Index: sodar/trunk/sodar/data.py
===================================================================
--- sodar/trunk/sodar/data.py (revision 66)
+++ sodar/trunk/sodar/data.py (revision 67)
@@ -111,5 +111,5 @@
         """Clean up data for analysis."""
         self._copy()
-        # self._convert()
+        self._convert()
         # compute time interval
         # correct for missing times
@@ -127,22 +127,18 @@
     
     def _copy(self):
-        """Create a deep copy of all the samples in this Data instance."""
-        self.samples = [(dict(sample.header), list(sample.body))
-                        for sample in self]
-        for sample in self.samples:
-            for altitude in sample[1]:
-                altitude = dict(altitude)
+        """Create a deep copy as a list of Sample copies."""
+        self.samples = [sample._copy() for sample in self]
     
     def _convert(self):
         """Convert to numbers and correct for invalid values."""
         INVALID = "-9999"
-        # convert to numbers and correct for invalid values
         for sample in self.samples:
-            for altitude in sample[1]:
-                for key in altitude.keys():
+            body = sample[1]
+            for altitude in body:
+                for key,value in altitude.items():
                     try:
-                        if altitude[key] == INVALID:
+                        if value == INVALID:
                             raise ValueError
-                        altitude[key] = float(altitude[key])
+                        altitude[key] = float(value)
                     except (ValueError, TypeError, KeyError):
                         altitude[key] = n.NaN
@@ -171,5 +167,5 @@
         if self.body is not None:
             self.body = Body(self.body)
-
+    
     def __getitem__(self, index):
         """Index Sample by body attribute."""
@@ -178,4 +174,8 @@
         except TypeError:   # sample.body may not exist
             raise IndexError('Sample index out of range')
+    
+    def _copy(self):
+        """Create a deep copy as a tuple of header and body copies."""
+        return (self.header._copy(), self.body._copy())
 
 
@@ -202,5 +202,9 @@
         self.update(dict(zip(" ".join(headerLines[::2]).split(),
                              " ".join(headerLines[1::2]).split())))
-        
+    
+    def _copy(self):
+        """Create a shallow copy as a dictionary."""
+        return self.copy()
+
 
 class Body(list):
@@ -239,8 +243,15 @@
     def _find(self, index):
         """Find altitude data in Body."""
-        for altitudeData in self:
-            if altitudeData['ALT'] != index: continue
-            return altitudeData
+        for altitude in self:
+            try:
+                if altitude['ALT'] != index: continue
+            except KeyError:
+                continue
+            return altitude
         raise IndexError('Body index, out of range')
+
+    def _copy(self):
+        """Create a deep copy as a list of dictionaries."""
+        return [altitude.copy() for altitude in self]