Are you using the 1-per-second recording setting? The older 705s could run out of memory, but the newer versions don't seem to have that problem. I always leave the 1-per-second turned on.

Garmins can calculate accurate elevations when you pick "Average" when saving a location. It usually takes at least 30 seconds for the displayed elevation to settle down. I save the location this way for commonly used starting points. Then the start elevation will be accurate. Otherwise, it's an estimate based on the air pressure only.

In general, my 705 seems to be correct within 50 feet or so. On a Blue Ridge Parkway ride last fall, I would often be within 20 feet of the elevation posted at the overlooks.

I save my rides on the free "My Tourbook". It has an option to correct the elevation data. I always keep the actual data instead of correcting it.

Here's the 705 recorded elevations, with a red line overlay of the SRTM satellite survey elevation data. It's pretty accurate for most of the ride. Each horizontal line is 50 feet. The red line actually shows more short, steep sections that don't actually exist on the Parkway. Since the road is along the side of a steep mountain, even a small shift in location off the road would make a big change in the elevation.

Even the fast downhills, with the extra wind pressure, are accurate.



Zoomed: