[FOSS-GPS] GPS issues for Ethiopia

[Danny Miller]

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 <andrea.antonello at gmail.com 
> <mailto:andrea.antonello at gmail.com>>
>
>     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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