Bogota 1975 (EPSG:4218)
Aug 27,2026

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Introduction

Bogota 1975 (EPSG:4218) is a national geodetic datum formerly used in Colombia. It employs the International 1924 ellipsoid and uses the meridian passing through the Bogotá Observatory as its prime meridian. It was primarily used for surveying and geolocation within mainland Colombia. It replaced three earlier adjustment versions from 1941, 1944, and 1951, and has since been superseded by the more modern MAGNA-SIRGAS datum (EPSG:4685).

Cobordinate System Composition

Reference Ellipsoid: Uses the International 1924 ellipsoid (Hayford ellipsoid), with a semi-major axis of 6,378,388 meters and a flattening of 1/297.
Datum: A geodetic datum defined based on the above ellipsoid, locally fitted with its origin at the Bogotá Observatory in Colombia.
Prime Meridian: Uses the meridian passing through the Bogotá Observatory as the zero meridian, rather than the Greenwich meridian. Its longitude value is -74°04′51.3″ (approximately -74.08091667°).
Angular Unit: Uses decimal degrees as the angular unit for longitude and latitude.
Coordinate Axes and Order: Latitude is defined relative to the Equator (positive northward, negative southward, range -90° to +90°), and longitude is defined relative to the Bogotá prime meridian (positive eastward, negative westward, range -180° to +180°).
Coordinate System Type: A geodetic coordinate reference system, describing the position of any point on the Earth's surface using two angular values (longitude and latitude). It does not involve map projection or linear units.

Pros

High local accuracy: Based on the International 1924 ellipsoid and local fitting to the Bogotá Observatory origin, it achieved planar positioning accuracy better than 10 meters over the core central regions of Colombia (such as along the Andes Mountains) under the ground triangulation networks of its time, meeting the needs of 20th-century surveying and engineering mapping.
Strong stability: Before the widespread adoption of GNSS technology, its fixed origin and regionalized triangulation network structure (such as the ARENA network) provided a long-term reusable control framework, supporting systematic construction of national topographic maps, roads, and infrastructure.
Regional zoning optimization: To address inconsistencies between the ellipsoid and the geoid across the national territory, IGAC divided the country into 8 conversion zones and calculated offset parameters for each, effectively mitigating systematic errors in peripheral areas.

Cons

Non-geocentric and globally incompatible: With Bogotá as the prime meridian (non-Greenwich) and the ellipsoid not aligned with the Earth's center of mass, its coordinates exhibit offsets of tens of meters relative to global systems such as WGS84 and ITRF. It cannot be directly used with modern GNSS devices such as GPS and BeiDou.
Lack of elevation system: Only planar coordinates are defined, with no unified orthometric or normal height system established. Elevation data must be obtained independently, resulting in inconsistent 3D positioning and affecting engineering and geological applications.
Complex conversion and high maintenance costs: Transitioning to MAGNA-SIRGAS (EPSG:4685) requires using the 10-parameter Molodenski-Badekas method, with parameters varying by region. GIS users must manually load transformation models, significantly increasing data processing complexity.
Technologically outdated: The 1924 ellipsoid parameters (semi-major axis: 6,378,388 m) lag behind those of WGS84 (6,378,137 m), and it does not incorporate gravity field models, making it unable to support modern geoid height calculations or integration with satellite altimetry data.

Application Scenario

Bogota 1975 (EPSG:4218), as Colombia's official national geodetic datum from 1975 to 2005, was widely applied in core areas such as national basic surveying, topographic map production, land tenure registration, and road and infrastructure engineering surveying. It supported IGAC (the Geographical Institute of Colombia) in nationwide geographic information collection and updating, particularly establishing high-precision regional control networks along the Andes Mountain range and in major economic zones. Its projected coordinate systems (such as EPSG:21897) were used to produce official topographic maps at scales from 1:50,000 to 1:100,000. These coordinates still exist in numerous historical cadastral archives, early road design blueprints, and public works records.