Ph.D Thesis

Ph.D StudentSarid Yechzkel
SubjectHybrid Geoid in Israel
Development of Analytical Tools and Methods for
the Calculation of an Accura and
Reliable Geoid
DepartmentDepartment of Civil and Environmental Engineering
Supervisor DR. Sagi Dalyot
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


The widespread use of GNSS for surveying and mapping allows the obtaining of accurate horizontal and vertical measurements (several centimeters), whereas the height component is measured above the mathematical reference ellipsoid surface. The height component requires the conversion to a height above sea level (orthometric) by using an accurate geoid model describing the spatial height relationship between the ellipsoid and the local geoid models. Since the local geoid model existing in Israel is mathematical, not based on physical data and observations, the height conversions do not always meet the accuracy standards required for land surveying and engineering purposes. The establishment of an accurate hybrid geoid model for Israel, which is based in part on gravimetric observations and spatial relations to the national heights, will allow accurate orthometric height calculations to be obtained from GNSS measurements that will meet the engineering standards.

The official geoid model in Israel - ILUM2.0 - is based on a Kriging interpolation that relies on control points, which comprise precise ellipsoidal and orthometric height measurements. This model relies on 850 control points spread along the main roads. The model accuracy can reach 10 cm and the grid resolution is 0.5 by 0.5 km, implying that reliance on the model cannot ascertain reliable undulation values. This research developed a methodology for the establishment of an accurate local geoid model that is based on various data sources for the area of Israel and its surroundings, namely: terrestrial and shipborne gravity measurements, satellite sea level altimetry, Digital Terrain Models, global gravity models and the Israeli vertical control points. The developed methodology also took under consideration that approximately 20% of Israel's land area is located beneath the sea level, and incorporated the use of various rock density values.

As a preliminary stage, a quasi-geoid gravity model is calculated, with a standard deviation value of 5.7 cm. The model is also successfully calculated in areas with a negative orthometric altitude, which proves for the first time the potential of the developed methodology of obtaining accurate geoid model. This is the first time in Israel that gravity data is used for research in the field of physical geodesy on a national scale. Using the data, the established hybrid geoid model was calculated with a standard deviation of 1.2 cm. The hybrid geoid model retains the orthometric Datum in the control points, and provides better slope information of the area based on the gravity data. It also allows the integration of the official model and the new gravity geoid model for supporting high accuracy GNSS measurements for infrastructure engineering projects.

This research will allow in the near future the gradual transition of working with the established hybrid geoid model, serving as the national model in Israel (and its close surroundings), which will allow on the one hand accurate height measurements required for engineering projects, as well as the establishment of a process for continuous updating and improvement through new gravimetric measurements and new and more accurate international gravity models.