diff --git a/quadkey/__init__.py b/quadkey/__init__.py
index 4c41fc0..abe4fb5 100644
--- a/quadkey/__init__.py
+++ b/quadkey/__init__.py
@@ -17,7 +17,6 @@ def __repr__(self):
 		return self.key
 
 	@classmethod
-	@precondition(TileSystem.valid_level)
 	def from_str(cls, key):
 		level = len(key)
 		lat, lon = QuadKey.get_coordinates(key)
diff --git a/quadkey/tile_system.py b/quadkey/tile_system.py
index 5048b40..4d7719b 100644
--- a/quadkey/tile_system.py
+++ b/quadkey/tile_system.py
@@ -1,5 +1,9 @@
 from util import precondition
-from math import cos
+from math import cos, pi
+
+def valid_level(level):
+	LEVEL_RANGE = (1,23)
+	return LEVEL_RANGE[0] <= level <= LEVEL_RANGE[1]
 
 class TileSystem:
 	"""
@@ -11,9 +15,6 @@ class TileSystem:
 	EARTH_RADIUS = 6378137
 	LATITUDE_RANGE = (-85.05112878, 85.05112878)
 	LONGITUDE_RANGE = (-180., 180.)
-	LEVEL_RANGE = (1,23)
-
-	valid_level = lambda level: TileSystem.LEVEL_RANGE[0] <= level <= TileSystem.LEVEL_RANGE[1]
 
 	@staticmethod
 	@precondition(lambda n, minMax: minMax[0] <= minMax[1])
@@ -28,14 +29,14 @@ def map_size(level):
 		return 256 << level
 
 	@staticmethod
-	@precondition(valid_level(*args[1]))
-	def ground_resolution(float lat, int level):
+	@precondition(lambda lat, lvl: valid_level(lvl))
+	def ground_resolution(lat, level):
 		"""Gets ground res in meters / pixel"""
-		lat = TileSystem.clip(lat, TileSytem.LATITUDE_RANGE)
-		return cos(lat * pi / 180) * 2 * pi * EARTH_RADIUS / TileSystem.map_size(level)
+		lat = TileSystem.clip(lat, TileSystem.LATITUDE_RANGE)
+		return cos(lat * pi / 180) * 2 * pi * TileSystem.EARTH_RADIUS / TileSystem.map_size(level)
 
 	@staticmethod
-	@precondition(valid_level(*args[1]))
+	@precondition(lambda lat, lvl, dpi: valid_level(lvl))
 	def map_scale(lat, level, dpi):
 		"""Gets the scale of the map expressed as ratio 1	: N. Returns N"""
 		return TileSystem.ground_resolution(lat, level) * dpi / 0.0254
diff --git a/quadkey/util.py b/quadkey/util.py
index 21b0a36..5d09271 100644
--- a/quadkey/util.py
+++ b/quadkey/util.py
@@ -2,7 +2,7 @@
 
 def condition(precondition=None, postcondition=None):
     def decorator(func):
-        @functools.wraps(func) # presever name, docstring, etc
+        @functools.wraps(func) # preserve name, docstring, etc
         def wrapper(*args, **kwargs): #NOTE: no self
             if precondition is not None:
                assert precondition(*args, **kwargs)
diff --git a/tests/tile_system.py b/tests/tile_system.py
index 7a0a155..fd02aa1 100644
--- a/tests/tile_system.py
+++ b/tests/tile_system.py
@@ -17,14 +17,26 @@ def testMapSize(self):
 		with self.assertRaises(AssertionError):
 			TileSystem.map_size(0)
 
-	def testFromString(self):
-		pass
+	def testGroundResolution(self):
+		TileSystem.ground_resolution(45, 7)
 
-	def testToString(self):
-		pass
+	def testMapScale(self):
+		TileSystem.map_scale(45, 7, 45)
 
-	def testGetQuadKey(self):
-		pass
+	def testGeoToPixel(self):
+		TileSystem.geo_to_pixel(geo, level)
 
-	def testGetCoordinates(self):
-		pass
+	def testPixelToGeo(self):
+		TileSystem.pixel_to_geo(pixel, level)
+
+	def testPixelToTile(self):
+		TileSystem.pixel_to_tile(pixel)
+
+	def testTileToPixel(self):
+		TileSystem.tile_to_pixel(tile)
+
+	def testTileToQuadkey(self):
+		TileSystem.tile_to_quadkey(tile, level)
+
+	def testQuadkeyToTile(self):
+		TileSystem.quadkey_to_tile(quadkey)