[FOSS-GPS] GPS issues for Ethiopia

António Pestana afsm.pestana at gmail.com
Thu Oct 25 05:10:21 PDT 2012

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


2012/10/25 andrea antonello <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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