# 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
76.8896835    62.61    63.80   0.12  0.03    120     0
76.8910724    64.46    63.83   0.10  0.03    119     0
76.8924613    62.81    65.44   0.10  0.04    119     0
76.8938501    64.01    62.00   0.12  0.03    119     0
76.8952390    61.36    61.00   0.17  0.03    119     0
76.8966280    62.76    65.50   0.19  0.03    119     0
76.8980168    66.49    73.55   0.17  0.03    119     0
76.8994056    74.03    74.65   0.12  0.03    119     0
76.9007945    79.38    88.70   0.09  0.02    119     0
76.9021834    99.18   107.50   0.06  0.03    119     0
76.9035722   112.66   117.00   0.03  0.03    119     0
76.9049611   120.32   122.04   0.05  0.03    119     0
76.9063499   119.30   114.04   0.05  0.03    101     0
76.9077388   106.51   100.50   0.06  0.03    119     0
76.9091277    98.83    97.36   0.08  0.03    108     0
76.9105165    95.69    93.44   0.05  0.02    118     0
76.9119054    91.63    90.70   0.08  0.03    118     0
76.9132943    91.83    92.50   0.11  0.02    118     0
76.9146831    93.78    95.40   0.11  0.02    118     0
76.9160720    96.35    96.80   0.10  0.03    118     0
76.9174609    95.48    94.50   0.13  0.03    117     0
76.9188497    95.26    95.80   0.14  0.03    118     0
76.9202386    95.52    95.30   0.14  0.03    118     0
76.9216275    96.43    97.60   0.15  0.03    118     0
76.9230163    95.29    92.10   0.17  0.03    118     0
76.9244052    88.98    85.90   0.15  0.03    118     0
76.9257941    83.45    81.39   0.16  0.03    118     0
76.9271829    80.55    80.70   0.17  0.03    118     0
76.9285718    81.64    81.90   0.17  0.03    118     0
76.9299606    82.27    85.19   0.16  0.03    118     0