[FOSS-GPS] GPS issues for Ethiopia

[julio menezes]

Dear Andrea !

--- Em qui, 25/10/12, andrea antonello <andrea.antonello at gmail.com> escreveu:

> De: andrea antonello <andrea.antonello at gmail.com>
> Assunto: [FOSS-GPS] GPS issues for Ethiopia
> Para: foss-gps at lists.osgeo.org
> Data: Quinta-feira, 25 de Outubro de 2012, 9:19
> Hi I need some information about the
> above subject.
> 
> We are planning a educational project for water management
> networks in
> Ethiopia 

Good luck for your project, especially because it is educational.

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"
> 


You said do not have a precise vertical network, even so you can use GPS and mix it with traditional topographic leveling for critical nstallations.

I do not know your project extension and precision requirements but i believe you can use GPS and a relative geoid undulation to correct the geometric height differences and get an almost orthometric height.
I have used EGM2008 and  this way and get realy good results.
Pay attention on EGM2008 if you have no gravitational anomalies in your working area and for sure this simple differential model will give you acceptable results.
Hj - Hi = (hj - hi) - ( Nj - Ni )
H=orthometric
h=geometric
N=geoid ondulation

http://www.fig.net/pub/athens/papers/ts07/ts07_8_roman.pdf

best regards,

julio menezes