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I am having trouble transforming data from GCS_Krasovsky_1940 to GCS_WGS_1984 in ArcGIS 10.4.1. I'm using the Project tool, but the geographic transformation parameter is left empty and I would expect this to be necessary to carry out the operation. If I run the transformation with the parameter remaining empty the resulting layer has the WGS84 coordinate system, but I do not trust that this is correct.

I read that the GCS_Krasovsky_1940 is really a spheroid and not a datum - is this important in this case?

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If a coordinate reference system (CRS) identifies itself as an ellipsoid (spheroid in Esri-speak) and there's no datum information, there are no available transformations.

Ellipsoids are used for multiple datums and geographic CRS. If you have the area of interest, you can find out what geographic CRS have been used in the area. If there are multiple ones, all based on Krasovky, which one do you choose?

You can look up possible CRS in the EPSG Geodetic Parameter Registry.

Disclosure: I'm on the subcommittee that maintains the EPSG Registry, and an Esri employee.

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What is the epsg code of your data that ArcGIS recognizes? It should be 4024 for the Krasovsky ellipsoid without datum. Try to set it to the data if it is not recognized and have a look if ArcGIS has a transformation then. Otherwise it would be best to have a look for reference points when you are not sure about the correct positions.

The ellipsoid, coordinate system and datum are together the coordinate reference. The ellipsoid is the "idea" of the earth shape and the datum is position, orientation and scale of the coordinate system that will be used for the projection of the data. As you don´t want to project it you don´t need the datum.

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