Transformation of the Croatian national navigation chart for the Danube river into WGS-84 compatible systems

The digital navigation map for the Croatian segment of the Danube River was created from the Danube Commission printed maps, which were geo-referenced using the Croatian national geodetic system and then transformed into geographic coordinates using the AutoCAD Map software. During the production of the navigation map, we discovered that the software used for the map production did not transform accurately Croatian national geodetic data into national and WGS-84 geographic coordinates. We designed a correction procedure, which could be efficiently used to obtain satisfactory results. We used the DAT ABMO software tool for transformation of WGS-84 compatible ETRS coordinates into the new Croatian state coordinate system (HDKS). Reverse calculations are inaccurate and cannot be used. Linear functions for determination of the differences between HDKS and European terrestrial reference system (ETRS) coordinates were used on the basis of 14 GPS control points, for which all coordinates had been measured (errors below 3 cm were obtained). In addition, we used the same procedure to reproduce the WGS-84 coordinates of two control points at the state border with Hungary. We compared them with the ones obtained earlier in Hungary. The accuracy of the Hungarian data is not known; however, the differences obtained are small enough (about 60 cm) to produce accurate digital maps of the 1:1000 scales.