I'm curious about this as well. I just noticed the 3D Speed and Distance options the other day and it seems to me that you would want them both on all the time. If you run up (or down) a hill, then you physically ran a greater distance than just the difference between the two points on flat ground. Why wouldn't you want the Garmin to give the distance you actually ran?
I'm very curious to see what others have to say about this.
It does not necessary work as you would think as the model is very vague and this causes GPS elevation to be very vague
This was really the only thing I could think of as a possible reason why you wouldn't want to use it... that it's only as reliable as the elevation data. On a typical run though your altitude measurement is probably varying up and down more due to measurement error than it is due to actual elevation change. The grades you encounter on a run are probably not steep enough to make 3-D measurements worth it. On a hike or something where you presumably have large elevation changes, you're well outside the margin of error on the altimeter so it's probably quite worth it.
Cycling is probably the biggest question mark. You can handle a bit steeper of grades on a bike, although still probably not steep enough to be worth it on 90% of rides... but if you have a speed sensor on your wheel, then the speed you get from that is effectively 3D. So would you not want to have 3D distance on too?
I've used it while trail running, both on the fenix 5 and on my Ambit3 Peak. Using 3D distance will seriously jack with distances, even with moderate elevation gain/loss. I'll typically run trails on the weekend, with 6 miles and around 1900 ft of elevation gain being fairly normal. With 3D distance, it was lots more distance than normal, on the same routes, and compared to previous runs and online route building tools.
I seem to remember reading that 3D distance really should be used for huge amounts of elevation gain or loss in short amounts of distance. Think mountaineering, or skiing really steep slopes.
I think the following post I found over in the F3HR forums does a good job summarizing why it doesn't matter for running...
I already did the math, on the Pikes Peak ascent where you climb 7,815 ft in a half marathon distance your average grade is 11% which will result in roughly 5% distance error. If you're "running" a race that is steeper than this you're probably hiking and not running. And we're still talking about only 5% error in this extreme scenario. I really don't know a lot of runners who can run up anything steeper than an average 10% grade and still call it running. To have a 10% error in distance you would have to average a 14.2% grade for the entire race. I dare you to find me a foot race with 14% average grade anywhere in the world then I dare you to actually "run" it.
Cycling is probably the biggest question mark. You can handle a bit steeper of grades on a bike, although still probably not steep enough to be worth it on 90% of rides... but if you have a speed sensor on your wheel, then the speed you get from that is effectively 3D. So would you not want to have 3D distance on too?
If you have the Garmin speed sensor on your wheel, then both speed and distance are derived from the speed sensor - no need for 3D settings on the watch to be enabled.
If you have the Garmin speed sensor on your wheel, then both speed and distance are derived from the speed sensor - no need for 3D settings on the watch to be enabled.
HTH
Interesting. I didn't know it used the sensor for distance too, although it makes sense.
Thanks, all, this is helpful. I guess Garmin's defaulting of 3D on in skiing but not MTB or trail running implies their recommendation (which seems consistent with what seems to be the consensus here)
Now this makes me wonder: when trails are marked as being a certain distance, does that indicate '3D' distance (as if you rolled a surveyor's wheel on the trail) or is it just 2D (as depicted on a flat map)?
