Re: Computations about geodesy
- From: jpwoodruff@xxxxxxxxx
- Date: Sun, 22 Feb 2009 15:50:12 -0800 (PST)
On Feb 22, 2:32 pm, "osmium" <r124c4u...@xxxxxxxxxxx> wrote:
<jpwoodr...@xxxxxxxxx> wrote:
I'm a hobbiest programmer interested in geodesy - computations on
near-spherical earth.
Not a project, but a question. How do they know where those GPS satellites
are? I can see how you can drive a single stake in the ground at Greenwich
and say that, *by definition*, this is zero degrees E-W and k degrees N
latitude. I can't imagine how you could drive a second stake anywhere in
the world with the required accuracy to meet the claims for GPS positioning.
I understand someone drove a stake in at the Meade Ranch in Kansas many
years ago. What is the distance in mm between these two stakes and the
absolute accuracy of that distance? How in the world was that answer
derived? Is the distance not a function of time?
I think if I knew how to establish two stakes, I could go on and drive a
third one, and so on.
There is a convention for associating land points with coordinates
from gps. Wikipedia on World Geodetic System explains.
The distances that I implemented are relative to the geoid WGS84. So
if one stipulates to use a map that is referenced to that surface, the
map has features and coordinates that agree. But the flat-paper map
won't tell you how far apart things are.
The rhumb lines I compute would be "great circles" if the earth were
spherical instead of WGS84. Instead they correspond to stretched
strings on that mathematical surface - altitude is always mean sea
level.
John
.
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