diff --git a/src/terraform_cmd.cpp b/src/terraform_cmd.cpp
--- a/src/terraform_cmd.cpp
+++ b/src/terraform_cmd.cpp
@@ -321,6 +321,86 @@ CommandCost CmdTerraformLand(TileIndex t
 		/* Finally mark the dirty tiles dirty */
 		for (TileIndexSet::const_iterator it = ts.dirty_tiles.begin(); it != ts.dirty_tiles.end(); it++) {
 			MarkTileDirtyByTile(*it);
+
+			int height = TerraformGetHeightOfTile(&ts, *it);
+
+			/* Now, if we alter the height of the map edge, we need to take care
+			 * about repainting the affected areas outside map as well.
+			 * Remember:
+			 * Outside map, we assume that our landscape descends to
+			 * height zero as fast as possible.
+			 * Those simulated tiles (they don't exist as datastructure,
+			 * only as concept in code) need to be repainted properly,
+			 * otherwise we will get ugly glitches.
+			 *
+			 * Furthermore, note that we have to take care about the possibility,
+			 * that landscape was higher before the change,
+			 * so also tiles a bit outside need to be repainted.
+			 */
+			int x = TileX(*it);
+			int y = TileY(*it);
+			if (x == 0) {
+				if (y == 0) {
+					/* Height of the northern corner is altered. */
+					for (int cx = 0; cx >= -height - 1; cx--) {
+						for (int cy = 0; cy >= -height - 1; cy--) {
+							/* This means, tiles in the sector north of that
+							 * corner need to be repainted.
+							 */
+							if (cx + cy >= -height - 2) {
+								/* But only tiles that actually might have changed. */
+								MarkTileDirtyByTileOutsideMap(cx, cy);
+							}
+						}
+					}
+				} else if (y < (int)MapMaxY()) {
+					for (int cx = 0; cx >= -height - 1; cx--) {
+						MarkTileDirtyByTileOutsideMap(cx, y);
+					}
+				} else {
+					for (int cx = 0; cx >= -height - 1; cx--) {
+						for (int cy = (int)MapMaxY(); cy <= (int)MapMaxY() + height + 1; cy++) {
+							if (cx + ((int)MapMaxY() - cy) >= -height - 2) {
+								MarkTileDirtyByTileOutsideMap(cx, cy);
+							}
+						}
+					}
+				}
+			} else if (x < (int)MapMaxX()) {
+				if (y == 0) {
+					for (int cy = 0; cy >= -height - 1; cy--) {
+						MarkTileDirtyByTileOutsideMap(x, cy);
+					}
+				} else if (y < (int)MapMaxY()) {
+					/* Nothing to be done here, we are inside the map. */
+				} else {
+					for (int cy = (int)MapMaxY(); cy <= (int)MapMaxY() + height + 1; cy++) {
+						MarkTileDirtyByTileOutsideMap(x, cy);
+					}
+				}
+			} else {
+				if (y == 0) {
+					for (int cx = (int)MapMaxX(); cx <= (int)MapMaxX() + height + 1; cx++) {
+						for (int cy = 0; cy >= -height - 1; cy--) {
+							if (((int)MapMaxX() - cx) + cy >= -height - 2) {
+								MarkTileDirtyByTileOutsideMap(cx, cy);
+							}
+						}
+					}
+				} else if (y < (int)MapMaxY()) {
+					for (int cx = (int)MapMaxX(); cx <= (int)MapMaxX() + height + 1; cx++) {
+						MarkTileDirtyByTileOutsideMap(cx, y);
+					}
+				} else {
+					for (int cx = (int)MapMaxX(); cx <= (int)MapMaxX() + height + 1; cx++) {
+						for (int cy = (int)MapMaxY(); cy <= (int)MapMaxY() + height + 1; cy++) {
+							if (((int)MapMaxX() - cx) + ((int)MapMaxY() - cy) >= -height - 2) {
+								MarkTileDirtyByTileOutsideMap(cx, cy);
+							}
+						}
+				  }
+			  }
+			}
 		}
 
 		if (c != NULL) c->terraform_limit -= ts.tile_to_new_height.size() << 16;
diff --git a/src/viewport.cpp b/src/viewport.cpp
--- a/src/viewport.cpp
+++ b/src/viewport.cpp
@@ -2139,6 +2139,20 @@ void MarkTileDirtyByTile(TileIndex tile)
 	);
 }
 
+void MarkTileDirtyByTileOutsideMap(int x, int y)
+{
+	Point pt = RemapCoords(x * TILE_SIZE, y * TILE_SIZE, GetTilePixelZOutsideMap(x, y));
+	/* Since tiles painted outside the map don't contain buildings, trees, etc.,
+	 * this reduced area for repainting should suffice. If not, adjust the offsets
+	 * used below. */
+	MarkAllViewportsDirty(
+		pt.x - TILE_SIZE + 1,
+		pt.y,
+		pt.x + TILE_SIZE - 1,
+		pt.y + TILE_SIZE + TILE_HEIGHT - 1
+	);
+}
+
 /**
  * Marks the selected tiles as dirty.
  *
diff --git a/src/viewport_func.h b/src/viewport_func.h
--- a/src/viewport_func.h
+++ b/src/viewport_func.h
@@ -80,6 +80,7 @@ extern Point _tile_fract_coords;
 void MarkTileDirtyByTile(TileIndex tile);
 
 int GetRowAtTile(int viewport_y, Point tile, bool bridge_correct);
+void MarkTileDirtyByTileOutsideMap(int x, int y);
 
 Point GetViewportStationMiddle(const ViewPort *vp, const Station *st);