Legacy Projection Handling

Projection support has improved with the integration of the Geocalc datasource in Global Mapper. This new method for coordinate reference system handling includes changes and improvements to the projection support included in legacy versions (26.2 and older). The following page outlines the key differences between legacy projection support, and supported map projections in the latest version. If a coordinate system is not listed on this page, then it should perform similarly to legacy versions, although transformations between it and other systems could be more accurate.

When loading a workspace from a version of Global Mapper 26.2 or older that was saved in any of the projections listed on this page, there may be an error stating: "The Coordinate Reference System could not be loaded into Global Mapper. Please review the supported Coordinate Reference Systems in the help documentation." The workspace will then load in the default geodetic system using WGS84. For specific instructions to restore the expected coordinate system/projection, look for the projection in the list below.

Updated projections

The following projections have been updated to adhere to the most accurate or best-practice mathematical formulas available. All formulas will match projections as defined in the GeoCalc projection engine. Users may notice positional differences when compared to legacy coordinate computation (Global Mapper 26.2 and below). Where possible, instructions are provided to users for reproducing legacy behavior if desired.

• Azimuthal Equidistant: Updated with more accurate formulas. Results are more accurate than previous releases, with potential differences of up to 50 meters.

• Cassini-Soldner: Now uses GeographicLib-based formulas, which provide superior round-trip accuracy compared to the Proj4/EPSG standards used previously. It remains best suited for long, narrow north-south regions near the natural origin.

• Hotine Oblique Mercator A: Replaced by Oblique Mercator Two Points. Updated to match USGS Professional Paper 1395 (Snyder). Users may see significant differences from legacy versions, but current results are more mathematically correct.

• Hotine Oblique Mercator B / Hotine Oblique Mercator Azimuth Natural Origin / Rectified Skew Orthomorphic: Replaced by Oblique Mercator Azimuth (optional parameters are used to distinguish variants). Results may differ from legacy versions in cases where zero-value parameters previously caused the projection to be treated as a Transverse Mercator.

• Laborde: Now defaults to the official Tananarive / Laborde Grid (EPSG 8441). To exactly replicate legacy results, users should manually select the"Madagascar Grid" projection instead.

• Lambert Conformal Conic: Legacy Global Mapper allowed for the user to specify a scale factor parameter. This parameter is no longer supported in GM 27.

• Orthographic: Transitioned from spherical to more accurate ellipsoidal formulas. To match legacy behavior, create a custom projection using a custom ellipsoid with spherical radius (legacy GM used average of semi-major and semi-minor axes).

• Robinson: Calculations have been adjusted to align with industry standards and ensure parity with other major software vendors.

• Stereographic: Transitioned from spherical to more accurate ellipsoidal formulas (USGS PP 1395). For EPSG Oblique Stereographic compliance, use the Double Stereographic projection.

• Swiss Grid (Swiss Oblique Mercator): Replaced legacy GM’s approximation with the official Swiss Oblique Mercator as specified in the EPSG guidance notes (9814). Legacy results can be replicated by creating an Oblique Mercator Azimuth projection with the original parameters.

• Two Point Equidistant: Updated to ensure mathematical correctness and ensure parity with other major software vendors.

• Spherical Projections (Van der Grinten, Mollweide, Miller Cylindrical, Hammer-Aitoff, Gnomonic): Legacy versions calculated spherical radius as the average of the semi-major and semi-minor axes when the base datum used an ellipsoid. The current version uses the semi-major axis by default (or an optional spherical_radius parameter). To match legacy behavior, users must define a custom ellipsoid where the semi-major and semi-minor axes equal the previously calculated average, then create a datum based on said ellipsoid, and then a geodetic coordinate system that uses said datum to use as the base for this projection.

• Mercator: Global Mapper now supports two variants. One is defined by a first standard parallel parameter (with scale = 1), and the other variant is defined by a scale that is not 1 (and a “0” first standard parallel). Legacy versions may define a Mercator projection with a non-zero first parallel, and upon import into the current version users will note the projection definition displays a calculated scale factor instead. Either is correct and will result in mathematically identical results.

Projections Requiring Manual Adjustment

These projections are still recognized, but in some cases may require user alteration to ensure correct implementation or full functionality.

