Resolve "Image creation bugs with 0 size windows"

Closes #21 (closed)

skais/ais/ais_trajectory.py

@@ -77,8 +77,8 @@ class AISTrajectory(AISPoints):
                                               kind='nearest', axis=0)(t_interp1d).astype(int)
 
             df = new_df
-
-        # self.df = df.dropna()
+        if 'sog' in df.columns:
+            df.loc[df['sog'] < 0, 'sog'] = 0
         AISPoints.__init__(self, df)
 
     def sliding_window(self, size=10, offset=1, fields=None):