On Feb 15, 4:32 pm, Jim Thill - Hiawatha Cyclery <thill....@gmail.com> wrote: > GPS should record elevation changes more accurately, and both GPS and > cycle computers should record distance traveled more accurately than > mapping software does. In the old days (10-15 years ago), GPS > elevation measurements were suspect because there weren't always > enough satellites to triangulate accurately in the 3rd dimension. I > think this has improved lately.
I've never used a bicycle computer, so I can't speak to comparing its data to web-based map service, but I can touch upon the GPS elevation issue. While it's true that the satellite constellation has filled out (and is currently being rearranged from a 21+3 to a 24+3 configuration over the next year or so), and GPS receivers are now running at least 12 channels or more, that accuracy has improved. Unfortunately, the vertical accuracy will always lag the horizontal, because the earth isn't transparent. If you were to stand at a window with a GPS unit and look at the 'skyplot' view, you would notice that the receiver is picking up signals from satellites out in front of you that you can see from the window (if you could actually 'see' them), but that the ones behind you are blocked by the building, which thus skews your calculated location. If you then went outside and were standing in the middle of a field, you could see satellites all around, then the calculated location can be more precise. The same thing is going on with elevations, but in that case, it's the earth that's obscuring signals from satellites on the other side of the earth. Because all of the satellites within view are being used are all overhead, and not 'behind' you, there's a bit of a skew; the smallest bit of error makes for a much larger change in elevation, than you would get for horizontal calculations. If the satellites that you're viewing/using are spread out around the horizon, then you'll be better off than if they were all clustered directly overhead - that would help minimize that error - but, that error will still be greater than your horizontal. FWIW, the maps that I usually make, I will pull my horizontal location off the GPS, plot it on a topo, then use the elevations off the map, in order to compare to other locations (that aren't GPS'd). [Unless, I can actually GPS the other locations myself, too, in which case I'll then emphasize their relative elevations over absolute elevations.] With that said, yes, it's improving. I've got a few Trimble units here in my office that are sub-meter, and I can pull off sub-foot with them, given enough time (and even cm-level accuracy w/ a LOT of time and post-processing); but, vertically, I wouldn't guarantee anything more than sub-foot, and that's w/ a long residence time, not an instantaneous value, as you would have if you're biking along. (Hope that's not totally confusing....) -L -- You received this message because you are subscribed to the Google Groups "RBW Owners Bunch" group. To post to this group, send email to rbw-owners-bu...@googlegroups.com. To unsubscribe from this group, send email to rbw-owners-bunch+unsubscr...@googlegroups.com. For more options, visit this group at http://groups.google.com/group/rbw-owners-bunch?hl=en.
