For a Ferro equidistant conic projection ArcGIS 10 still expect that in "new" projection files the central meridian is specified in Greenwich degrees rather than Ferro degrees as it was the case for the "old" projection files.
This is contrary to current rules.
For reference check http://spatialreference.org/ref/epsg/31251/
PARAMETER["central_meridian",28] is universally expressed in Ferro degrees and not Greenwhich degrees
Mapping Center Answer:
It has to do with what prime meridian is being used. Ferro (El Hierro) is at 17deg40min West. Austria often uses it rather than Greenwich as the prime meridian. When defining a projected coordinate system, angular projection parameters like the central meridian are given relative to the connected GCS's prime meridian (thus 28E of Ferro rather than 10deg20min East of Greenwich) AND in the unit of the GCS. In this case, MGI (Ferro) uses degrees.
Because you refer to the equidistant conic projection, I think you might be defining a custom coordinate system. If so, and the definition uses MGI (Ferro), the central meridian must be relative to Ferro, not to Greenwich. I wonder if you tried to use MGI as the GCS. It uses Greenwich.
Note that we have not changed how the units of the angular/linear projection parameters are defined.
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