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In ArcGIS, there are two datums they look like same except their names; GCS_European_1950 and GCS_International_1924. Where I started to work, I found out that they are using both datums. Since they are two different datums, ARCGIS asks for Geographic Transformation Parameters and there are no predefined any parameters. But I realized somebody had defined a custom transformation parameters(with Coordinate_Frame Method) in the past and its parameters were defined as 0. Is it OK to do transformation this way between these two datum from GIS point of view?

It seems like use of GCS_International_1924 is not common and I am quite confuse about the difference between those two geographic coordinates?

GCS_European_1950 WKID: 4230 Authority: EPSG

Angular Unit: Degree (0,0174532925199433) Prime Meridian: Greenwich (0,0) Datum: D_European_1950 Spheroid: International_1924 Semimajor Axis: 6378388,0 Semiminor Axis: 6356911,946127947 Inverse Flattening: 297,0

GCS_International_1924 WKID: 4022 Authority: EPSG

Angular Unit: Degree (0,0174532925199433) Prime Meridian: Greenwich (0,0) Datum: D_International_1924 Spheroid: International_1924 Semimajor Axis: 6378388,0 Semiminor Axis: 6356911,946127947 Inverse Flattening: 297,0

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International 1924 is an ellipsoid (spheroid, in ArcGIS), not a true geodetic datum nor geographic coordinate reference system. EPSG has defined many geographic coordinate reference systems that are based on an ellipsoid, rather than on a true datum. This was done for data where the datum is not known but the ellipsoid is. If you know that a dataset is really using ED 1950, the definition should be changed to that, and not left as International 1924.

Disclosure: I'm on the IOGP subcommittee that maintains the EPSG Geodetic Parameter Registry. Also I work for Esri.

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