diff --git a/src/heightmap.cpp b/src/heightmap.cpp
--- a/src/heightmap.cpp
+++ b/src/heightmap.cpp
@@ -364,8 +364,7 @@ static void GrayscaleToMapHeights(uint i
 				SetTileHeight(tile, map[img_row * img_width + img_col] / 16);
 			}
 			/* Only clear the tiles within the map area. */
-			if (TileX(tile) != MapMaxX() && TileY(tile) != MapMaxY() &&
-					(!_settings_game.construction.freeform_edges || (TileX(tile) != 0 && TileY(tile) != 0))) {
+			if (IsInnerTile(tile)) {
 				MakeClear(tile, CLEAR_GRASS, 3);
 			}
 		}
diff --git a/src/tgp.cpp b/src/tgp.cpp
--- a/src/tgp.cpp
+++ b/src/tgp.cpp
@@ -950,8 +950,7 @@ static void TgenSetTileHeight(TileIndex 
 	SetTileHeight(tile, height);
 
 	/* Only clear the tiles within the map area. */
-	if (TileX(tile) != MapMaxX() && TileY(tile) != MapMaxY() &&
-			(!_settings_game.construction.freeform_edges || (TileX(tile) != 0 && TileY(tile) != 0))) {
+	if (IsInnerTile(tile)) {
 		MakeClear(tile, CLEAR_GRASS, 3);
 	}
 }
diff --git a/src/tile_map.cpp b/src/tile_map.cpp
--- a/src/tile_map.cpp
+++ b/src/tile_map.cpp
@@ -22,11 +22,7 @@ Slope GetTileSlope(TileIndex tile, int *
 {
 	assert(tile < MapSize());
 
-	uint x = TileX(tile);
-	uint y = TileY(tile);
-
-	if (x == MapMaxX() || y == MapMaxY() ||
-			((x == 0 || y == 0) && _settings_game.construction.freeform_edges)) {
+	if (!IsInnerTile(tile)) {
 		if (h != NULL) *h = TileHeight(tile);
 		return SLOPE_FLAT;
 	}
diff --git a/src/tile_map.h b/src/tile_map.h
--- a/src/tile_map.h
+++ b/src/tile_map.h
@@ -78,6 +78,23 @@ static inline TileType GetTileType(TileI
 }
 
 /**
+ * Check if a tile is within the map (not a border)
+ *
+ * @param tile The tile to check
+ * @return Whether the tile is in the interior of the map
+ * @pre tile < MapSize()
+ */
+static inline bool IsInnerTile(TileIndex tile)
+{
+	assert(tile < MapSize());
+
+	uint x = TileX(tile);
+	uint y = TileY(tile);
+
+	return x < MapMaxX() && y < MapMaxY() && ((x > 0 && y > 0) || !_settings_game.construction.freeform_edges);
+}
+
+/**
  * Set the type of a tile
  *
  * This functions sets the type of a tile. If the type
@@ -95,7 +112,7 @@ static inline void SetTileType(TileIndex
 	/* VOID tiles (and no others) are exactly allowed at the lower left and right
 	 * edges of the map. If _settings_game.construction.freeform_edges is true,
 	 * the upper edges of the map are also VOID tiles. */
-	assert((TileX(tile) == MapMaxX() || TileY(tile) == MapMaxY() || (_settings_game.construction.freeform_edges && (TileX(tile) == 0 || TileY(tile) == 0))) == (type == MP_VOID));
+	assert(IsInnerTile(tile) == (type != MP_VOID));
 	SB(_m[tile].type_height, 4, 4, type);
 }