Vance,
The headings normally shown on any aviation GPS are magnetic headings. In aviation, we use angular relationship to magnetic north, stated in degrees, for almost everything -- headings, orientation of runways, orientation of navaids. Instructions from controllers are always in magnetic: "Turn left 320 degrees, intercept the localizer."
The north-south orientation of the aeronautical chart is based on true north, which is why the 0º/360º radial of a VOR on the chart doesn't align with the north-south grid lines. (Technically, the gridlines are oriented neither to magnetic nor true north, but to a third measure, "grid north." We can ignore that distinction). There is only one other exception that I can think off off the top of my head (I am on the road and away from most aviation references). The exception is that wind direction in written forecasts and reports (such as METARS) is given in degrees from true north.
Unlike these written publications, SPOKEN forecasts and reports are usually in magnetic. (They're supposed to always be in magnetic, but controllers get a bit loose and sometimes just read you the METAR).
From NOAA (
here):
WIND and TEMPERATURE ALOFT FORECASTS (FD) are 6, 12, and 24-hour forecasts of wind direction, speed, and temperatures for selected altitudes to 53,000 feet MSL at specified locations. Direction is relative to true north rounded to the nearest 10 degrees. Speed is in knots. Temperatures aloft (in degrees Celsius) are included with wind data for all but the 3000-foot MSL level and those levels within 2500 feet of the ground. Temperatures above 24,000 feet MSL are always negative. Winds at other locations and altitudes can be obtained by interpolation.
Note that while Garmin GPSes like the GPS III Pilot can be set to read true heading, the default or factory setting is to read headings in magnetic degrees. That's logical, because these are the headings that controllers will understand you to be on.
Finally, unlike true headings, magnetic headings
change -- not from minute to minute, but as the years go by. This is because Earth's magnetic field is in motion. This motion is fairly stable and highly predictable (if you have a USGS (USA) or Ordnance Survey (UK) map, it has a small grid-magnetic angle diagram on it with a date, and information about how the angle changes from year to year. Why don't aviation charts have that? There are two reasons. First, aviation charts don't last for years, they are quickly replaced (every 56 days). Second, aviation charts cover a much larger area than a hiker's map (because aircraft cover more ground than hikers!) This means that the isogonic lines might be moving in one direction at one point on a sectional chart, but in another direction entirely a few hundred miles away (at abnother point on that same chart).
Here is a page explaining some of the entanglements in GPS and headings... it also has a very cool time-lapse video of the isogonic lines in motion over the years!
http://www.cockpitgps.com/VOR_magnetic_variation/vor_out_of_alignment_gps_and_obs.htm
I am afraid I may have made a jumble of this explanation. Vance, is it clear enough for you?
They key fact is this: unless you manually changed your GPS to display headings in degrees from true north, they will be displayed in degrees from magnetic north. This change is not something that is likely to happen by accident. So your GPS is almost certainly using degrees magnetic.
You can check it by finding a place on the sectional where a line (a power line, say) oriented to true north intersects a line oriented to magnetic north. Fly there and see which one aligns with north on your GPS... I bet it'll be magnetic north.
cheers
-=K=-