Hello Forum,
I am looking for the method CR23X and CR1000 loggers use to compute a daily average for the wind direction std collected at a finer resolution (ie. 5min or hourly). Is it something other than a daily average?
Below are the relevant program parts for the CR23x logger.
;MEASURE WIND SPEED
21: Pulse (P3)
1: 1 Reps
2: 1 Pulse Channel 1
3: 21 Low Level AC, Output Hz
4: 21 Loc [ WINDSPEED ]
5: 0.098 Mult
6: 0 Offset
;MEASURE WIND DIRECTION
22: Excite-Delay (SE) (P4)
1: 1 Reps
2: 15 5000 mV, Fast Range
3: 2 SE Channel
4: 4 Excite all reps w/Exchan 4
5: 10 Delay (units 0.01 sec)
6: 5000 mV Excitation
7: 24 Loc [ WIND_DIR ]
8: 0.071 Mult
9: 0 Offset
84: If time is (P92)
1: 0 Minutes (Seconds --) into a
2: 1440 Interval (same units as above)
3: 10 Set Output Flag High
101: Wind Vector (P69)^20591
1: 1 Reps
2: 0 Samples per Sub-Interval
3: 2 S, U, éu, å(éu) Polar
4: 21 Wind Speed/East Loc [ WINDSPEED ]
5: 24 Wind Direction/North Loc [ WIND_DIR ]
What do you mean by "wind direction std". If you mean wind speed standard deviation then you can use code similar to that posted with the relevant setting to output standard deviation.
However, I should warn you that the calculation of standard deviation for wind direction is not really standard statistics partly because of the issue of the wind veering through North (360 to 0 or 0 to 360). There are also issues with interpreting the meaning of standard deviations over long periods.
There is a detailed discussion about these issues, along with some references, in the datalogger manual which describes the two alternative calculations of standard deviation for direction. There is also some detail of the "samples per sub-interval" parameter in the instruction which some users prefer to use to get a better estimate of short term variations in wind speed, when calculated over longer periods.
Thanks for this reply. I found the section of manual that deals with the wind dir std as you noted in your reply. My initial observation was the difference between the daily wind dir std value output by the logger and the average of the 5min wind dir std as computed in our software (daily averages of the 5min wind dir std).