Merge pull request #124 from heathhenley/north_south_fix

Rebase and tweaks to "Fix forcing zones around equator and add force_northern in from_latlon"

test/test_utm.py

@@ -381,6 +381,12 @@ def test_force_zone(lat, lon, utm, utm_kw, expected_number, expected_letter):
     assert result[3].upper() == expected_letter.upper()
 
 
+def assert_equal_lat(result, expected_lat, northern=None):
+    args = result[:3] if northern else result[:4]
+    lat, _ = UTM.to_latlon(*args, northern=northern, strict=False)
+    assert lat == pytest.approx(expected_lat, abs=0.001)
+
+
 def assert_equal_lon(result, expected_lon):
     _, lon = UTM.to_latlon(*result[:4], strict=False)
     assert lon == pytest.approx(expected_lon, abs=0.001)
@@ -396,6 +402,67 @@ def test_force_west():
     assert_equal_lon(UTM.from_latlon(0, -179.9, 60, "N"), -179.9)
 
 
+def test_force_north():
+    # Force southern point to northern zone letter
+    assert_equal_lat(UTM.from_latlon(-0.1, 0, 31, 'N'), -0.1)
+
+    # Again, using force northern
+    assert_equal_lat(
+        UTM.from_latlon(-0.1, 0, 31, force_northern=True), -0.1, northern=True)
+
+
+def test_force_south():
+    # Force northern point to southern zone letter
+    assert_equal_lat(UTM.from_latlon(0.1, 0, 31, 'M'), 0.1)
+
+    # Again, using force northern as False
+    assert_equal_lat(
+        UTM.from_latlon(0.1, 0, 31, force_northern=True), 0.1, northern=True)
+
+
+@pytest.mark.skipif(not use_numpy, reason="numpy not installed")
+def test_no_force_numpy():
+    # Point above and below equator
+    lats = np.array([-0.1, 0.1])
+    with pytest.raises(ValueError,
+                       match="latitudes must all have the same sign"):
+      UTM.from_latlon(lats, np.array([0, 0]))
+
+
+@pytest.mark.skipif(not use_numpy, reason="numpy not installed")
+@pytest.mark.parametrize("zone", ('N', 'M'))
+def test_force_numpy(zone):
+    # Point above and below equator
+    lats = np.array([-0.1, 0.1])
+
+    result = UTM.from_latlon(
+        lats, np.array([0, 0]), force_zone_letter=zone)
+    for expected_lat, easting, northing in zip(lats, *result[:2]):
+        assert_equal_lat(
+            (easting, northing, result[2], result[3]), expected_lat)
+
+
+@pytest.mark.skipif(not use_numpy, reason="numpy not installed")
+@pytest.mark.parametrize("force_northern", (True, False))
+def test_force_numpy_force_northern_true(force_northern):
+    # Point above and below equator
+    lats = np.array([-0.1, 0.1])
+
+    result = UTM.from_latlon(
+        lats, np.array([0, 0]), force_northern=force_northern)
+    for expected_lat, easting, northing in zip(lats, *result[:2]):
+        assert_equal_lat(
+            (easting, northing, result[2], result[3]), expected_lat,
+            northern=force_northern)
+
+
+def test_force_both():
+    # Force both letter and northern not allowed
+    with pytest.raises(ValueError, match="set either force_zone_letter or "
+                                         "force_northern, but not both"):
+        UTM.from_latlon(-0.1, 0, 31, 'N', True)
+
+
 def test_version():
     assert isinstance(UTM.__version__, str) and "." in UTM.__version__
 

utm/conversion.py

@@ -66,12 +66,6 @@ def mixed_signs(x):
     return use_numpy and mathlib.min(x) < 0 and mathlib.max(x) >= 0
 
 
-def negative(x):
-    if use_numpy:
-        return mathlib.max(x) < 0
-    return x < 0
-
-
 def mod_angle(value):
     """Returns angle in radians to be between -pi and pi"""
     return (value + mathlib.pi) % (2 * mathlib.pi) - mathlib.pi
@@ -97,7 +91,8 @@ def to_latlon(easting, northing, zone_number, zone_letter=None, northern=None, s
             designators can be seen in [1]_
 
         northern: bool
-            You can set True or False to set this parameter. Default is None
+            You can set True (North) or False (South) as an alternative to
+            providing a zone letter. Default is None
 
         strict: bool
             Raise an OutOfRangeError if outside of bounds
@@ -116,7 +111,6 @@ def to_latlon(easting, northing, zone_number, zone_letter=None, northern=None, s
     """
     if not zone_letter and northern is None:
         raise ValueError('either zone_letter or northern needs to be set')
-
     elif zone_letter and northern is not None:
         raise ValueError('set either zone_letter or northern, but not both')
 
@@ -184,7 +178,7 @@ def to_latlon(easting, northing, zone_number, zone_letter=None, northern=None, s
             mathlib.degrees(longitude))
 
 
-def from_latlon(latitude, longitude, force_zone_number=None, force_zone_letter=None):
+def from_latlon(latitude, longitude, force_zone_number=None, force_zone_letter=None, force_northern=None):
     """This function converts Latitude and Longitude to UTM coordinate
 
         Parameters
@@ -204,6 +198,11 @@ def from_latlon(latitude, longitude, force_zone_number=None, force_zone_letter=N
             You may force conversion to be included within one UTM zone
             letter.  For more information see utmzones [1]_
 
+        force_northern: bool
+            You can set True (North) or False (South) as an alternative to
+            forcing with a zone letter. When set, the returned zone_letter will
+            be None. Default is None
+
         Returns
         -------
         easting: float or NumPy array
@@ -227,6 +226,8 @@ def from_latlon(latitude, longitude, force_zone_number=None, force_zone_letter=N
         raise OutOfRangeError('latitude out of range (must be between 80 deg S and 84 deg N)')
     if not in_bounds(longitude, -180, 180):
         raise OutOfRangeError('longitude out of range (must be between 180 deg W and 180 deg E)')
+    if force_zone_letter and force_northern is not None:
+        raise ValueError('set either force_zone_letter or force_northern, but not both')
     if force_zone_number is not None:
         check_valid_zone(force_zone_number, force_zone_letter)
 
@@ -243,11 +244,16 @@ def from_latlon(latitude, longitude, force_zone_number=None, force_zone_letter=N
     else:
         zone_number = force_zone_number
 
-    if force_zone_letter is None:
+    if force_zone_letter is None and force_northern is None:
         zone_letter = latitude_to_zone_letter(latitude)
     else:
         zone_letter = force_zone_letter
 
+    if force_northern is None:
+        northern = (zone_letter >= 'N')
+    else:
+        northern = force_northern
+
     lon_rad = mathlib.radians(longitude)
     central_lon = zone_number_to_central_longitude(zone_number)
     central_lon_rad = mathlib.radians(central_lon)
@@ -274,10 +280,10 @@ def from_latlon(latitude, longitude, force_zone_number=None, force_zone_letter=N
     northing = K0 * (m + n * lat_tan * (a2 / 2 +
                                         a4 / 24 * (5 - lat_tan2 + 9 * c + 4 * c**2) +
                                         a6 / 720 * (61 - 58 * lat_tan2 + lat_tan4 + 600 * c - 330 * E_P2)))
-
-    if mixed_signs(latitude):
+    check_signs = force_northern is None and force_zone_letter is None
+    if check_signs and mixed_signs(latitude):
         raise ValueError("latitudes must all have the same sign")
-    elif negative(latitude):
+    elif not northern:
         northing += 10000000
 
     return easting, northing, zone_number, zone_letter