3d speed and distance: advantages / disadvantages?

Yes, but a 2D distance will not be correct if there are elevation changes on the course.

AndersB said:

Yes, but a 2D distance will not be correct if there are elevation changes on the course.

Right, and in most cases that's why you'd want to use 3d speed/distance on the watch. But the use case for leaving it in 2d mode is if, for example you're through-hiking a very long trail and using a guidebook and milemarkers to see where you are, you'd want the distance on your watch to match the distances listed in the book and on the posted signs (which are all 2d distances), NOT the distance actually covered over ground (which is 3d).

Also, measured races such as marathons or ultra trail runs are typically measured using a map (2d), and you'd probably want your distances measured by the watch to match what the race is using, not the actual 3d ground distance covered.

That false, altimeter must be calibrated by barometric pressure

C.sco said:

(14 miles, your actual ground distance traveled), whereas in 2d mode it will measure only the 2d distance covered (10 miles, your distance traveled from a 2d "bird's eye" perspective).

This is absolutely correct.  I always get a chuckle when people don't use the feature and they're running in the hills with a lot of elevation changes and they complain that the distance is off.  GPS measures distance by line of sight from GPS points recorded.  A 100 meter straight line with GPS doesn't mean 100 meters travel if the start and end point has a depression in between. 

Hmm, it needs to be calibrated with GPS coordinates, DEM and an verified correct barometric pressure, 

It does if it's detected. If a 100 meter point A to B has slight rolling hills of 1-2 meters the barometer may not detect that minor change at the speed you're traveling. What may actually be 125 meters traveled may show as 100 meters.  This can add up the greater the distance traveled. 3D is supposed to record those minor changes. 

So here's a real world example so people can get an idea of the magnitude of difference between 2d and 3d and determine if it's meaningful for them.  A popular hike in Colorado is Mt. Elbert - the highest point in the state.  It's 4.45 miles from trailhead to summit, with 4410 feet (0.84 miles) of elevation gain - so fairly steep and not really any descending on the climb. 

Measured in 2d, we'd expect the watch to say you hiked 4.45 miles when you reached the summit. 

Measured in 3d, we'd expect the watch to say you travelled 4.53 miles when you reached the summit.  That's 0.08 miles difference, or 1.8%.  Your call if that's important to you or not.

Also, assuming you came back down the same way, the difference of course doubles to 0.16 miles, but still a 1.8% difference from 2d.

My take is yeah, you could see some benefit of 3D in mountainous situations.  The steeper the grade (up or down) and longer the distance, the more difference it makes.  The example above shows the magnitude of the difference for a reasonably steep hike/run.  For typical road runs, including marathons, 3d seems unlikely to make much difference.

This of course doesn't account for the accuracy of barometric pressure readings, GPS. 

Link to climb details:  www.hikingproject.com/.../mt-elbert

Agreed. And the greater the distance traveled with elevation changes, the bigger the difference in distance recorded. 

Cool thanks, that makes sense put in perspective.