## 2 Inherited and Official Horizontal and Vertical Reference Systems of the Republic of Croatia  
As a former member of Austrian-Hungarian Monarchy, the Republic of Croatia historically inherited the horizontal (2D) coordinate reference system (in 1901) called Croatian State Coordinate System (cro. _Hrutski drzawni koordinantni sustav_--HDKS), _Hermannskagel_ or HR1901. HDKS was realized by astro-geodetic measurements of the former Military Geographic Institute (MGI) from Vienna in a 1 st-order triangulation network, adjusted in seven separate blocks (2 nd, 5 th, 6 th, 22 nd, 23 rd, 24 th, and 25 th) [16]. The origin (so-called _fundamental point_) of HDKS was situated at _Hermannskagel_ hill, near Vienna with orientation to the nearby hill _Hundsheiner Berg_. HDKS utilizes a non geocentric geodetic datum that is defined by five parameters: two for surface definition that is a mathematical approximation of the earth shape--ellipsoid--and three positional parameters--coordinates of the fundamental point. For HDKS, the _Bessel 1841_ rotation ellipsoid was adopted (a = 6,377,397.15500 m, 1/f = 299.15281886). At the fundamental point, surfaces of the selected ellipsoid and geoid coincide, which causes vertical deflection components as well as geoid undulation to be zero. Due to the coincidence of the ellipsoid and geoid at the fundamental point, vertical to geoid and normal to ellipsoid coincide as well, so ellipsoidal and orthometric heights are equal [19]. Along with HDKS, the Republic of Croatia inherited vertical reference system based on one year (1875) tide-gauge measurements in Trieste (Italy) at mole _Sartorio_, which was realized by Austrian Precise Levelling (cro. _Austrijski precizni nivelman_--APN) and 1 st High Accuracy Levelling (cro. _I. nivelman visoke locnosti_--INVT), called Croatian Height Reference System 1875 (cro. _Hrutski visinski referentni sustav 1875_--HVRS1875). The HVRS1875 utilizes normal-orthometric heights [20]. Therefore, the inherited compound coordinate reference system (CCRS) of the Republic of Croatia includes horizontal coordinates in HDKS with a vertical component in HVRS1875. Following modern European trends, as well as the development of satellite positioning systems (primarily GPS and GLONASS), the poor documentation of inherited reference system inhomogeneity of HDKS (due to adjustment of separate block) was more than a good reason to redefine the national horizontal as well as height (and gravimetric) reference systems [21]. On 4 August 2004, the Government of the Republic of Croatia brought the _Decree on establishing new official geodetic data and map projections of Republic of Croatia_[16]. According to the Decree, the Republic of Croatia adopted a new horizontal reference system called Croatian Terrestrial Reference System 1996 (cro. _Hrutski terestriki referentni sustav 1996_--HTRS96), based on the European Terrestrial Reference System 1989 (ETRS89) with GRS80 level-ellipsoid (a = 6,378,137.00000 m, 1/f = 298.2572210). The reference frame of HTRS96 is represented with the network of 78 1 st-order trigonometric points (a part of the former Austrian-Hungarian 1 st-order trigonometric network over the Croatian territory), with coordinates determined within combined solutions of GPS measuring campaigns: CROSLO'94 (ITRF96, e1994.4), CROREF'95 (ITRF96, e1995.7), and CROREF'96 (ITRF96, e1996.7) in mutual epochs of measurements 1995.55 with transformation to ETS89 using EUREF Memo specifications [22]. Based on the same decree [16], Croatia adopted a new vertical reference system as well, called Croatian Height Reference System 1971 (cro. _Hrutski visinski referentni sustav 1971_--HVRS71). HVRS71 was realized with 2 nd High Accuracy Levelling (cro. _II. nivelman visoke locnosti_--IINVT), which was stretched over the territory of former Yugoslavia. Mean seal level was determined by measurements at five tide gauges on the eastern shore of the Adriatic Sea: Koper (Slovenia), Rovinj, Bakar, Split, and Dubrovnik for a period of 18.6 years.