Researchers who are faced with the task of transforming the free-air gravity anomalies from real space to the Helmert or the NT space would have met with a term called Secondary Indirect Topographical Effect, SITE for brief. Here the spaces are defined by their distribution of topographical mass density and they share the normal gravity field as well as the coordinate systems used for describing the processes that take place in these spaces. The Helmert space is defined by the two-dimensional distribution of a condensed topographical mass layer on the geoid in terms of Helmert's second method of condensation. Another space is the NT space in which the entire topographical mass is removed, i.e. it has a zero topographical density distribution. The SITE is one term in the sequence of terms applied to a gravity anomaly in the “from space", say the real space, to obtain gravity anomaly in the “into space", say, the Helmert space. The origin of all the terms in the sequence can be intuitively understood, except for the SITE, which is also missing in the transformation equations/sequences for other gravity related quantities.

On the other hand, people who deal with these different spaces, speak of the direct transformation from the “from space" to the “into space" and the inverse transformation from the “into space" to the “from space". The direct transformation is associated with the “direct effects" while the inverse transformation is associated with the “indirect effects". So, is the SITE associated with the inverse transformation? It turns out that it is not. Instead, the SITE must be understood as transforming from one space to another the rule of computing gravity anomalies. This means transforming the rule of identifying the “corresponding" equipotential surfaces in the real gravity field and the normal gravity field. This study explains the physical meaning and formulates mathematical transformations of the SITEs between the Helmert and NT spaces.