NGA Advisory Notice on "Web Mercator"

Application Risks to Operations and Adherence to DoD World Geodetic System 1984 (WGS 84)

  1. This message outlines the risks associated with the use of Web Mercator applications for DoD and GEOINT user community operations. The NGA Geomatics Office has assessed the use of Web Mercator and other non-WGS 84 spatial reference systems may cause geo-location / geo-coordinate errors up to 40,000 meters. This erroneous geospatial positioning information poses an unacceptable risk to global safety of navigation activities, and department of defense, intelligence community, and allied partner systems, missions, and operations that require accurate and precise positioning and navigation information. The NGA Geomatics Office reminds the community to use DoD approved World Geodetic System 1984 (WGS 84) applications for all mission critical activities.

  2. Many popular commercial visualization, mapping and mobile web device applications use a Web Mercator spherical earth reference. These applications are not an equal alternative to DoD WGS 84, as they do not provide the same geo-coordinates or geo-location accuracies. Common terms associated with non-WGS 84 applications include but are not limited to: Web Mercator, WGS 84 Web Mercator, WGS 1984 Web Mercator (auxiliary sphere) and European Petroleum Survey Group (EPSG) Spatial Reference System Identifier (SRID) (such as) EPSG: 3857. Users are advised that many projected, un-projected, and local spatial coordinate system definitions do not conform to WGS 84.

  3. A spherical or Web Mercator spatial reference system uses mathematical formulas and parameters that are incompatible with and non-compliant to the WGS 84. Noted defects extend to erroneous calculation of ground control points, positional system data, and variance with WGS 84 GEOINT (imagery, geospatial information, and imagery intelligence).

  4. Authority: in accordance with Chairman of the Joint Chiefs of Staff Instruction, (CJCSI 3900.01C), 30 June 2007, Position (Point and Area) Reference Procedures, WGS 84 is directed for standardization by the National System for Geospatial-Intelligence (NSG), Allied System for Geospatial Intelligence (ASG), North Atlantic Treaty members, and Intelligence Community (IC) members, to fully support geographic coordinate accuracy and interoperability requirements.

  5. Recommended course of action: in accordance with CJCSI 3900.01C, users and associated programs will continue to reference coordinates (horizontal and vertical) to DoD approved WGS 84 spatial reference system. Additional references on the Web Mercator issue and the use of WGS 84 are available at the following URLs:

  6. ·    JWICS: http://www.geoint.nga.ic.gov/products/gandg/

    ·    SIPRNET: http://www.geoint.nga.smil.mil/products/gandg/

    ·    NIPRNet: http://osis.nga.mil/GandG/index.htm

    ·    Internet: http://earth-info.nga.mil/GandG/

  7. For assistance please contact the NGA Geomatics Office (SN) using the point of contact information provided below:

  8. ·    NGA Geomatics Office

    ·    Phone: (314) 676-9123, DSN: 846-9123

    ·    Email: 

  9. Background Information:
  10. Implementation Practice Web Mercator Map Projection
    1. "Technical information on the Web Mercator map projection to include its relationship to the World Geodetic System 1984 (WGS 84) and the risks and impacts associated with its usage as a positioning reference tool for the National System for Geospatial-Intelligence (NSG)."

    DoD Instruction 5000.56
    1. "All acquisition programs relying on GEOINT data shall be designed to use standard GEOINT data whose content, formats, and standards conform to those established by the National System for Geospatial Intelligence (NSG) through community government processes."
    2. "Navigation and positioning systems shall be based on NGA standards for conversion into the Universal Transverse Mercator System, Military Grid Reference System, and the World Geodetic System 1984 datum or follow-on system."

    Chairman Joint Chiefs of Staff Instruction 3900.01C
    1. "World Geodetic System 1984 (WGS 84) is the official DOD positional reference system. Universal use of the WGS 84 positional reference system (datum) will eliminate confusion regarding which system is being used in reporting positions."

    DoD Standard Practice 2401
    1. "WGS 84 shall provide the basic reference frame (coordinate system), geometric figure for the earth (ellipsoid), earth gravitational model, and means to relate positions on various geodetic datums and systems for DoD operations and applications."
    2. "WGS 84 shall be the primary geodetic system used in DoD systems and Mapping, Charting and Geodesy (MC&G) products."

    NGA Standard Practice 0019.2.0
    1. "Unambiguous expression of the values of these characteristics requires the identification of a robust coordinate reference system, such as those associated with the World Geodetic System 1984 (WGS 84), and the ability to specify the values of such characteristics with respect to that coordinate reference system."

    NIMA Technical Report 8350.2
    1.  WGS 84 represents the current state-of-the-art DoD operational reference system which has been derived from the best available satellite tracking, satellite altimeter and surface gravity data. It supports the most stringent accuracy requirements for geodetic positioning, navigation and mapping within DoD. It is the policy of NIMA to continually improve components of the WGS 84 system to maintain it as a state-of-the-art DoD operational World Geodetic System. 

Point of Contact: Office of Geomatics
Phone NCW: (314) 676-9123, DSN NCW: 846-9123

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