diff --git a/changelog.txt b/changelog.txt
--- a/changelog.txt
+++ b/changelog.txt
@@ -1,3 +1,6 @@
+0.4.0
+- Switch quat2dcm mode to avoid divide by 0, add Ord constraint
+
 0.2.0
 - convert to using `linear` V3, M33, Quaternion types
 - doctests
diff --git a/spatial-math.cabal b/spatial-math.cabal
--- a/spatial-math.cabal
+++ b/spatial-math.cabal
@@ -1,5 +1,5 @@
 name:                spatial-math
-version:             0.3.1.0
+version:             0.4.0.0
 synopsis:            3d math including quaternions/euler angles/dcms and utility functions
 description:         This is a port of my 'mathlib' C library: `https://github.com/ghorn/mathlib`
 license:             BSD3
diff --git a/src/SpatialMath.hs b/src/SpatialMath.hs
--- a/src/SpatialMath.hs
+++ b/src/SpatialMath.hs
@@ -174,28 +174,75 @@
 -- | convert a DCM to a quaternion
 --
 -- >>> quatOfDcm $ V3 (V3 1 0 0) (V3 0 1 0) (V3 0 0 1)
--- Quaternion 1.0 (V3 (-0.0) (-0.0) (-0.0))
+-- Quaternion 1.0 (V3 0.0 0.0 0.0)
 --
 -- >>> quatOfDcm $ V3 (V3 0 1 0) (V3 (-1) 0 0) (V3 0 0 1)
--- Quaternion 0.7071067811865477 (V3 (-0.0) (-0.0) 0.7071067811865474)
+-- Quaternion 0.7071067811865476 (V3 0.0 0.0 0.7071067811865475)
 --
 -- >>> let s = sqrt(2)/2 in quatOfDcm $ V3 (V3 s s 0) (V3 (-s) s 0) (V3 0 0 1)
--- Quaternion 0.9238795325112868 (V3 (-0.0) (-0.0) 0.3826834323650898)
-quatOfDcm :: Floating a => M33 a -> Quaternion a
+-- Quaternion 0.9238795325112867 (V3 0.0 0.0 0.3826834323650898)
+quatOfDcm :: (Floating a, Ord a) => M33 a -> Quaternion a
 quatOfDcm
   (V3
-   (V3 r11 r12 r13)
-   (V3 r21 r22 r23)
-   (V3 r31 r32 r33)) = Quaternion q0 (V3 qi qj qk)
-  where
-    q0 = 0.5 * sqrt (1e-15 + (1 + r11 + r22 + r33))
-    qi = negate (r32 - r23) / fourQ0
-    qj = negate (r13 - r31) / fourQ0
-    qk = negate (r21 - r12) / fourQ0
-    fourQ0 = 4 * q0
+    (V3 r11 r12 r13)
+    (V3 r21 r22 r23)
+    (V3 r31 r32 r33)
+  )
+  | r11 + r22 + r33 > 0 =
+      let sqtrp1 = sqrt (r11 + r22 + r33 + 1)
+          q0 = 0.5*sqtrp1
+          qx = (r23 - r32)/(2.0*sqtrp1)
+          qy = (r31 - r13)/(2.0*sqtrp1)
+          qz = (r12 - r21)/(2.0*sqtrp1)
+      in Quaternion q0 (V3 qx qy qz)
+  | (r22 > r11) && (r22 > r33) =
+      let -- max value at r22
+          sqdip1' = sqrt (r22 - r11 - r33 + 1)
 
+          qy = 0.5*sqdip1'
 
