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Batavia (Batavia Datum | EPSG:4211) is a region-specific, non-geocentric datum used primarily in the former Dutch East Indies (present-day Indonesia) during the early 20th century. Established to support colonial administration, topographic surveying, cadastral mapping, and infrastructure development, it served as a unified reference for surveying and mapping centered around Java. Today, it has been entirely replaced by modern geocentric coordinate systems such as ITRF and WGS84. Its practical relevance is now largely confined to the conversion and analysis of historical maps and legacy survey data.

Lisbon 1890 (EPSG:4803) is a regional non-geocentric datum established by Portugal at the end of the 19th century. It primarily provided a unified coordinate framework for the country's early terrestrial surveying and topographic mapping (for example, when combined with the Bonne projection to form EPSG:2963). As a national standard from a specific historical period, it has been completely superseded by modern geocentric datums (e.g., ETRS89). Its current value is limited solely to processing and converting legacy Portuguese data such as historical maps and cadastral archives from the late 19th to mid-20th century, which, after professional conversion, can be used for digital archiving and historical geographic analysis.

Roma40 (Rome 1940, EPSG:4806) is a regional, historical geodetic datum designed specifically for Italy and its surrounding regions (such as Sicily and Sardinia), and belongs to the geocentric coordinate system. Its core purpose was to provide a benchmark for official Italian surveying from the 1940s to the 1990s.

ED87 (European Datum 1987, EPSG:4231) is a regional geodetic datum established in 1987 specifically for Europe, primarily Western Europe. It is a traditional geodetic datum designed to align the coordinate framework as closely as possible with the European geoid at the time and served as a key intermediate achievement in the modernization of European geodesy. Its primary application is in processing historical surveying data, topographic maps, and engineering drawings from Western Europe predating the 1990s. However, as a transitional datum based on terrestrial triangulation, its accuracy and global compatibility are now insufficient. Today, it is mainly used for scenarios such as historical data integration, digital archiving, and specialized research after being transformed into modern geocentric coordinate systems like WGS84 or ETRS89.

NAD27 (North American Datum 1927) is a geodetic datum established in 1927 for North America, defined based on the Clarke 1866 ellipsoid with its origin at Meades Ranch in Kansas, USA. This datum served as the official mapping and surveying standard for the United States, Canada, and Mexico for decades, widely used in historical topographic maps, land ownership records, and traditional geographic data. Although its accuracy is limited by early surveying technology and it has been superseded by more modern datums (e.g., NAD83), a significant number of historical archives and legacy systems still rely on NAD27. Therefore, special attention must be paid to datum transformations when dealing with older geographic data in North America.

Eckert IV (Eckert's Fourth Projection) is an equal-area map projection proposed in 1906 by German mathematician Max Eckert. Designed for representing the entire Earth on a world map, it accurately preserves area while distributing shape distortions relatively evenly. It is commonly used in GIS and thematic mapping for visually representing statistical data and distribution information. It is implemented by ESRI as ESRI:54012.

A Compound CRS achieves unified representation of multi-dimensional spatial data by combining multiple coordinate reference systems (CRS). For example, it integrates a planar coordinate system (e.g., EPSG:7661) with an elevation system (e.g., EPSG:3855) to form a complete "planar + elevation" spatial description. Its core features include ensuring consistency between horizontal and vertical coordinate systems through associated parameters (such as ellipsoids and datums), making it suitable for fields requiring simultaneous expression of planar positions and elevations, such as terrain modeling and 3D GIS analysis. Technically, a Compound CRS typically comprises a horizontal CRS and a vertical CRS, unified through nested or associated parameters.

Albers equal-area conic projection is an equal-area conic projection that utilizes two standard parallels. Compared to projections with only one standard parallel, this approach helps reduce distortion to some extent. It is best suited for mapping large continental areas extending predominantly in an east-west direction in the mid-latitudes and is commonly used for maps of the contiguous United States, Europe, and Australia.

Robinson projection is a widely used map projection method developed since the 1960s, designed to represent the entire world on a single map. It seeks a practical compromise for displaying the whole Earth on a flat surface. Introduced in 1963 by Dr. Arthur H. Robinson, this projection features straight latitude lines and evenly spaced meridians, classifying it as a pseudocylindrical projection. While the central meridian is straight, all other meridians are curved. Robinson defined this projection by constructing a lookup table of Cartesian coordinates through specific interpolation of latitudes and longitudes, rather than using mathematical formulas.

Mercator projection is a conformal tangent cylindrical projection devised by the Flemish cartographer Gerardus Mercator in 1569. It conceptually projects the Earth's surface onto a cylinder tangent at the equator, with a light source at the Earth's center casting surface features onto the cylinder. When unrolled, this creates a world map with the standard parallel at zero degrees (the equator).