GPS coordinates can be provided in any number of formats, WGS84
ellipsoid is very common. That doesn't take into account local
gravitational anomalies, it's just a ellipse with like 3 parameters to
describe the planet.
Geodetic format DOES use a map of arbitrary, measured data points of
gravitational anomalies across the globe.
Note the geodetic height is not "the ground". Topographical maps are a
different thing. Geodetic height is gravitational, showing, yes, what
direction water will flow between levels. It is NOT typically mapped
with great precision because it doesn't vary much in, say, 50 miles. Of
course in reality the mass of your body next to a receiver "affects" it,
but this has no practical effect on water flow. A hill doesn't either,
a mountain range DOES.
As such, I'm not an expert here, but I'm guessing your water system
would only go a few miles. The geodetic deviation in water leveling
from the WGS85 ellipsoid solution sounds pretty darn small, probably
much much less than a meter.
There's a large number of possible formats, though. Including ones
which aren't lat-long-alt at all. X/Y/Z ECEF is just a 3D Cartestian
coordinate system based on the center of the Earth, the north pole, and
the International Reference Meridian, and rotates with the Earth.
But the point is NONE of this matters for actually getting a GPS fix and
getting GPS data. It's just an output format, and as long as enough
digits are represented, you can swap from one to another. It does NOT
affect the fixing process.
As far as not having geodetic surveys of Ethiopia, I DOUBT that. Well
for one, I mean, you can Google a geoid chart, and it covers the entire
world. The military demanded this stuff decades ago for long-range
missiles. I would guess it would be accurate for Ethiopia.
Like this:
http://www.pha.jhu.edu/~hanish/seminarmain.html
<http://www.pha.jhu.edu/%7Ehanish/seminarmain.html>
or
http://www.google.com/imgres?um=1&hl=en&safe=off&sa=N&biw=1920&bih=936&tbm=isch&tbnid=AG0-K-fk45o7OM:&imgrefurl=http://www.esri.com/news/arcuser/0703/geoid1of3.html&docid=W-fUkuUF2OizmM&imgurl=http://www.esri.com/news/arcuser/0703/graphics/geoid3_lg.jpg&w=1096&h=567&ei=jIOJUOrkCsmkqgGm64HIBQ&zoom=1&iact=hc&vpx=474&vpy=519&dur=360&hovh=161&hovw=312&tx=119&ty=100&sig=108368022690907845995&page=1&tbnh=134&tbnw=260&start=0&ndsp=38&ved=1t:429,r:15,s:0,i:118
-105m to 85m deviations, and yes, Ethiopia is covered.
Danny
On 10/25/2012 7:10 AM, António Pestana wrote:
GPS geodetic ellipsoidal coordinates (latitude, longitude and height)
are relative to an ellipsoid (WGS84). Computation of GPS height
differences is not a true topographic levelling because GPS heights
are relative to the ellipsoid surface (+ above or - below).
Topographic levelling uses gravitic equipotential surfaces (the Geoid
is one of this kind). The water flows (when not using water pumps) due
to gravity. So for open channel water supply systems true levelling is
almost mandatory.
Best regards
Antonio
2012/10/25 andrea antonello <[email protected]
<mailto:[email protected]>>
Hi I need some information about the above subject.
We are planning a educational project for water management networks in
Ethiopia and we applied to get a precision GPS to do the surveys.
We just got hit by one very upset persons responsible for the project
that told us that a professor of topography explained them that:
"since in Africa there is not geoid, the GPS is useless"
Apart of the citation, which may rise some comments on its own, I was
wondering if there is something I am not considering and should really
know. We have used gps in Rwanda without problems for the same
purposes and I am quite puzzled about what I am missing.
Any advice or link to documentation is very appreciated,
Thanks,
Andrea
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