The definition of a common global vertical coordinate system is nowadays one of the key points in Geodesy. With the advent of GNSS, a coherent global height has been made available to users. The ellipsoidal height can be obtained with respect to a given geocentric ellipsoid in a fast and precise way using GNSS techniques. On the other hand, the traditional orthometric height is not coherent at global scale. Spirit levelling allows the estimation of height increments so that orthometric heights of surveyed points can be obtained starting from a benchmark of known orthometric heights. As it is well known, this vertical coordinate refers to the geoid, which is assumed to be coincident to the mean sea level. By means of a tide gauge, the mean sea level is estimated and thus a point of known orthometric height is defined. This assumption, which was acceptable in the past, became obsolete given the level of precision which is now required. Based on the altimetry observation, one can precisely quantify the existing discrepancy between geoid and mean sea level that can amount to 1 ÷ 2 m at global scale. Therefore, different tide gauges provide biased estimates of the geoid, given the discrepancy between this equipotential surface and the mean sea level. Also, in the last years, another vertical coordinate was used, the normal height that was introduced in the context of the Molodensky theory. In this paper, a review of the existing different height systems is given and the relationships among them are revised. Furthermore, an approach for unifying normal height referring to different tide gauges is presented and applied to the Italian test case. Finally, a method for defining a physical height system that is globally coherent is discussed in the context of the definition of the International Height Reference System/Frame, a project supported by the Global Geodetic Observing System of the International Association of Geodesy (IAG). This project was established in 2015 during the XXVI IAG General Assembly in Prague as described in IAG Resolution no. 1 that was presented and adopted there.

当今，通用的全球垂直坐标系的定义是大地测量学的关键点之一。随着GNSS的出现，用户可以使用一个连贯的全球高度。可以使用GNSS技术以快速，精确的方式相对于给定的地心椭球获得椭球高度。另一方面，传统的正高在全球范围内并不一致。水平仪允许估算高度增量，以便可以从已知正交高度的基准开始获取被测点的正交高度。众所周知，该垂直坐标指的是大地水准面，假定它与平均海平面重合。借助于潮汐计，可以估计平均海平面，从而定义已知的正高。这个假设 鉴于现在需要的精确度，过去可以接受的方法已过时。根据高程观测，可以精确量化大地水准面与平均海平面之间的差异，全球范围内的差异可以达到1÷2 m。因此，考虑到该等势面与平均海平面之间的差异，不同的潮汐仪可提供对大地水准面的偏差估计。同样，在最近几年中，使用了另一个垂直坐标，这是在莫洛登斯基理论的背景下引入的正常高度。本文对现有的不同高度系统进行了综述，并修正了它们之间的关系。此外，提出了一种统一参考不同潮汐仪的正常高度的方法，并将其应用于意大利测试用例。最后，在国际大地测量学协会（IAG）的全球大地观测系统支持的项目“国际高度参考系统/框架”的定义中，讨论了一种定义全局上连贯的物理高度系统的方法。该项目于2015年在第二十六届IAG大会在布拉格期间建立，如IAG第13号决议所述。在那里介绍并采用了1。