## 3 Review of Previous Studies  
The first scientific studies of coordinate transformation procedures between different terrestrial reference frames appeared in the 1960s [2; 5; 6], so scientists like [PERSON] (1909-1991), [PERSON] (1929-2019), and [PERSON] (1910-1994) can be considered as pioneers of coordinate transformation. Development and usage of global navigation satellite systems (GNSS), primarly NAVSTAR GPS in the 1980s [23; 24], for solving everyday geodetic tasks required linking national coordinate reference frames to global ones [3; 7] and pointed out that, often, inherited (historical) coordinate reference systems are inappropriate for the application of new technologies. Therefore, finding the appropriate coordinate transformation procedure between those coordinate reference systems is essential [10; 11; 12; 13; 25].  
For the territory of the Republic of Croatia, coordinate transformation procedure also became important with the usage of NAVSTAR GPS as a measuring technique in the late 1980s and early 1990s within first regional geodynamic (CRODYN) and EUREF GPS measuring campaigns [26; 27]. Later, two different transformation models were established to ensure the coordinate transformation procedure between HDKS and ETRS89/HTRS96, called DAT_ABMO and, currently, the official one, T7D, both of which are based on a 7-parameter transformation [14; 18; 21; 28; 29; 30; 31; 32; 33].  
The previous studies [15; 18; 19; 27; 28; 34] have shown that transformation parameters between the inherited (HDKS) and new coordinate system (HTRS96) have been calculated multiple times since the establishment of the Republic of Croatia. The transformation parameters for the territory of the Republic of Croatia were first calculated in the year 2000 as part of the \"Geodetic and Cartographic Datum of Republic of Croatia\" project based on 120 identical points in HDKS and HTRS96 coordinate systems [19; 28]. In 2002, a new set of transformation parameters was calculated based on 241 identical points in two coordinate systems as part of the \"Unique adjustment of 10 km GPS network of Republic of Croatia\" project [34], resulting in improved 3D accuracy compared to the previous effort in 2000. The next set of transformation parameters was calculated in 2006 as part of the \"Development of unique transformation model--HTRS96/HDKS\" project, where a total of 1780 identical points in the inherited and new coordinate systems of the Republic of Croatia were used in the calculation, resulting in the transformation model T7D2006 [15]. In order to improve the T7D2006 transformation model, a project, \"New geoid model of Republic of Croatia and improvement of T7D transformation model\", was initiated in 2009, which resulted in the new T7D2009 transformation model [18]. Unique transformation parameters were calculated for this model based on 5200 points distributed across the entire territory of the Republic of Croatia.  
The purpose of all the mentioned projects was to calculate transformation parameters for coordinate transformation in the territory of the Republic of Croatia, without the need for detailed explanations of the calculation procedure. For this reason, the aim of this article is to provide a detailed explanation of the procedure for calculating transformation parameters of all types of transformations, with transformation parameters given as solutions of numerical examples.