Yes it has been an interesting thread! Thanks to Matthew for his generous sharing of knowledge on the subject (and to Alan C for the additional datapoint).
On Fri, Sep 13, 2013 at 11:15 AM, Alan C <[email protected]> wrote: > This has been a very interesting thread. I know that when road runners use > GPS to measure distances run, they are generally a bit short (because of the > elevation issue?). However, when they are within range of the GPS mast on > top of the local mine headgear, the distances are almost spot on. > > Alan C > > -----Original Message----- From: Darren Addy > Sent: Friday, September 13, 2013 4:56 PM > To: Pentax-Discuss Mail List > Subject: Re: Incentive to get busy learning how to use my O-GPS1 > > > Good explanation, Matthew. I suppose that means that systems that can > take advantage of the GROUND-BASED WAAS stations should perform a bit > better on the elevation question because you have another point to > calculate the triangulation/time measurements from. I have to believe > that you would need a LOT of ground-based stations for that to work or > the curvature of the earth would again get in the way of the > ground-based signals (guessing here). > > On Fri, Sep 13, 2013 at 9:32 AM, Matthew Hunt <[email protected]> wrote: >> >> On Fri, Sep 13, 2013 at 10:13 AM, Darren Addy <[email protected]> >> wrote: >> >>> Elevation of location. (ASIDE: This seems to be where "all GPS units >>> exhibit a weakness" but I'm not sure why that is. According to this >>> PDF, >>> >>> http://www.nasa.gov/centers/johnson/pdf/584738main_Wings-ch5c-pgs360-369.pdf >>> the space shuttle created topographical data sets that can give the >>> elevation for virtually any location. Perhaps it is just not built >>> into the GPS system yet, or there is no method for the GPS to "look it >>> up" and enter it into the calculations.) >> >> >> GPS receivers, in general, calculate elevation the same way they >> calculate position, from the difference in arrival times of the GPS >> signals. While it is conceivable to use SRTM (the Shuttle data) or >> another elevation dataset, I would be surprised of the O-GPS1 were >> doing so. (If you hoist the O-GPS1 up a flagpole or something, you >> should be able to tell.) Also, SRTM is itself somewhat coarse in terms >> of the post spacing on the ground (30 meters or worse), so I'm not >> sure if it would be an improvement or not. >> >> Why do GPS receivers do worse with elevation than with lat/lon? The >> reason is geometric. When measuring position via arrival times, you >> get the best results from having satellites all around you. If you're >> measuring your latitude, you'd like a satellite to the north, and >> another to the south. That ways if you move a meter north, you've >> decreased the distance to the north satellite by a meter, and >> increased the distance to the south satellite by a meter, and thus >> changed the difference between them by 2 meters, which is reflected in >> your difference of arrival time measurement. >> >> With elevation, the stupid earth is annoying in the way, preventing >> you from seeing a satellite beneath you. They're all above you, and >> thus somewhat correlated in the up-down direction. When you move up, >> you move closer to all of them. They're not all directly above you, so >> there is some variation to measure, but it's a harder measurement. >> >> There are various "dilution of precision" metrics that the GPS >> receiver calculates to estimate these uncertainties based on the >> geometry of the satellites at any particular time. I don't know that >> the O-GPS1 makes those visible to the user, though. >> >> -- >> PDML Pentax-Discuss Mail List >> [email protected] >> http://pdml.net/mailman/listinfo/pdml_pdml.net >> to UNSUBSCRIBE from the PDML, please visit the link directly above and >> follow the directions. > > > > > -- > Nothing is sure but death and Pentaxes. > > -- > PDML Pentax-Discuss Mail List > [email protected] > http://pdml.net/mailman/listinfo/pdml_pdml.net > to UNSUBSCRIBE from the PDML, please visit the link directly above and > follow the directions. > > -- > PDML Pentax-Discuss Mail List > [email protected] > http://pdml.net/mailman/listinfo/pdml_pdml.net > to UNSUBSCRIBE from the PDML, please visit the link directly above and > follow the directions. -- Nothing is sure but death and Pentaxes. -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