-quatOfDcmB2A :: Floating a => M33 a -> Quaternion a
+          sqdip1
+            | sqdip1' == 0 = 0
+            | otherwise = 0.5/sqdip1'
+
+          q0 = (r31 - r13)*sqdip1
+          qx = (r12 + r21)*sqdip1
+          qz = (r23 + r32)*sqdip1
+
+      in Quaternion q0 (V3 qx qy qz)
+  | r33 > r11 =
+      let -- max value at r33
+          sqdip1' = sqrt (r33 - r11 - r22 + 1)
+
+          qz = 0.5*sqdip1'
+
+          sqdip1
+            | sqdip1' == 0 = 0
+            | otherwise = 0.5/sqdip1'
+
+          q0 = (r12 - r21)*sqdip1
+          qx = (r31 + r13)*sqdip1
+          qy = (r23 + r32)*sqdip1
+
+      in Quaternion q0 (V3 qx qy qz)
+  | otherwise =
+      let -- max value at r11
+          sqdip1' = sqrt (r11 - r22 - r33 + 1)
+
+          qx = 0.5*sqdip1'
+
+          sqdip1
+            | sqdip1' == 0 = 0
+            | otherwise = 0.5/sqdip1'
+
+          q0 = (r23 - r32)*sqdip1
+          qy = (r12 + r21)*sqdip1
+          qz = (r31 + r13)*sqdip1
+
+      in Quaternion q0 (V3 qx qy qz)
+
+
+quatOfDcmB2A :: (Floating a, Ord a) => M33 a -> Quaternion a
 quatOfDcmB2A = quatConjugate . quatOfDcm
 
 -- | Convert DCM to euler angles
diff --git a/src/SpatialMathT.hs b/src/SpatialMathT.hs
--- a/src/SpatialMathT.hs
+++ b/src/SpatialMathT.hs
@@ -134,7 +134,7 @@
 dcmOfEuler321 = Rot . O . SM.dcmOfEuler321 . unRot
 
 
-quatOfDcm :: Floating a => Rot f g (V3 :. V3) a -> Rot f g Quaternion a
+quatOfDcm :: (Floating a, Ord a) => Rot f g (V3 :. V3) a -> Rot f g Quaternion a
 quatOfDcm = Rot . SM.quatOfDcm . unO . unRot
 
 quatOfEuler321 :: Floating a => Rot f g Euler a -> Rot f g Quaternion a
diff --git a/tests/Tests.hs b/tests/Tests.hs
--- a/tests/Tests.hs
+++ b/tests/Tests.hs
@@ -22,16 +22,32 @@
 main :: IO ()
 main = defaultMainWithOpts tests opts
 
-close :: forall f . (F.Foldable f, Applicative f) => Double -> f Double -> f Double -> Maybe Double
-close eps f0 f1
+closeEuler :: Double -> Euler Double -> Euler Double -> Maybe Double
+closeEuler eps f0 f1
   | all (\x -> abs x <= eps) deltas = Nothing
   | otherwise = Just $ maximum $ map abs deltas
   where
-    delta :: f Double
+    delta :: Euler Double
     delta = (-) <$> f0 <*> f1
 
     deltas = F.toList delta
 
+closeQuat :: Double -> Quaternion Double -> Quaternion Double -> Maybe Double
+closeQuat eps f0 f1
+  | worstDelta <= eps = Nothing
+  | otherwise = Just worstDelta
+  where
+    deltas0 :: Quaternion Double
+    deltas0 = (-) <$> f0 <*> f1
+
+    deltas1 :: Quaternion Double
+    deltas1 = (-) <$> f0 <*> (negate <$> f1)
+
+    worstDelta =
+      min
+      (maximum (map abs (F.toList deltas0)))
+      (maximum (map abs (F.toList deltas1)))
+
 closeDcm :: Double -> M33 Double -> M33 Double -> Maybe Double
 closeDcm eps f0 f1
   | all (\x -> abs x <= eps) deltas = Nothing
@@ -80,73 +96,61 @@
 instance Arbitrary (V3 (V3 Double)) where
   arbitrary = dcmOfEuler321 <$> arbitrary
 
