# dh is 'reliable' (usually a gyro) minus 'accurate'
# reliable instrument, message = gyro,hdg
# accurate instrument, message = posmv,pmv
# dday      mean_hdg last_hdg mean_dh std_dh num badflag
330.8939104    29.73    29.80  -0.19  0.02    120     0
330.8942438    29.80    29.70  -0.22  0.03     28     0
334.6570274   113.95   113.00  -0.20  0.04    119     0
334.6584254   113.41   112.53  -0.20  0.04    120     0
334.6598143   113.34   114.15  -0.21  0.04    119     0
334.6612032   113.44   114.32  -0.20  0.04    119     0