diff --git a/skais/ais/ais_trajectory.py b/skais/ais/ais_trajectory.py
index db3e64eccdc17b248544f47c3d3174c29bd7cdb0..de9b6b0532c830b4dde010d4f96707f912a128a9 100644
--- a/skais/ais/ais_trajectory.py
+++ b/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):