Do the differences between these numbers below (i.e. Standard_Parallel_1 & _2) matter at all when importing a .shp file exported from AutoCAD. We were thinking we used to just import .shp files and they would reflect the right WA SPN projection upon import, but now GIS is calling anything we import from CAD a "Custom" projection, presumably because ArcMap cannot relate to the way the .prj file is written from CAD. We can simply c/p the text from the GIS .prj into the CAD exported .prj - but people are asking me if it matters -

can anyone explain the differences in these nos. and whether we should be concerned or not?



CAD (Map 3D)


1 Answer 1


The order of the standard parallels doesn't matter. When checking for equality, we will swap the two standard parallels as part of the process.

The value that's causing the definition to be considered a custom definition is this one:


Internally, we've converting the base value that's defined in meters to US survey feet, and using full double precision. The equality check compared at almost double precision, so this coordinate system definition doesn't match our internal definition.

Edit: discuss effect of swapping the CAD prj file for the Esri prj file.

There will be sub-millimeter differences if you reproject the data to lat/lon or another projected coordinate reference system.

Disclosure: I'm an Esri employee who works on coordinate reference systems.

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