Precise height definition of Nisyros island in Greece
20/09/2016 | 16:45 | Session 3: Local/regional geoid determination methods and models
Author(s): Evangelia Lambrou
"The global geoid model EGM08 consists nowadays one of the best tools for the transformation of geometric heights provided by GNSS measurements to orthometric ones. Nevertheless there are some areas worldwide, where due to the strong variations of their terrain (high mountains, spread islands etc.), there is not a satisfied adaptation of the EGM08. However the determination of accurate height referred to an equipontential surface arises as one of the major problems of the geodetic community as demanding constructions (bridges, railways, dams, etc.) and their monitoring required it. Thus, the well spread use of GNSS needs a perfect geoid correction equation in order to derive reliable orthometric height from the geoidal ones.
This study presents the improvement of the EGM08 model on Nisyros Island, which is located at the southeast edge of the Aegean Sea. Its area is about 41Km2. It has almost round shape, with radius of about 4Km, as it was created by the volcano eruptions, which is situated almost at its center. A network of 13 points was established, in order to cover all the area. GNSS and Accurate Trigonometric Heighting measurements were carried out connecting the network’s points. The orthometric and geometric heights of each point were determined with rms ±5mm and ±10mm respectively by appliying least square adjustments. The appropriate best adapding equation is determined as well as the contour lines of the derived local geoid undulation are drawn. Additionally the corresponding values of the geoid undulation N were calculated by the EGM08 geoid model. Thus, a new equation was formed for their correction in order to be more feasible for the concrete area. Moreover the zero-level geopotential value (Wo) of the island, has been determined and compared to the corresponding one of the Greek Vertical Datum (GVD)."