الفهرس 
INTRODUCTIONOnly 14 pages are availabe for public view
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Abstract
Nowadays, it is importantly and urgently needed in Egypt the existence of precise geoid for many applications. For that reason, lot of terrestrial gravity data could be collected as gravity
values, gravity anomalies, and GPS leveling undulations. These data might be collected from governmental agencies, petroleum companies, and research institutes and centers. The collected
data covers large area of the Egyptian territory but they have been taken with different references, strategies, accuracies, and methodologies.
In the same time, satellite missions concerning the gravity field of the earth started to yield
global gravity models free from the mentioned defects of the terrestrial observations. Recently,
many global geoid models have been computed based on satellite data. Meanwhile global satellite only models still suffer from the resolution issue. Those global models need to be
verified and investigated against terrestrial (GPS/Leveling) data. Therefore, this thesis contains
two main parts. The first one is evaluating the whole released, until 2019, satellite based data
global models against GPS/Leveling data in addition to EGM2008. The second part is improving some of the best resulted models beside the highest resolution model (EGM2008) and two models having low d/o. Concerning the first part; ninety-three satellite data based global models are evaluated against
three GPS/Leveling data sets. EGM2008 as famous common global model is also evaluated. The results showed that the best ten satellites based data global models have data either from Goce
only or Goce plus other missions like Grace and SLR. Three of them are Goce only data, three of them are (Goce + Grace), and four of them are (Goce + Grace + SLR). The best ten models have
d/o ranges from 240 to 300. They considered recent models released from 2014 to 2019. The results showed also that the mean value of the undulation differences ranged from 53 to 56 cm
for all of the ten models while it was 63 cm in the case of EGM2008. The standard deviation values are ranged from 24.2 to 28.1 cm while it is 25.8 cm in the case of EGM2008. The best ten
models have close results to each other and most of them are better than EGM2008. Concerning the second part, three of the ten best models are subjected to the improvement
process. Two methods are applied, one is the traditional first order polynomial and the second is proposed simple shifting method. Regarding that the GPS/leveling points in the three data sets have a longitudinal extension, the results showed that the proposed simple shift method with the three data sets gave around 8 cm as a mean and 11 cm as a standard deviation using one seventh
of all data points corresponding to 15, 30, and 25 km as radii around the center point in the three data sets respectively. The results of the simple shift method when using one eleventh and one fifteenth of the data points corresponding to 50 and 60 km as radii around the center point are not
very far from the results in the previous notice. The same can be said about the results of the polynomial method but the mean values in the shifting solutions are noticeably smaller than their corresponding values from the polynomial solutions To assure the previous results of the improving process, EGM2008 and other two low d/o models are improved. One of them has 180d/o and the other as 120 d/o. They have been improved very significantly.