On Fri, Mar 02, 2018 at 05:57:26PM +0100, Radek via Therion wrote: > Here's more accurate > > https://www.ngdc.noaa.gov/geomag-web/#declination
That uses the same model as Therion does, so I can't see why it should be more accurate (unless you're using a version of therion more than a year old, as before https://github.com/therion/therion/pull/52 Therion calculated the fraction of the year to use in a slightly odd way, though even then the difference is tiny if you're specifying a full date). Also, if you look up values by hand and specify them explicitly to therion you are hard coding the answers for the current version of the IGRF model. But each generation of the IGRF model is necessarily predictive for dates after it was issued and is only declared to be definitive for dates more than 5 years before it was issued. So each new generation of the model can change the declination figures reported for the last 10 years compared to the previous generation (and the new answers should be expected to be better). As Xavier suggests, the table of reported declinations in the log is for the start of each year, but declinations used for surveys are calculated for the actual survey dates. Maybe this needs making clearer somehow, as this isn't the first time this summary table has led someone to think that the declination was only being calculated for the start of each year: https://mailman.speleo.sk/pipermail/therion/2017-February/006241.html Cheers, Olly _______________________________________________ Therion mailing list [email protected] https://mailman.speleo.sk/listinfo/therion
