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I'm trying to view 3 .dwg group layers in ArcMap 10.1. Whenever I add them to the map, I get an error message that says they are missing a spatial reference. That's fine. I'm just trying to view them, but I can't seem to figure out how.

Is there an extension that needs to be enabled? The .dwg files contain contour data, if that makes a difference.

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    Do you get an 'Unknown Spatial Reference' dialog box? You should be able to just click OK to dismiss it (there should also be checkboxes to dismiss the warning for the session or permanently).
    – DWynne
    May 11, 2015 at 23:06
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    Is the problem that they're not added to ArcMap or that they don't line up with other data? CAD data and Raster data can survive quite peacefully without a spatial reference (coordinate system) but can also be without georeference (real world coordinates) - both rasters and CAD support world file creation to assist with this. The problem arises when you try to view data in a different spatial reference to what it is, ArcMap doesn't know how to transform the data from 'undefined' to 'something'. May 11, 2015 at 23:07
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    Since you specifically mention contour lines, in CAD lines aren't always just lines. If these are Civil objects of type contour, they may or may not display correctly from dwg format. They might have to be exploded/exported down to dxf to be brought in. If/when you dismiss the warning, do they not show up at all? Tried zooming to extents? Tried opening the layer attribute table if they do get added, selecting a record, and zooming to feature?
    – Chris W
    May 12, 2015 at 0:26
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    @ChrisW, I once did my head in for hours trying to figure out why contours weren't being imported but were visible in a Microstation file when viewed in Microstation... as it turned out they were a 'reference' file in the file and the actual file contained absolutely nothing except for a link to where the contours were on the network... GeoNeo, are you 100% certain they're there and are of type polyline? May 12, 2015 at 1:02

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Often draftsperson add other items in the model space besides data; title blocks. profile graphs, external blocks, etc, etc. which are not necessarily near the actual source data. When you right-click on the layer in the TOC in ArcMap and chose "Zoom to Layer" and there is still nothing visible you can:

Open the attribute layer of the "line" CAD layer and make a selection query to just select the contours. Hopefully the DWG contains a layer to which the contours are assigned; typically topo, contour, etc. With all contours selected Zoom to Selected Features.

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