• Interrupted Goode Homolosine (now Goode Homolosine Interrupted): Files created in legacy versions will mistakenly load as Goode Homolosine (an uninterrupted variant now supported). Users need to manually edit the system to change to Goode Homolosine Interrupted. Additionally, this projection is spherical. Legacy versions used the average of axes to calculate the radius; current behavior simply uses the semi-major axis. To match legacy behavior, users would need to create a custom ellipsoid, datum and geodetic coordinate system to use as the base for their projection, in which the custom ellipsoid is defined with a semi-major and semi-minor equal to the calculated average.

• State Plane Coordinate Systems (USA): Previously, Global Mapper's definitions for State Plane Coordinate Systems using Lambert Conformal Conic projections had technically non-standard definitions. While data accuracy is unaffected, workspaces saved in those systems in Legacy versions of Global Mapper may not display an EPSG code in the Coordinate Reference System section of the Configuration menu, and a custom system will be created that matches the Legacy Global Mapper definition, which has several extra decimal places when describing the false easting. Best practice is to change the workspace coordinate system to the new definition, so that the metadata contains standard information.

  Additionally, Global Mapper previously supported a number of optional parameters for State Plane coordinate systems (e.g., extra false easting and northing that default to “0”). These extra, non-standard parameters are no longer supported and will be ignored when the file is opened in the current version. Note: an extra shift or rotation can be applied to a layer in the Selected Layers Context Menu.

• Minnesota County: Legacy Global Mapper supported a version of MN County projections that were defined by a custom ellipsoid for each county. The current version instead supports the county projections as an extended LCC projection with an additional parameter to account for elevation adjustment. Legacy MN county data that uses a custom ellipsoid will load as a custom coordinate system with a custom datum. As a result, if a datum transformation is needed, typically none will be found. For a workaround, users can override the auto-created custom system and instead choose the new GeoCalc equivalent for the county; all use GRS 1980 datum, making those datum transformations accessible.

Projections Requiring Manual Entry

The current version of Global Mapper will not automatically read the projection information from files using these projections if they were created in legacy versions of Global Mapper, because version 26.2 and below created non-standard PRJ strings for these projections. However, these types of projections are supported in the current version, and users can manually set up systems matching legacy behavior by creating user-defined custom projected coordinate systems. Note the updated names for some of these projections.

These projections have also been updated to adhere to the most accurate or best-practice mathematical formulas available. All formulas will match projections as defined in the GeoCalc projection engine. Users may notice small positional differences when compared to legacy coordinate computation.

• Bipolar Oblique Conformal Conic (now Bipolar Oblique Conic West): Manual recreation required; results match within error tolerance.

• Lambert Azimuthal Equal Area (now Azimuthal Equal Area): A Lambert Azimuthal Equal Area projection that includes a rotation parameter may not import correctly. If so, the parameters can be manually edited.

• Polyconic: Manual recreation required. Results match legacy behavior within error tolerance, except when the legacy projection used a rotation parameter. The rotation parameter is not currently supported.

• Times: Manual recreation required; results will vary slightly from legacy versions, as the formula has been adapted from the original source (Snyder) to ensure parity with other major software vendors.

• Wagner VII: Manual recreation required. Formulas have been updated for higher accuracy and use more accurate constants matching the original source; differences may be as much as 20 meters. When the base datum used an ellipsoid, legacy versions used the average of axes to calculate the radius; current behavior simply uses the semi-major axis. To match legacy behavior, users would need to create a custom ellipsoid, datum and geodetic coordinate system to use as the base for their projection, in which the custom ellipsoid is defined with a semi-major and semi-minor equal to the calculated average.

• Vertical Near-Sided Perspective (now Vertical Perspective): When the base datum used an ellipsoid, legacy versions used the average of axes to calculate the radius; current behavior simply uses the semi-major axis. To match legacy behavior, users would need to create a custom ellipsoid, datum and geodetic coordinate system to use as the base for their projection, in which the custom ellipsoid is defined with a semi-major and semi-minor equal to the calculated average.

• Winkel I & Winkel Tripel: Manual recreation required; results match within error tolerance.

Depracated/Unsupported Projections

These projections were available in Global Mapper 26.2 and older, but are not currently supported in the GeoCalc integration. Please reach out to technical support if you require any of the following projections:

• Aitoff-Wagner

• Alaska Conformal

• Equidistant Conic A

• Marta Software Satellite

• Mercator (Lowrence Special)

• Mercator (Navionics Special)

• Miller Oblated Sterographic

• Sinusoidal Integerized