-testConversion :: (F.Foldable f, Applicative f, Show (f Double))
-                  => Double -> (f Double -> f Double) -> f Double
+testConversion :: (Show a, Show b)
+                  => (b -> b -> Maybe Double)
+                  -> (a -> b) -> (a -> b) -> a
                   -> Property
-testConversion eps f x0 = counterexample msg ret
+testConversion toErr f0 f1 x = counterexample msg ret
   where
-    (ret, errmsg) = case close eps x0 x1 of
+    y0 = f0 x
+    y1 = f1 x
+    (ret, errmsg) = case toErr y0 y1 of
       Nothing -> (True, [])
       Just worstErr -> (False, [printf "worst error: %.3g" worstErr])
     msg = init $ unlines $
-          [ "original:  " ++ show x0
-          , "converted: " ++ show x1
+          [ "original:  " ++ show x
+          , "first route:  " ++ show y0
+          , "second route: " ++ show y1
           ] ++ errmsg
-    x1 = f x0
 
+-- inverses
 prop_e2q2e :: Euler Double -> Property
-prop_e2q2e = testConversion 1e-9 (euler321OfQuat . quatOfEuler321)
+prop_e2q2e = testConversion (closeEuler 1e-9) id (euler321OfQuat . quatOfEuler321)
 
 prop_e2d2e :: Euler Double -> Property
-prop_e2d2e = testConversion 1e-9 (euler321OfDcm . dcmOfEuler321)
+prop_e2d2e = testConversion (closeEuler 1e-9) id (euler321OfDcm . dcmOfEuler321)
 
-testDoubleConversion :: (Show f, Show g) => f -> g -> g -> Maybe Double -> Property
-testDoubleConversion orig res0 res1 err = counterexample msg ret
-  where
-    (ret, errmsg) = case err of
-      Nothing -> (True, [])
-      Just worstErr -> (False, [printf "worst error: %.3g" worstErr])
-    msg = init $ unlines $
-          [ "original: " ++ show orig
-          , "first route:  " ++ show res0
-          , "second route: " ++ show res1
-          ] ++ errmsg
+prop_d2e2d :: M33 Double -> Property
+prop_d2e2d = testConversion (closeDcm 1e-9) id (dcmOfEuler321 . euler321OfDcm)
 
+prop_d2q2d :: M33 Double -> Property
+prop_d2q2d = testConversion (closeDcm 1e-9) id (dcmOfQuat . quatOfDcm)
+
+prop_q2e2q :: Quaternion Double -> Property
+prop_q2e2q = testConversion (closeQuat 1e-9) id (quatOfEuler321 . euler321OfQuat)
+
+prop_q2d2q :: Quaternion Double -> Property
+prop_q2d2q = testConversion (closeQuat 1e-9) id (quatOfDcm . dcmOfQuat)
+
+-- two routes
 prop_e2d_e2q2d :: Euler Double -> Property
-prop_e2d_e2q2d euler = testDoubleConversion euler dcm0 dcm1 (closeDcm 1e-9 dcm0 dcm1)
-  where
-    dcm0 = dcmOfEuler321 euler
-    dcm1 = dcmOfQuat (quatOfEuler321 euler)
+prop_e2d_e2q2d = testConversion (closeDcm 1e-9) dcmOfEuler321 (dcmOfQuat . quatOfEuler321)
 
 prop_e2q_e2d2q :: Euler Double -> Property
-prop_e2q_e2d2q euler = testDoubleConversion euler quat0 quat1 (close 1e-9 quat0 quat1)
-  where
-    quat0 = makeScalarPositive (quatOfEuler321 euler)
-    quat1 = quatOfDcm (dcmOfEuler321 euler)
+prop_e2q_e2d2q =
+  testConversion (closeQuat 1e-9) (makeScalarPositive . quatOfEuler321) (quatOfDcm . dcmOfEuler321)
 
 prop_q2e_q2d2e :: Quaternion Double -> Property
-prop_q2e_q2d2e quat = testDoubleConversion quat euler0 euler1 (close 1e-9 euler0 euler1)
-  where
-    euler0 = euler321OfQuat quat
-    euler1 = euler321OfDcm (dcmOfQuat quat)
+prop_q2e_q2d2e = testConversion (closeEuler 1e-9) euler321OfQuat (euler321OfDcm . dcmOfQuat)
 
