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I am trying to understand why the degree unit is different between ESRI/Mapinfo and QGIS:

ESRI/Mapinfo PROJCS["GDA_1994_MGA_Zone_56",GEOGCS["GCS_GDA_1994",DATUM["D_GDA_1994",SPHEROID["GRS_1980",6378137.0,298.257222101]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Transverse_Mercator"],PARAMETER["False_Easting",500000.0],PARAMETER["False_Northing",10000000.0],PARAMETER["Central_Meridian",153.0],PARAMETER["Scale_Factor",0.9996],PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0]]

QGIS PROJCS["GDA94_MGA_zone_56",GEOGCS["GCS_GDA_1994",DATUM["D_GDA_1994",SPHEROID["GRS_1980",6378137,298.257222101]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",153],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",10000000],UNIT["Meter",1]]

I believe all software packages suggest I am using EPSG 28356. Is it possible that for a large spatial dataset, the small difference in units can lead to very slight shifts in the datasets?

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    And here's another variation. I wouldn't worry, the value is the degree to radian conversion and the difference is just a rounding effect. – user2856 Apr 10 '15 at 9:59
  • For some reason the link @Luke posted is 404ing, but if you use spatialreference.org/ref/epsg/28356 instead you can see the parameters in a variety of formats (I believe he's linking directly to the Well Known Text as HTML version). As stated, 'same' number rounded to different number of digits, and is probably up to how many the software was programmed to write out. I imagine it would only come into play at levels beyond the tolerance of GIS data storage. Luke might turn that into a formal answer. – Chris W Apr 10 '15 at 20:32
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    Esri originally wrote out the 2nd number (...3295) then later decided to make it more true to double precision so we now write out (...33). In our testing, we found no differences in results using either number. – mkennedy Apr 16 '15 at 18:03
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As commented by @mkennedy:

Esri originally wrote out the 2nd number (...3295) then later decided to make it more true to double precision so we now write out (...33). In our testing, we found no differences in results using either number.

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