Downward continuation of gravity field

重力是在地球表面观测到的，但对于大地水准面的计算，它需要在海平面（在大地水准面上）：它必须通过地形质量“继续向下”到海平面。在我们的大地水准面确定方法中，从实际地形中减去地形质量的最佳近似值，我们认为结果（没有地形的地球）代表“NT空间”中的情况。 实际上，我们用大地水准面上的凝结层来代替地形，并把这个新的空间称为“赫尔默特空间”。 因此，赫尔默特的空间是由凝结层增加的NT空间。 在NT和赫尔默特的空间中，向下的延续是通过一部分空间进行的，这部分空间是我们尽可能使它成为无质量的。

Gravity is observed on the earth's surface but for the geoid computation it is needed at the sea level (on the geoid): it has to be "continued downward" onto the sea level through topographical masses. In our approach to geoid determination, the best approximation of topographical masses is subtracted from actual topography and we consider the result (Earth without topography) to represent the situation in the "NT space". In fact, we replace topography by a condensation layer on the geoid and call this new space the "Helmert space". Helmert's space is thus the NT space augmented by the condensation layer. In both the NT and Helmert's space, the downward continu-ation is carried out through a part of space that is as void of mass as we can possibly make it.