I have a weather station that reports the wind direction and speed on a regular basis via publishing to MQTT topic that openHAB is subscribed to.
This works just fine, I have one channel for each value and items linked to them allow me to see the persisted values.
Getting the average speed over a given period of time is easily done with the Javascript persistence extension method averageBetween as described here.
And if I’m not mistaken, it’s already a time-weighted average which is fine by me as the speed does not change every time a report is sent by the weather station.
Now, I’d like to be able to get a mean wind direction over a given period of time, but I’m quite sure averageBetween does not compute a circular mean as it would need to know the stored values are angles.
And even if it did, it wouldn’t give me a mean weighted both by time and wind speed, as it’s quite important that the higher wind speed get the most impact on the wind direction mean over a given period of time.
Looking at the ItemHistory class, I see that I can get all recorded steps for a given period via the getAllStatesBetween method.
But this gives me the exact recorded elements, without any synchronization between two items.
Here is an example
| Time | Wind speed (km/h) | Wind direction (°) |
|---|---|---|
| 00:00 | 50 | |
| 00:01 | 12 | 10 |
| 00:10 | 21 | 8 |
| 00:12 | 2 | |
| 00:20 | 170 | |
| 00:30 | 25 | |
| 00:42 | 350 | |
| 01:01 | 1 |
Blanks in that table are when there is no data given in the returned HistoricItem array.
This is where I’m struggling a bit with how to reconstruct the history so that I always have the two values for any given time.
I thought about repeatedly calling historicState with regular time intervals in the past, but it’s quite slow and does not seem very effecient when I could get all values at once and compute elements along the way.
I looked here and could not find any similar discussion with a solution.
Any suggestion is most welcome.