<html>
<head>
<meta content="text/html; charset=ISO-8859-1"
http-equiv="Content-Type">
</head>
<body bgcolor="#FFFFFF" text="#000000">
<div class="moz-cite-prefix">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. <br>
<br>
Geodetic format DOES use a map of arbitrary, measured data points
of gravitational anomalies across the globe.<br>
<br>
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.<br>
<br>
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.<br>
<br>
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.<br>
<br>
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.<br>
<br>
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.<br>
<br>
Like this:<br>
<a href="http://www.pha.jhu.edu/%7Ehanish/seminarmain.html">http://www.pha.jhu.edu/~hanish/seminarmain.html</a><br>
or<br>
<a
href="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">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&
amp;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</a><br>
-105m to 85m deviations, and yes, Ethiopia is covered.<br>
<br>
Danny<br>
<br>
On 10/25/2012 7:10 AM, António Pestana wrote:<br>
</div>
<blockquote
cite="mid:CACHwcg+AF4rLvqMkRFV-e4ETwZnZK=92B3mCQ=r4vAqQqxbRJQ@mail.gmail.com"
type="cite">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.
<div>
<br>
</div>
<div>Best regards</div>
<div><br>
</div>
<div>Antonio<br>
<br>
<div class="gmail_quote">2012/10/25 andrea antonello <span
dir="ltr"><<a moz-do-not-send="true"
href="mailto:andrea.antonello@gmail.com" target="_blank">andrea.antonello@gmail.com</a>></span><br>
<blockquote class="gmail_quote" style="margin:0 0 0
.8ex;border-left:1px #ccc solid;padding-left:1ex">Hi I need
some information about the above subject.<br>
<br>
We are planning a educational project for water management
networks in<br>
Ethiopia and we applied to get a precision GPS to do the
surveys.<br>
<br>
We just got hit by one very upset persons responsible for
the project<br>
that told us that a professor of topography explained them
that:<br>
"since in Africa there is not geoid, the GPS is useless"<br>
<br>
Apart of the citation, which may rise some comments on its
own, I was<br>
wondering if there is something I am not considering and
should really<br>
know. We have used gps in Rwanda without problems for the
same<br>
purposes and I am quite puzzled about what I am missing.<br>
<br>
Any advice or link to documentation is very appreciated,<br>
Thanks,<br>
Andrea<br>
_______________________________________________<br>
This message is sent to you from <a moz-do-not-send="true"
href="mailto:FOSS-GPS@lists.osgeo.org">FOSS-GPS@lists.osgeo.org</a>
mailing list.<br>
Visit <a moz-do-not-send="true"
href="http://lists.osgeo.org/mailman/listinfo/foss-gps"
target="_blank">http://lists.osgeo.org/mailman/listinfo/foss-gps</a>
to manage your subscription<br>
For more information, check <a moz-do-not-send="true"
href="http://wiki.osgeo.org/wiki/FOSS-GPS" target="_blank">http://wiki.osgeo.org/wiki/FOSS-GPS</a><br>
</blockquote>
</div>
<br>
</div>
<br>
<fieldset class="mimeAttachmentHeader"></fieldset>
<br>
<pre wrap="">_______________________________________________
This message is sent to you from <a class="moz-txt-link-abbreviated" href="mailto:FOSS-GPS@lists.osgeo.org">FOSS-GPS@lists.osgeo.org</a> mailing list.
Visit <a class="moz-txt-link-freetext" href="http://lists.osgeo.org/mailman/listinfo/foss-gps">http://lists.osgeo.org/mailman/listinfo/foss-gps</a> to manage your subscription
For more information, check <a class="moz-txt-link-freetext" href="http://wiki.osgeo.org/wiki/FOSS-GPS">http://wiki.osgeo.org/wiki/FOSS-GPS</a>
</pre>
</blockquote>
<br>
</body>
</html>