Conference Paper

EU Commission Regulations implementing Directive 2007/2/EC as regards interoperability of spatial data sets and services, as well as the guidelines for the INSPIRE Specification on Coordinate Reference Systems recommend that any spatially-enabled INSPIRE mandated dataset be provided in the ETRS89 datum or an ITRF-based geodetic system (for areas outside of ETRS89’s definition).
Romania, as an EU member state, is bound to follow these guidelines. The Romanian law mandates the use of a national reference system, commonly referred to as System 42 (S-42), for all topographic and geodetic measurements. This reference system is only valid for areas within Romania, and was designed in the 1970s, long before the adoption of ETRS89. Consequently, no simple mathematical formula exists for projecting coordinates between the two systems.
Various approximations have been used, with varying degrees of success. Recently, the Romanian Cadastre Agency has made available a computer application (TransDatRo) that allows transformation between the two systems with a claimed accuracy of ±10-15 cm. Unfortunately, the application has some major limitations, including operating system constraints, input and output format restrictions, and a cumbersome workflow.
Our works seeks to identify the most appropriate software to use for general, high volume transformations between the two reference systems. To this end, we studied several proprietary GIS software, as well as the PROJ.4 library, used extensively in open source systems. Taking the values obtained through TransDat as reference, we found out that the transformations defined by current EPSG codes (used by PROJ4) provide inferior results when compared with proprietary software. However, we were able to increase the accuracy of PROJ4 by using a custom 7 parameter transformation, using a set of parameters provided by the Romanian Cadastre Agency. In order to bring the quality of PROJ4 transformations in line with proprietary software, we recommend adopting these parameters into one of the EPSG definitions.
While redefining the parameters does produce a significant improvement in accuracy (up to 1.3 m), the transformation still falls short of the results produced by TransDatRO. In a bid to reach better accuracy, we resolved to generate a custom grid shift file based on the results produced by TransDatRO. We thus computed the datum shift between System 42 and ETRS89 across Romania and saved the results in a format amenable to use in PROJ4 (NTv2). We were thus able to reach an accuracy of 3 cm within the TransDatRO results. Considering PROJ4 uses WGS84 (identical with ETRS89 in the current PROJ4 implementation) as an intermediary reference system, the increase in accuracy will be observed across all transformations involving System 42. Furthermore, considering the versatility of the library, such accuracy can be obtained no matter the input or output format (e.g. by using OGR/GDAL).
Our results show that an accurate transformation between System 42 and other reference systems can be obtained using exclusively open-source tools, eluding the costs of expensive proprietary software. Moreover, our methodology for generating the grid shift file could also be used in other countries where officially-sanctioned transformations are similarly limited in functionality.