# [FOSS-GPS] GPS issues for Ethiopia

Gordon Williams gwilliams at ncf.ca
Thu Oct 25 06:12:01 PDT 2012

There are converters that will take GPS ellipsoidal height and give you
Orthometric height based on the Geoid for a specific Long/Lat.  UNAVCO has
one http://www.unavco.org/community_science/science-support/geoid/geoid.html

Example output
Your Input Coordinates and GPS Height:

Latitude = 9.026958° N = 9° 1' 37.05" N
Longitude = 38.72371° E = 38° 43' 25.36" E
GPS ellipsoidal height = 100 (meters)
Geoid height = -7.392 (meters)
Orthometric height (height above mean sea level) = 107.392 (meters)
(note: orthometric Height = GPS ellipsoidal height - geoid height)

This is based on a mathematical model of the Geoid, so you would have to
look into how accurate that model is and if your region fits the model well.
The Geoid is affected locally by gravitational changes from mountains and
ground density.  Check out wikipedia for geoid and orthometric height.

You should get a book on surveying and talk to your local university
surveying department.

Gordon Williams

----- Original Message -----
From: "andrea antonello" <andrea.antonello at gmail.com>
To: <foss-gps at lists.osgeo.org>
Sent: Thursday, October 25, 2012 7:19 AM
Subject: [FOSS-GPS] GPS issues for Ethiopia

> 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.
>