I have a Garmin 17X GPS receiver, and am trying to determine how it calculates Magnetic Variation (table lookup, WMM, etc). More importantly, I need to know how accurate this value is. I'm at a Variation of 9.2 degrees, but the unit is reporting 8.9 degrees (receiver connected to laptop). I don't know if it is using an old WMM model, or if it is doing a crude lookup without considering the current date. I am considering buying a Furuno GP-330B and seeing what it displays. Any information on these two models would be useful. I have tech support cases open for both, but am not getting useful answers.
