementById("submapShiftY").value = 0;
+    document.getElementById("submapMirrorH").checked = false;
+    document.getElementById("submapMirrorV").checked = false;
     $("#remapOptionsDialog").dialog({
       title: "Resampler options",
       resizable: false,
@@ -50,26 +56,32 @@ window.UISubmap = (function () {
     const angle = Number(document.getElementById("submapRotationAngle").value) / 180 * Math.PI;
     const shiftX = Number(document.getElementById("submapShiftX").value);
     const shiftY = Number(document.getElementById("submapShiftY").value);
+    const mirrorH = document.getElementById("submapMirrorH").checked;
+    const mirrorV = document.getElementById("submapMirrorV").checked;
+    if (!cellsDensityConstants[cellNumId]) {
+      console.error("Unknown cell number!");
+      return;
+    }
+
     const [cx, cy] = [graphWidth / 2, graphHeight / 2];
     const rot = alfa  => (x, y) => [
       (x - cx) * Math.cos(alfa) - (y - cy) * Math.sin(alfa) + cx,
       (y - cy) * Math.cos(alfa) + (x - cx) * Math.sin(alfa) + cy
     ];
     const shift = (dx, dy) => (x, y) => [x + dx, y + dy];
+    const flipH = (x, y) => [-x + 2 * cx, y];
+    const flipV = (x, y) => [x, -y + 2 * cy];
     const app = (f, g) => (x, y) => f(...g(x, y));
-    if (!cellsDensityConstants[cellNumId]) {
-      console.error("Unknown cell number!");
-      return;
-    }
+    const id = (x, y) => [x, y];
+    let projection = id, inverse = id;
 
-    let projection, inverse;
-    if (angle || shiftX || shiftY) {
-      projection = app(shift(shiftX, shiftY), rot(angle));
-      inverse = app(rot(-angle), shift(-shiftX, -shiftY));
-    } else {
-      projection = (x, y) => [x, y];
-      inverse = (x, y) => [x, y];
+    if (angle) [projection, inverse] = [rot(angle), rot(-angle)];
+    if (shiftX || shiftY) {
+      projection = app(shift(shiftX, shiftY), projection);
+      inverse = app(inverse, shift(-shiftX, -shiftY));
     }
+    if (mirrorH) [projection, inverse] = [app(flipH, projection), app(inverse, flipH)];
+    if (mirrorV) [projection, inverse] = [app(flipV, projection), app(inverse, flipV)];
 
     changeCellsDensity(cellNumId);
     WARN && console.warn("Resampling current map");