I need to incorporate CAD data into my geodatabase but the usual steps seem to require defining a projection or projecting the data in some way.

All of my geodatabase's feature classes are in GCS_WGS_1984 which is technically not projected, so I want the CAD data to be in GCS_WGS_1984 as well.

Is there a way to georeference the CAD data and convert (ie CAD to GIS) it in this case?

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    What are some sample coordinates from your CAD data? If you were working in lat/longs, you probably just need to define your CAD data's projection as GCS_WGS_1984 and import as normal. – Mintx Jul 9 '15 at 20:59
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    Depends what you have available on the CAD side. You can set a coordinate system/projection if you have the right flavor of CAD, just as you can save as a shapefile. You can also come at it from the GIS side and import from a dwg or dxf. Once you have it imported, you can then reference it. In Arc, this is known as a Spatial Adjustment rather than georeferencing (which is for raster data). – Chris W Jul 9 '15 at 21:00
  • What AutoCAD version do you have? is it AutoCAD map? – Shefqet LULJA Jul 9 '15 at 21:18
  • @Mintx - technically speaking, when we say "the projection" we really mean "the projected coordinate system". So can I truly define or project GCS_WGS_1984 which is a geographic coordinate system, but not really a true projected coordinate system? – Joseph Jul 12 '15 at 1:45
  • @ Chris W & Shefqet LULJA - I am using AutoCAD 2012. – Joseph Jul 13 '15 at 13:57

Because you're using vanilla ACAD, you don't have access to coordinate reference systems or the ability to save as a shapefile - that requires Map or Civil (so far as I know). This means you have plain CAD data, which uses a local, Cartesian coordinate system (ie there's an origin at 0,0 and then x/y coordinates on a flat plane). This means you'll have to address the conversion/alignment on the GIS side.

In ArcGIS, that means the Spatial Adjustment toolbar. This is georeferencing for vector data rather than raster (image) data. You'll first import the data and set the CRS to WGS84. Obviously since the units and origin and everything else are different, it will come in at the wrong place. You'll then use the tools under spatial adjustment to create link points between features in the CAD data and where they should be relative to your other layers. Then, when you run the spatial adjustment it will scale/transform/rotate/move/warp/etc. the CAD data to fit. Note this can and probably will introduce distortions, so lines that were exactly 5 meters or feet long in the CAD file may not be exactly that after adjustment. Particularly going from a planar (projected) to a ellipsoidal coordinate system.

There is a very slim chance that your CAD data is already on a projected coordinate system even if you have plain CAD. Since projected CRSs are planar, it's just a question of whether the units and origin match a known projected CRS (for instance a state plane or UTM zone). Almost if not all of them use false eastings and northings, so if your CAD data is anywhere near the origin of 0,0 it's a good sign it won't match a known system. But if the coordinates are out in the hundreds of thousands range and you can identify the projection, you would simply define the CRS as that projection when you import the CAD data. It would be reprojected on the fly in your dataframe, but to make it permanently match your other layers you'd need to use the Project tool on it (not Define Projection).

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