Redefining of “EPSG” and “PROJ” for current State Coordinate Reference System of the Republic of North Macedonia

The state Coordinate Reference System (CRS) of the Republic of North Macedonia (RNM) has been established a century ago, by the Military Geographic Institute of the Yugoslavia Kingdom. It is in official usage entire period up to day. In international public EPSG registry of geodetic datums, spatial reference systems, Earth ellipsoids, coordinate transformations and related units of measurement, CRS for RNM is recognizable within 3 EPSG codes 6204, 6316 and 8679. First code EPSG 6204 represents current state CRS for the entire country area, based on current law, however unfortunately this CRS is official by the law but it is not used for developing the official spatial data published in geoportals of Agency for Real Estate Cadastre (AREC) and NSDI geoportal of RNM. The second code EPSG 6316 is defined to be used for 6 countries of former Yugoslavia that covers area between 19.5°E up to 22.5°E longitude, which does not correspond with the practical and official usage of CRS for working with spatial data in RNM and CRS law definition in RNM. Third code EPSG 8679 has never been used in RNM, which covers eastern part of RNM and Serbia beginning from 22.5°E. Beside of problems with EPSG codes, default transformation parameters of EPSG 6316 have low accuracy and can not be used for data overlapping with open layers. Therefore, redefined new EPSG codes for state CRS of RNM are proposed in this paper.


Introduction
Coordinate Reference Systems (CRS) represents the mathematical model for location defining of spatial entities within the particular referent frame, which contain geodetic datum, and may contain map projection as well additional parameters. In practice it is known as spatial reference system (SRS) also, which is synonym of CRS [9]. Geographic and projected coordinate systems are two types of CRS's that are used in practice and coded for standardized usage in GI software (Idrizi 2020). Each CRS is associated with a local or global datum and should support an operation that exposes the domain of validity of that system (Staudinger 1999). The State Plane Coordinate Refence System is one or a set of geographic zones or coordinate systems designed for specific regions within national area, associated with a geodetic datum. Standardized usage of CRS in GIS software gives opportunity to reach uniform and easier usage of same CRS by all users, without need for deep knowledge on CRS parameters. CRS's are specified in ISO 19111:2019 "Geographic information -Referencing by coordinates" [1], defined by the OGC [2] simple feature access using well-known text representation of coordinate reference systems, and referred to using EPSG codes defined by the International Association of Oil and Gas Producers [3]. It is very important to know that ISO 19111 was not defined for geodetic experts, it was made for producers and users of GIS, therefore the structure shall be clear and easy, as well as correct on a common level of abstraction (Ihde et all 2000). The EPSG dataset conforms to ISO 19111:2019, that is distributed as EPSG registry, EPSG database and in a relation model as SQL scripts (IOGP 2019), as Spatial Reference System Identifiers (SRIDs) and data definition for identifying coordinate reference systems, projections, and performing transformations between them, used by most of geographic information systems and GIS libraries as tool for standardized developing and usage of spatial data by all users. In parallel, PROJ (former PROJ.4) as generic coordinate transformation software that includes map projections and geodetic transformations (PROJ 2021)  By reviewing current CRS's in EPSG database that covers national area of the Republic of North Macedonia, none of them can be fully used, and are not appropriate for part of official spatial data (Idrizi 2019), which prove the need for necessity changes in EPSG coding of North Macedonian CRS's. Requesting feedback by the North Macedonian national mapping agency AREC before applying changes is of big importance and is very valuable step, however if the national responsible institution does not support or ignore necessary changes, IOGP should give opportunities for changes in cases when request comes from third parties (non-official institution) but proven with proof in practical and/or scientific level. Proposed CRS's for North Macedonia should be compiled correctly on a common level of abstraction with the clear structure for wide usage by geo community and other experts who use spatial data, without need for detailed knowledge for CRS, according to basic rule of ISO 19111:2019.

Nonformal CRS's of North Macedonia
In practice, beside formal CRS, two CRS's are being defined due to Cartesian coordinate values used for developing spatial data, with the difference on using or excluding first digits 7 for Easting and/or 4 for Northing coordinate.

