You can determine the Magnetic Declination of a location with a compass and a sunPATH‚Ñ¢ report for the location based on True North bearings. (Set Azimuth Bearings to True North in the Preferences under the Edit menu.)
Using your compass enter the azimuth (horizontal position) bearing of the sun into the first edit box. (The box near the Compass Icon.) This is the current sun position based on Magnetic North.
Look at your sunPATH‚Ñ¢ report for this location and date. The report must be calculated for True North. Find the Azimuth bearing from the report for the day and time that you took your compass reading above. Enter the Azimuth bearing into the 2nd edit box. The program will calculate the Magnetic Declination based on this information.
