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 <m...@pobox.com> wrote:
On Fri, Sep 13, 2013 at 10:13 AM, Darren Addy <pixelsmi...@gmail.com>
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.
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