GNSS 2D positioning accuracy is well defined. According to the receiver model, the software, the positioning Mode, the available corrections used, the 2D positioning accuracy vary from few meters to mm. However the altimetry positioning with GNSS is less accurate than 2D one. Without an accurate Geoid reference earth surface, the levelling survey by GNSS couldn’t be done with sufficient accuracy needed for several fields of survey like construction or water management projects.