 prop_q2d_q2e2d :: Quaternion Double -> Property
-prop_q2d_q2e2d quat = testDoubleConversion quat dcm0 dcm1 (closeDcm 1e-9 dcm0 dcm1)
-  where
-    dcm0 = dcmOfQuat quat
-    dcm1 = dcmOfEuler321 (euler321OfQuat quat)
+prop_q2d_q2e2d = testConversion (closeDcm 1e-9) dcmOfQuat (dcmOfEuler321 . euler321OfQuat)
 
 prop_d2e_d2q2e :: M33 Double -> Property
-prop_d2e_d2q2e dcm = testDoubleConversion dcm euler0 euler1 (close 1e-7 euler0 euler1)
-  where
-    euler0 = euler321OfDcm dcm
-    euler1 = euler321OfQuat (quatOfDcm dcm)
+prop_d2e_d2q2e = testConversion (closeEuler 1e-7) euler321OfDcm (euler321OfQuat . quatOfDcm)
 
 prop_d2q_d2e2q :: M33 Double -> Property
-prop_d2q_d2e2q dcm = testDoubleConversion dcm quat0 quat1 (close 1e-5 quat0 quat1)
-  where
-    quat0 = quatOfDcm dcm
-    quat1 = makeScalarPositive (quatOfEuler321 (euler321OfDcm dcm))
+prop_d2q_d2e2q = testConversion (closeQuat 1e-5) quatOfDcm (makeScalarPositive . quatOfEuler321 . euler321OfDcm)
 
 makeScalarPositive :: Quaternion Double -> Quaternion Double
 makeScalarPositive quat0'@(Quaternion q0 _)
@@ -156,16 +160,20 @@
 tests :: [Test]
 tests =
   [ testGroup "inverses"
-    [ testProperty "(euler -> quat -> euler) == euler" prop_e2q2e
-    , testProperty "(euler -> dcm -> euler) == euler" prop_e2d2e
+    [ testProperty "euler == (euler -> quat  -> euler)" prop_e2q2e
+    , testProperty "euler == (euler -> dcm   -> euler)" prop_e2d2e
+    , testProperty "dcm   == (dcm   -> euler -> dcm  )" prop_d2e2d
+    , testProperty "dcm   == (dcm   -> quat  -> dcm  )" prop_d2q2d
+    , testProperty "quat  == (quat  -> euler -> quat )" prop_q2e2q
+    , testProperty "quat  == (quat  -> dcm   -> quat )" prop_q2d2q
     ]
   , testGroup "two routes"
-    [ testProperty "(euler -> dcm) == (euler -> quat -> dcm)" prop_e2d_e2q2d
-    , testProperty "(euler -> quat) == (euler -> dcm -> quat)" prop_e2q_e2d2q
-    , testProperty "(quat -> euler) == (quat -> dcm -> euler)" prop_q2e_q2d2e
-    , testProperty "(quat -> dcm) == (quat -> euler -> dcm)" prop_q2d_q2e2d
-    , testProperty "(dcm -> euler) == (dcm -> quat -> euler)" prop_d2e_d2q2e
-    , testProperty "(dcm -> quat) == (dcm -> euler -> quat)" prop_d2q_d2e2q
+    [ testProperty "(euler -> dcm  ) == (euler -> quat  -> dcm  )" prop_e2d_e2q2d
+    , testProperty "(euler -> quat ) == (euler -> dcm   -> quat )" prop_e2q_e2d2q
+    , testProperty "(quat  -> euler) == (quat  -> dcm   -> euler)" prop_q2e_q2d2e
+    , testProperty "(quat  -> dcm  ) == (quat  -> euler -> dcm  )" prop_q2d_q2e2d
+    , testProperty "(dcm   -> euler) == (dcm   -> quat  -> euler)" prop_d2e_d2q2e
+    , testProperty "(dcm   -> quat ) == (dcm   -> euler -> quat )" prop_d2q_d2e2q
     ]
   ]
 
@@ -181,4 +189,5 @@
 my_test_opts =
   Mo.mempty
   { topt_timeout = Just (Just 15000000)
+  , topt_maximum_generated_tests = Just 1000
   }