Inherited CRS from Yugoslavia
All spatial data for the area of North Macedonia before its independence on year 1991, have been compiled based on 7 th projecting zone of the Former Yugoslavia with given parameters in the

Differences between formal and nonformal CRS's of North Macedonia
Three CRS's used in North Macedonia differs between them on false easting and false northing CRS values, which effects on 7000km distance along easting and 4000km distance along northing axis, between spatial data developed in three CRS's. In the next figure, distances between, and positions of three CRS's in plane coordinate system are shown.

EPSG codes for CRS of North Macedonia
In international public EPSG registry of geodetic datums, spatial reference systems, Earth ellipsoids, coordinate transformations and related units of measurement, CRS for RNM is recognizable within 3 codes 6204, 6316 and 8679.

EPSG 6204 -Macedonia State Coordinate System
First code EPSG 6204 represents current state CRS for entire country area, based on current law with abovementioned CRS parameters in table 1, however unfortunately this CRS is official by the law but it is not used for developing the official spatial data published in geoportals of Agency for Real Estate Cadastre (AREC) and NSDI geoportal of RNM. Therefore, EPSG 6204 can not be used as CRS for working with official spatial data of RNM, although it is fully compatible with the law for real estate cadastre and entitled in EPSG registry as "EPSG 6204 Macedonia State Coordinate System" [10]. In next figure 5, structure of EPSG 6204 is given, while in figure  6 area covered is given.   figure 9 the structure of EPSG 8679 is given, while in figure 10 area covered in EPSG 8679 is shown.

Problems of using datum transformations
Beside above-mentioned problems with CRS definitions and practical usage by AREC and geo community in RNM given in head 3, datum transformation between MGI 1901 and WGS84/ETRS89 given in head 4 is one of the main limitations in everyday work with spatial data of RNM. Transformation parameters of EPSG 6204 based on PROJ.4 (+ellps=bessel +towgs84=521.748,229.489,590.921,4.029,4.488,) are well defined, however this CRS cannot be used for processing of North Macedonian official spatial data due to problem with false easting value. On the other side, default transformation parameters of EPSG 6316 based on PROJ.4 (+ellps=bessel +towgs84=682,480,0,0,0,0) have low accuracy and can not be used for data transformation aimed for overlapping with open layers by transformation of spatial data between North Macedonian and other CRS based on WGS84. Because of this limitations, existing EPSG coding of NRM state CRS's should be redefined and adopted to the current conditions with spatial data in RNM.

Redefinition of EPSG and PROJ.4 codes for CRS's of North Macedonian spatial data based on state CRS parameters and official datum transformation parameters
Above-mentioned problems with CRS definitions of North Macedonia in EPSG coding, datum transformations and current status of three type of official spatial data, itself states the need for defining new EPSG codes, in order to enable easy work with spatial data developed in different CRS's. This could be achieved with establishing two new CRS's and by keeping EPSG 6204 as it is established at this moment. By this system, EPSG 6316 which is currently in use in official geoportals of state and local self-government institutions will be used as regional CRS, instead current usage as national CRS for North Macedonia. The EPSG 6316 should not be redefined, but it should remain as a regional CRS that covers areas of six countries from ex-Yugoslavia, with cross-referred available options for datum transformation. In next table 7, which CRS's shown in Proj.4 format are given as CRS's that are necessary to be established, that will avoid current accuracy problems by geo community during using EPSG 6316 and overlapping with open layers.

Conclusions
New proposed CRS's for North Macedonian spatial data developed by AREC and other institutions in central and local level, is more than necessary for practical issues which will simplify usage of GIS tools for overlapping spatial data developed in different CRS's, even in WGS84 and ETRS89 geodetic datums. Proposed options are in use provided by Geo-SEE Institute to its stakeholders as custom defined CRS's; however, those should be officialised by establishing new EPSG code structures for each purpose with single available geodetic datum transformation. With this kind of definition, GIS users will be able to use/overlap spatial data from different CRS's, even from different geodetic datums, without requirement for deep knowledge on CRS's and transformations between them. Although North Macedonia in near future will change its datum to ETRS89, for the historical purposes and further usage of spatial data developed in past period, new EPSG codes are more than necessary to be established as soon as possible with above given parameters. All proposed new EPSG codes are in line with ISO 1911, developed for producers and users of GIS, and not only for geodetic experts, with clear, easy and correct CRS structure on a common level of abstraction for usage by non-professional users.