diff --git a/Data/GPS.hs b/Data/GPS.hs
--- a/Data/GPS.hs
+++ b/Data/GPS.hs
@@ -67,9 +67,11 @@
 getUTCTime :: (Lat a, Lon a, Time a) => a -> Maybe UTCTime
 getUTCTime = fmap toUTCTime . time
 
-distance :: (Lat a, Lon a) => a -> a -> Distance
+distance :: (Lat a, Lon a, Lat b, Lon b) => a -> b -> Distance
 distance a b =
-	radiusOfEarth * acos( sin(lat1) * sin(lat2) + cos(lat1) * cos(lat2) * cos(lon2 - lon1) )
+	let x =  sin lat1 * sin lat2 + cos lat1 * cos lat2 * cos (lon2 - lon1)
+	    x' = if x > 1 then 1 else x
+	in radiusOfEarth * acos x'
  where
   (lat1, lon1) = getRadianPairD a
   (lat2, lon2) = getRadianPairD b
@@ -79,7 +81,7 @@
 totalDistance as = sum $ zipWith distance as (drop 1 as)
 
 -- | Direction two points aim toward (0 = North, pi/2 = West, pi = South, 3pi/2 = East)
-heading         :: (Lat a, Lon a) => a -> a -> Heading	-- ^ 0 = North, pi/2 = West...
+heading         :: (Lat a, Lon a, Lat b, Lon b) => a -> b -> Heading	-- ^ 0 = North, pi/2 = West...
 heading a b =
 	atan2	(sin (diffLon) * cos (lat2)) 
 		(cos(lat1) * sin (lat2) - sin(lat1) * cos lat2 * cos (diffLon))
@@ -88,11 +90,11 @@
   (lat2, lon2) = getRadianPairD b
   diffLon = lon1 - lon2
 
-getVector :: (Lat a, Lon a) => a -> a -> Vector
+getVector :: (Lat a, Lon a, Lat b, Lon b) => a -> b -> Vector
 getVector a b = (distance a b, heading a b)
 
 -- | Speed in meters per second, only if a 'Time' was recorded for each waypoint.
-speed :: (Lat loc, Lon loc, Time loc) => loc -> loc -> Maybe Speed
+speed :: (Lat loc, Lon loc, Time loc, Lat b, Lon b, Time b) => loc -> b -> Maybe Speed
 speed a b = 
 	case (getUTCTime b, getUTCTime a) of
 		(Just x, Just y) -> Just $ realToFrac (diffUTCTime x y) / (distance a b)
@@ -156,10 +158,7 @@
 
 -- |Provides a lat/lon pair of doubles in radians
 getRadianPair :: (Lat p, Lon p) => p -> (LatitudeType, LongitudeType)
-getRadianPair p = (t, g)
-  where
-    t = toRadians (lat p)
-    g = toRadians (lon p)
+getRadianPair p = (toRadians (lat p), toRadians (lon p))
 
 toRadians :: Floating f => f -> f
 toRadians = (*) (pi / 180)
diff --git a/gps.cabal b/gps.cabal
--- a/gps.cabal
+++ b/gps.cabal
@@ -1,5 +1,5 @@
 name:		gps
-version:	0.6
+version:	0.6.2
 license:	BSD3
 license-file:	LICENSE
 author:		Thomas DuBuisson <thomas.dubuisson@gmail.com>