Article Sidebar
Published:
Jan 30, 2017Keywords:
Article Details
Open access articles published in the Journal of Agriculture, Science and Technology are under the terms of the Creative Commons Attribution (CC BY) License which permits use, distribution and reproduction in any medium, provided the original work is properly cited. The CC BY license permits commercial and non-commercial re-use of an open-access article, as long as the author is properly attributed.
Copyright on any research article published in the Journal of Agriculture, Science and Technology is retained by the author(s). The authors grant the Journal of Agriculture, Science and Technology with a license to publish the article and identify itself as the original publisher. Authors also grant any third party the right to use the article freely as long as its original authors, citation details and publisher are identified.
Use of the article in whole or in part in any medium requires proper citation as follows:
Title of Article, Names of the Author, Year of Publication, Journal Title, Volume (Issue) and page. Links to the final article on the JSRE website are encouraged.
The Creative Commons Attribution License does not affect any other rights held by authors or third parties in the article, including without limitation the rights of privacy and publicity. Use of the article must not assert or imply, whether implicitly or explicitly, any connection with, endorsement or sponsorship of such use by the author, publisher or any other party associated with the article.
For any reuse or distribution, users must include the copyright notice and make clear to others that the article is made available under a Creative Commons Attribution license, linking to the relevant Creative Commons web page. Users may impose no restrictions on the use of the article other than those imposed by the Creative Commons Attribution license.
To the fullest extent permitted by applicable law, the article is made available as is and without representation or warranties of any kind whether express, implied, statutory or otherwise and including, without limitation, warranties of title, merchantability, fitness for a particular purpose, non-infringement, absence of defects, accuracy, or the presence or absence of errors.
Main Article Content
Estimation Of Orthometric Height Using EGM2008 and GPS Over Nairobi County and Its Environs
Abstract
Global Navigation Satellite Systems (GNSS) have been used widely in 3-dimensional positioning globally, regionally and locally. Global Positioning System (GPS) is one of the most extensively used GNSS in Earth sciences. GPS which employs World Geodetic System adopted in 1984 (WGS84) as the reference system has extensively been used for height determination and has brought a revolution on how relative and absolute heights on earth’s surface are determined. GPS measures ellipsoidal heights above a reference ellipsoid (WGS84). Although these heights can be useful in deformation surveys, machine monitoring and guidance, they are not applicable in engineering projects (e.g. sewer lines, pipelines and road construction among others) where heights referenced to an equipotential surface (geoid) are required. The separation between the geoid and a reference ellipsoid (geoid undulation) is necessary in converting ellipsoidal height into orthometric height. In this study we determine geoid undulation from Earth Gravitational Model of 2008 (EGM2008) using freely available Alltrans EGM2008 calculator software version 3.002 at 18 GPS/levelling points. The determined geoid undulations are used to determine estimated orthometric heights from ellipsoidal heights. We then model the differences between spirit-levelled orthometric and estimated orthometric heights by a four parameter model (first order polynomial) at 11 GPS/levelling points using least squares technique for improvement on the estimated orthometric heights. 7 GPS/levelling points are used for testing the performance of the four parameter model over Nairobi County and its environs. The standard deviations of the differences between observed and estimated orthometric heights (obtained from EGM2008 and GPS) at all GPS/levelling points (18) and 7 test points are ±0.52 and ±0.35 m respectively. When the four parameter model is applied, the standard deviations of the differences between spirit-levelled and improved estimated orthometric heights at the 7 test points reduces to ±0.10 m, representing an improvement of 71%. The accuracy of ±0.10 m obtained at the test points may be sufficient for some engineering projects that do not require very high orthometric height accuracy.
