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--- cs_lang:python:things-to-know:geographic [2012/06/20 11:16] – created cedric
+++ cs_lang:python:things-to-know:geographic [2012/06/20 11:21] (current) – cedric
@@ Line 1: / Line 1: @@
 <code python>
+def Deg2Rad(x):
+    """
+    Degrees to radians.
+    """
+    return x * (math.pi/180)
+def Rad2Deg(x):
+    """
+    Radians to degress.
+    """
+    return x * (180/math.pi)
+def CalcRad(lat):
+    """
+    Radius of curvature in meters at specified latitude.
+    """
+    a = 6378.137
+    e2 = 0.081082 * 0.081082
+    # the radius of curvature of an ellipsoidal Earth in the plane of a
+    # meridian of latitude is given by
+    #
+    # R' = a * (1 - e^2) / (1 - e^2 * (sin(lat))^2)^(3/2)
+    #
+    # where a is the equatorial radius,
+    # b is the polar radius, and
+    # e is the eccentricity of the ellipsoid = sqrt(1 - b^2/a^2)
+    #
+    # a = 6378 km (3963 mi) Equatorial radius (surface to center distance)
+    # b = 6356.752 km (3950 mi) Polar radius (surface to center distance)
+    # e = 0.081082 Eccentricity
+    sc = math.sin(Deg2Rad(lat))
+    x = a * (1.0 - e2)
+    z = 1.0 - e2 * sc * sc
+    y = pow(z, 1.5)
+    r = x / y
+    r = r * 1000.0      # Convert to meters
+    return r
 def EarthDistance((lat1, lon1), (lat2, lon2)):
     """
@@ Line 20: / Line 60: @@
         a = -1
     return CalcRad((lat1+lat2) / 2) * math.acos(a)
+def MeterOffset((lat1, lon1), (lat2, lon2)):
+    """
+    Return offset in meters of second arg from first.
+    """
+    dx = EarthDistance((lat1, lon1), (lat1, lon2))
+    dy = EarthDistance((lat1, lon1), (lat2, lon1))
+    if lat1 < lat2:
+        dy *= -1
+    if lon1 < lon2:
+        dx *= -1
+    return (dx, dy)
 </code>