Journal of Civil Engineering and Architecture 15 (2021) 437-445 doi: 10.17265/1934-7359/2021.08.005 Local Geoid Model Generation Using the Geometrical Approach Mohammed Anwer Jassim 1 and Mohanad Mohsen Yousef 2 1. Geomatics Engineering Department, College of Engineering, Salahaddin University-Erbil, Erbil 44001, Iraqi Kurdistan Region, Iraq 2. Duhok Polytechnical University—Zakho Technical Institute, Duhok 42001, Iraqi Kurdistan Region, Iraq Abstract: The global navigation satellite systems (GNSS) provide an accurate three-dimensional positioning including the geodetic (ellipsoidal) height (h), which is, in most cases, must be transformed to the local/regional orthometric height (H) to have physically the desired meaning of the elevation above the national vertical datum (e.g., MSL (Mean Sea Level)). Usually, the local orthometric heights are determined based on the value of geoid undulation (N), which is obtained by methods of gravimetrical observations or derived by methods of interpolation utilizing the local geoid models (LGM). The current paper highlights one of the methods of generating LGM that is based on the geometrical relationship between the global datum WGS84 and the Iraqi datum Karbala 1979. The DGPS (Differential GPS) method is used to get the geodetic coordinates of about 97 selected points to cover the experimental area, within the boundaries of Erbil municipality, which has been partitioned into four parts. Therefore, four LGM were generated individually for each one of the four parts with an estimated uncertainty equal to ±0.076 m. Key words: Geoid undulation, LGM, ellipsoidal height, Karbala 1979. 1. Introduction The determination of the undulation of the geoid (N) has always been one of the main goals of researchers in the field of geodesy and surveying sciences. According to the widespread use of global positioning system (GPS) in geodetic applications, great attention is paid to the precise determination of local/regional geoid with an aim to replace the geometric levelling, which is very onerous measurement work, with GPS surveys [1]. Although the global navigation satellite system (GNSS) observations provide the position including the geodetic (or ellipsoidal) height (h) according to the reference global datum WGS84, the observed position can be also given as horizontal coordinates with the orthometric height (H) referring to the local datum utilizing the seven transformation parameters and the local geoid undulation (N). However, the value of the geoid undulation can be determined even from the Corresponding author: Mohammed Anwer Jassim, Dr., Asst. Prof., research fields: geomatics engineering, adjustment theory. gravimetric observations or using the geoid model. Depending on data availability and accuracy requirements, there are two principal approaches for determining the geoid models. These approaches include the gravimetric method and interpolation between geometrically derived geoid heights using the benchmarks of which three-dimensional coordinates and orthometric heights have been determined according to GPS and levelling measurements [1]. 2. Geometrical Relationship between Global and Local Datums With many different realizations of terrestrial reference systems, as well as local or regional datums, it is important for many geodetic applications to know the relationship between these frames [2]. The geometrical relationship between different datums is usually understood according to the elements of geometry of those datums (size & flattening) and the degree of orientation compatibility (parallelism). Therefore, the position’s transformation from one datum to another will be based upon this D DAVID PUBLISHING